#27 - Charles Weschler: Unpacking the Impact of Ozone on Indoor Air Quality and Health

A converation with Charles Weschler

After completing his Ph.D. in chemistry at the University of Chicago, Dr. Weschler did postdoctoral studies with Prof. Fred Basolo at Northwestern University.

In 1975 he joined Bell Laboratories as a  research scientist in the Physical Chemistry Division. He conducted research at Bell Labs and its successor institutions until 2001 being named a Distinguished Member of Technical Staff (1986).

In 2001 he retired  from Bellcore/Telcordia and accepted positions at the Environmental & Occupational Health Science  Institute, Rutgers University, and the International Centre for Indoor Environment and Energy, Technical  University of Denmark. He has continued in those positions through the present. In 2010 he joined the  faculty of the Building Science department at Tsinghua University (Beijing) as an ongoing Visiting  Professor.

He is also an Adjunct Professor in the Rutgers School of Public Health.

He was a Member of  the Committee on Air Quality in Passenger Cabins in Commercial Aircraft, National Academy of Sciences,  2000-2001;

Advisor on Strategies to Protect the Health of Deployed US Forces, National Academy of  Sciences, 1998-2000;

Member of the Committee to Review the Structure and Performance of the Health  Effects Institute, National Academy of Sciences, 1991-1993;

And Member of the Committee on Advances  in Assessing Human Exposure to Airborne Pollutants, National Academy of Sciences, 1987-1990.

From  1999-2005 he served on the US EPA’s Science Advisory Board, and from 2012-2023 was an advisor for  the Sloan Foundation’s Chemistry of Indoor Environments program.

He was elected to the International  Academy of Indoor Air Sciences in 1999 and received the Pettenkofer Award, its highest honor, in 2014. 

He has been conferred the 2017 Haagen-Smit Prize from Atmospheric Environment; “Distinguished  Visiting Professor” at Tsinghua University (2018); “Doctor Technices Honoris Causa” from the Technical  University of Denmark (2018); and was elected a Fellow of the American Association for the  Advancement of Science (2020, AAAS).

We spoke about Ozone, it impacts on human health and the air chemistry in our indoor environments.

And needless to say, if you go looking for information and papers on Ozone Charles Weschler if not an author will be cited somewhere. 

So it was a pleasure and honour to talk to Charles, who is an absolute gentleman and so generous with his time.

Ozone is fascinating, never far from controversy and quite a naughty little pollutant by all accounts. It plays a fascinating role in our indoor environments.

This is a really insightful episode and full of information, so strap in, get ready to Google some chemistry names, and enjoy.

Charles Weschler - Linkedin
 

Transcript

Simon: 0:13

Welcome to Air Quality Matters, and this is a conversation with Charles Weschler. After completing his PhD in chemistry at the University of Chicago, he did his postdoctoral studies with Professor Fred Basalo at the Northwestern University. In 1975, he joined Bell Laboratories as a research scientist in the physical chemistry division. Chemistry division. He conducted research at the Bell Labs and its successor institutions until 2001,. Being named as a distinguished member of the technical staff In 2001, he retired from Bell Corp and Telcordia and accepted positions at the Environmental and Occupational Health Science Institute in Rutgers University and the International Centre for Indoor Environment and Energy at Technical University of Denmark. He has continued in those positions through to the present day and in 2010 he joined the faculty of the Building Science Department at Tengchua University in Beijing as an ongoing visiting professor. He is also an adjunct professor in the Rutgers School of Public Health and has served on National Academies of Science committees several times. He was elected to the International Academy of Indoor Air Sciences in 1999 and received the Penton Coffer Award, its highest honour, in 2014, amongst many others.

Simon: 1:51

I'll link her full bio in the show notes. It's quite a read, I can tell you. And, needless to say, if you go looking for information or papers on ozone. Charles, if not an author, will be cited somewhere, I can tell you. So it was an absolute pleasure and honour to spend time talking to Charles. He's an absolute gentleman and so generous with his time. Ozone is absolutely fascinating, never far from controversy, and quite a naughty little pollutant by all accounts, and plays a fascinating role in our indoor environments. I think this is a really insightful episode and full of information, so strap in and get ready to Google some chemistry names and enjoy this one. Thanks for taking time out of your day to listen. As always, this is Air Quality Matters and a conversation with Charles Weschler. What actually is ozone? We hear it spoken about a lot. I don't think many of us, including myself, really understand it that well, and why does it cause so much trouble?

Charles: 3:10

cause so much trouble. Ozone is a molecule that consists of three oxygen atoms linked to one another. Let's contrast ozone with oxygen in the air, the oxygen we think of when we breathe, that's involved in respiration. That oxygen in air, 21% of air, that's two oxygen molecules linked together. Ozone is three oxygen molecules linked together. Ozone is much less abundant in air than dioxygen. It's a more powerful oxidant than dioxygen, the oxygen that's important for our respiration.

Charles: 3:47

But ozone can only react in the gas phase rapidly with certain molecules. It's not like ozone reacts quickly with everything. There's a subset of chemicals that have carbon-carbon double bonds for those of your listeners who are chemists and ozone reacts fast enough with those compounds to compete with typical air change rates in a building. But there's other compounds that don't have those carbon-carbon bonds and ozone reacts with them relatively slowly. Bonds and ozone reacts with them relatively slowly. Crudely speaking, about 10% of the organic pollutants indoors react with ozone fast enough for that reaction to be significant. We can get back to that point. I want to say something about the hydroxyl radical, but we're not there yet. I want to say something about the hydroxyl radical, but we're not there yet. I want to say a little bit more about ozone itself.

Charles: 5:02

So outdoors ozone is formed primarily by the action of the sun on a mix of organic compounds and nitrogen oxides. In outdoor air Ozone is formed, created when the sun's shining during the day. The sun rises and the chemistry starts. The sun sets and the chemistry shuts down. Ozone is not created during the night, ozone is created during the day. So sunshine, that photochemistry is very important. So let's think about that a moment. That implies that ozone chemistry is more important in the summer, when the flux from the sun is more intense than in the winter, and that's certainly the case. And the higher the sun is during the day, the greater the flux and the more ozone is generated. So typically outdoors ozone peaks in the mid-afternoon. It's climbing during the morning. It's still climbing. At noon there's a bit of a lag, maybe around one, two. In the afternoon it peaks and then it starts to go down as the sun starts to go down.

Charles: 6:08

Now, if you're in an urban area and you have a lot of automobile emissions, a lot of motor vehicle traffic during the night, that ozone will go almost to zero because nitric oxide and motor vehicle exhaust will titrate that ozone. Because nitric oxide and motor vehicle exhaust will titrate that ozone. If you're in a suburb or a rural area, the ozone probably will not go down to zero overnight. It will go down but probably not to zero Over the night. If you're living in a suburb, you might actually see your ozone go up and you're saying what's going on? Well, probably you're downwind of the city. That had higher ozone during the day than you did, and that air that's coming across you from the urban area at night, that's elevating your ozone. That's just a little background on outdoor ozone. But it's important to understand outdoor ozone if you want to understand indoor ozone, because in most situations our indoor ozone comes from outdoors. Outdoor to indoor transport of ozone is very important.

Simon: 7:18

Is this something that generally causes us harm out in the ambient air? And the other question I had for you is is it something that's created equally in all parts of the atmosphere? Because you know, for those of us that are old enough, we remember an emergency several decades ago about a hole appearing in the ozone layer.

Charles: 7:46

A hole appearing in the ozone layer, so I'm assuming there's a story to tell about where we find ozone and how much good or harm it does on balance, generally in the ambient space you're taking us into some very interesting areas and I'll try not to spend too much time on them, but I can get carried away In terms of ozone being a natural constituent of the atmosphere. Yes, humans have probably always lived with ozone, but the levels of ozone in our outdoor air, at ground level, have likely gone up since the Industrial Revolution. We didn't have the ability chemically to measure ozone until 1850, 1860, 1870. When we first learned how to do it, ozone measurements were made in cities like London and Paris. They were also made on certain mountains. That was of interest because of change with elevation. Well, if you look at those early measurements of ozone, which are chemically sound, which were well done, they're telling us that ozone levels in urban areas today are substantially higher than they were in London or Paris in 1870. What was observed when they made those measurements on mountains was that ozone was higher at altitude than at sea level, and that remains the case today. If you're in an airplane at 10,000 to 11,000 meters you're in an airplane at 10,000, 11,000 meters the ozone concentrations outside the aircraft cabin can easily be larger than 500 ppb, whereas at ground level it's more likely to be around 50 ppb during the summer in a city like Paris or Atlanta or Washington DC.

Charles: 9:45

Then you alluded to the ozone hole. The ozone hole refers to ozone in the stratosphere. There's a little saw. How does it go? It's basically saying that ozone is good up high and bad down low. When we can breathe it, it's bad when it's up high. It protects us from UV radiation. The ozone concentrations in the stratosphere are hundreds thousands of times larger than the ozone concentrations at ground level and, yes, they're very important for protecting us from UV radiation. Of course, uv can contribute to skin cancer. It can also drive chemistry that we might not want to happen outdoors. The Montreal Protocol forbade the use of certain chlorofluorocarbons that were getting into the stratosphere, where they were being busted apart and chlorine atoms were chewing up ozone and you could actually see this so-called hole in the ozone layer up there in the stratosphere. Fortunately, the hole has healed since the montreal protocol, um, but it's still a concern. Does that distinction make sense to you, simon, between ozone up high and ozone down here where we're breathing it?

Simon: 11:16

I, I think it does, and I think it's an important distinction, because I you know um, and it probably is a generational thing, like if you're of my age, um, when you were young and very open to this kind of thing, the ozone, the hole in the atmosphere, was a real thing.

Simon: 11:35

As a kid you know it was something that was found out.

Simon: 11:38

It was serious and there was a global effort in the production of products to change our ways to have an impact on the environment.

Simon: 11:47

It was one of the first experiences, perhaps aside from removing lead from petrol, which was probably a little bit before that, but it was one of the first kind of global environmental things that was happening for me as a kid going, kid going, wow, like there's something in the air that is important to us and we can have a big impact. And you have to kind of remind people of that every now and then that as a species, we can actually see us, see ourselves going down the wrong path and actually make a correction if we get organized and do it. Because I asked somebody the other day is is the um, is the issue with lead in the air still a thing? Even anymore? Have we managed to eradicate, because you know, when I was even younger, it was damaging sandstone and historic buildings in urban areas and things, and we seem to have fixed that as well. So we have a track record, actually, and it's quite encouraging sometimes to know that we can actually recognize this stuff and have an impact.

Charles: 12:50

Yes, yes, there's other examples as well where we really have improved things and actually ozone is one of those. Simon, ironically, about the same time you're talking about the concern regarding ozone in the stratosphere and the ozone hole, there was considerable concern about ozone pollution at ground level. If you lived in greater Los Angeles, it was not at all unusual to have outdoor ozone levels routinely above 100 ppb ozone levels routinely above 100 ppb. We made detailed measurements of outdoor and indoor ozone at a telephone switching office in. It was around 1994. And for 14 months we continuously measured outdoor and indoor ozone at that office, burbank, california, and it was not at all unusual to see the outdoor ozone over 100 ppb, especially during the ozone season from about April perhaps to October. We've improved things. The ozone levels don't get that high anymore.

Charles: 14:04

I live in New Jersey. When I first came here, ozone and smog were a serious problem and we just didn't see blue skies during the summer days. Today we have mostly blue skies in the summer, if it's not cloudy, and our ozone levels are so much lower than they were in the mid-70s, in the early 80s, and that's because of steps that have been taken to reduce the emissions of certain organic compounds and nitrogen oxides from the tailpipe of motor vehicles. You can't change the sun. The sun's going to be doing its thing, but if you reduce the precursors, you'll reduce the amount of ozone formed when the sun is shining. So that's another success story, and it's not just in the United States, but it's also in many large European cities. Ozone is less of a concern for your listeners who live in london than your listeners who live in paris or athens. Uh, but I don't. I don't know what the ozone is in london, what it got up to today.

Simon: 15:13

Um, yeah, that's, that's a miss for me. I should have checked before the call. Actually, it might be a good point to do just a couple of reference points for people. Um, when we're talking about parts, uh, pbb, we're talking about parts per billion, by the way, which are, which is an incredibly small amount of something in something, um, it's very hard to describe how small that is, but parts per billion of anything is really quite, quite small, um, but you thought you've thrown out a couple of numbers there that I think might be worthwhile as just putting a reference point in for people for later on in the discussion.

Simon: 15:50

What's our current kind of WHO type target to keep ozone below generally? What are we trying to do these days? What are the numbers?

Charles: 16:00

I'm embarrassed. I should remember that.

Simon: 16:19

I'm embarrassed. I should remember that the current National Ambient Air Quality Standard in the US is 70 ppb, as I recall, and I'd have to double check on the WHO outdoor standard. It's in that ballpark. It might be a little lower and you were talking about seeing measurements regularly up at the 100 or plus historically. So we've managed to bring that down somewhat in the environment.

Charles: 16:33

If you look at these maps of the United States, which show regions that are in violation of the National Amphion Air Quality Standard for ozone, of the National Ambient Air Quality Standard for ozone, those maps used to have many, many regions in the United States that were out of compliance and today far fewer regions are out of compliance and the standard has gone down during that period. It's easier to go out of compliance today than it was in the 70s and 80s because the bogey, the bogey number, was higher in the 70s and 80s, 1970s, 1980s so, if I get you right, from a, from a what is ozone perspective?

Simon: 17:21

um, it's one of those things that is going to be there to some degree. We've probably always, always existed around it to some degree. Um, it's a bit like baking a cake in some way. We're always going to have the sun applying the heat. We've just got to not provide it with the ingredients to to rise. You know, it's one of those kind of one.

Charles: 17:44

That's a good analogy, but we do want to remember that ozone levels in urban areas today are probably substantially higher than ozone levels in urban areas in the 1700s or early 1800s. It's not like humans have evolved experiencing high levels of ozone. I think it's important to make the point. We're talking about ozone and the reason we're talking about it is because of concerns regarding ozone and human health, and I should just, in broad strokes, talk about the evidence there. Brilliant and there was a very large, well-done study by authors Turner et al came out in 2016. And it looked at the effects of long-term exposure to ozone. It's an epidemiological study. The ozone in the study was measured at different outdoor monitoring stations or it was inferred by calculations If they didn't have outdoor monitoring stations close enough to the individuals they were following. They followed a huge number of people. They followed them over a period of time and they looked how a 10 ppb increase in ozone outdoors how that affected different health endpoints. They found that that 10 ppb increase in outdoor ozone resulted in a 12% increase in respiratory mortality deaths from something like COPD 12% increase with a 10ppb increase in ozone. They also looked at cardiovascular effects circulatory mortality. That went up 3% over the period of the study. Ozone goes up 10 ppp. Circulatory mortalities increase 3%. If we look at mortality from all causes, mortality from all causes went up 2% for that 10% increase in ozone.

Charles: 20:04

If you dig more deeply into the data, you'll find that if you're exposed to PM2.5, it increases the adverse effect of ozone. Now people can explain that. The explanation goes something like the PM2.5 causes irritation in the respiratory tract and that irritation makes you more susceptible to what ozone does People, the epidemiologists. They correct for confounders when they do these studies and you can deliberately look at how a pollutant like PM2.5 influences the effects of ozone. You can treat it as a confounder In terms of causation. Chamber studies have been really valuable in terms of helping us better understand mechanistically why ozone is bad. If I put you in a chamber and the chamber contains clean air, but we deliberately generate ozone with something like a UV lamp and I ask you have you ever done one of those tests with a ping pong ball and a tube? You blow into the tube. How high can you get the ping pong ball?

Simon: 21:32

I've not done it personally, but I've seen them. Yes.

Charles: 21:35

Okay, it's a simple measure of your lung capacity. Yeah Well, if you're in a chamber and you're breathing 100 ppb of ozone for a couple of hours, you're not going to be able to get that ping pong ball this high. I don't know if you were you ever a runner.

Simon: 21:56

No, but I'd have played a lot of sports when I was younger, so I would have done a lot of running.

Charles: 22:01

But again, you were probably playing in a region where you didn't have high ozone. Personally, I used to run quite a bit. We had a race that went on every Monday night during the summer a 5K, and some of those summer nights the ozone would be high. You'd finish those races and you'd be coughing. You'd take a shower later on and you try to take a deep breath and you caught your breath. Now that that's ozone affecting our respiratory system. So I'm again. I'm making this too long, but we do have mechanistic explanations for why ozone is bad.

Simon: 22:44

I think it answers a fundamental question, though, charles, and that is what is ozone and what does it do that we're concerned about? Because, as somebody said on the podcast earlier last year, we're not concerned about the pollutants that don't cause us harm. So it clearly does something to us. So there's a physical reaction at a cellular level, at a physiological level, to ozone. Is that correct?

Charles: 23:18

It's something that is harmful to us to us Now understand the causation issue Much better for ozone and respiratory health than we do for ozone and cardiovascular health. As a matter of fact, the studies are somewhat conflicting there. There's studies that show ozone adversely affects cardiovascular health. There's other studies that have not observed that. A question is what's going on there? I think the evidence is firming up that when outdoor ozone rises, there's more bad things happening in terms of individuals' cardiovascular health, but it might not be ozone per se that's doing it, and that's another topic that I'd like to talk about and now's probably a good time to introduce it.

Charles: 24:20

That large epidemiology study I told you about looking at increases in ozone and increases in different health outcomes. That study was using ozone monitored outdoors at central locations In the United States. It might have been 100 different central locations. However, that study, although all they're measuring is outdoor ozone, it's capturing three types of exposures. It's capturing inhalation of ozone when the people were outdoors. It's capturing inhalation of ozone when the people were indoors and those levels are much slower, much lower, than outdoors.

Charles: 25:06

I'd like to talk about that. And it's capturing inhalation of ozone products when people are indoors. I'll say that again. The exposures responsible for these long-term health effects. It's three different types of exposures, if you will Inhalation of ozone when you're outdoors, inhalation of ozone when you're indoors and inhalation of ozone products when you're indoors. Why have I left out inhalation of ozone products when you're outdoors? That would be the symmetric right. The products outdoors are much smaller in concentration than the products indoors. We can, to a first approximation, ignore inhalation of ozone oxidation products indoors. Most of our inhalation of products derived from ozone chemistry occurs indoors as a consequence of ozone initiated chemistry indoors interesting and this goes back to a fundamental point we often make on this podcast is because we spend so much of our time indoors.

Simon: 26:22

Even our exposure to outdoor pollutants often occurs indoors, and ozone, I'm guessing, is a key example of that that a large percentage of our exposure to out unless we're a very outdoors person in a job specifically, a large degree of our exposure to that pollutants is going to happen in our own homes and in our workplaces.

Charles: 26:50

You're taking me good places. I want to say something there. Indoor ozone levels are lower than outdoor ozone if there's no indoor source. Median indoor ozone levels. Typical indoor ozone levels are somewhere around 5 ppb. In homes, in schools, in offices, indoor ozone levels are typically 15 to 30 percent of outdoor ozone levels if there's no indoor sources. So let's pretend that the products of ozone chemistry have absolutely no effect, that it's only ozone that's bad.

Charles: 27:37

Ozone indoors is identical to ozone outdoors. It's the same chemical. It doesn't change. Ozone is ozone. So let's say indoor ozone is 5 ppb and outdoor ozone is 35 ppb Very common numbers. And let's say it's a day where you spend three hours outdoors and 21 hours indoors. So 3 times 35, that's 105, right. And 21 times 5, that's 105, right. So for that simple example, you inhale as much ozone indoors as outdoors. I say again we assumed that indoors was five and outdoors was 35. We assumed we were outdoors for three hours and indoors for 21. Now we should refine it and say what was ozone when you were outdoors, because ozone has these diurnal variations, also happens indoors. But you see, the point I'm making related to your point, even though indoor ozone concentrations are only a fraction of outdoor concentration. Because we spend so much more time indoors, we're likely to inhale comparable levels of ozone indoors to what we inhale outdoors that's interesting.

Simon: 29:07

Yet I suppose, if we've always been around ozone and ozone was probably very low in the past and we were much more outdoors historically we've probably always been around five and ten pbbs of ozone, so maybe that isn't so bad. But I think we're probably going to completely bust people's noodles here, to excuse, uh, an english colloquialism, but I'd say we're probably going to fry people's brains when I ask the question why is indoor ozone so much lower than outdoor ozone? If we're changing our indoor air frequently enough let's say, half an air change an hour, an air change an hour we're completely replacing the air in our buildings frequently enough. Why would ozone be so much lower indoors than it is outdoors?

Charles: 30:07

Because ozone reacts in indoor environments. Ozone reacts on indoor surfaces. Ozone reacts with chemicals in the air. Let's assume the air change rate is half an air change an hour, what you just mentioned. You said a half to one. We can use either one of those numbers. The rate at which ozone is removed indoors because of reaction with chemicals on surfaces and chemicals in the air, that rate is about three per hour. So if you simply compare the rate at which ozone is reacting to the air change rate three per hour compared to half per hour, that's a factor of six difference, right? You're making products and I want to emphasize, you make stuff when ozone reacts with indoor surfaces. You make stuff when ozone reacts with something in the air.

Charles: 31:09

The rate at which you're making these products, it's easy for that to compete with an air change rate of half an air change an hour or even an air change an hour. But you are right, simon, as the air change rate goes up you have less time for that chemistry to affect the indoor environment. For the gas phase chemistry it's kind of fun to think about this. For gas phase chemistry, imagine we have ozone reacting with molecule A. If that reaction is too slow, they're swept off the stage before they can talk to one another. Okay, that reaction has to be fast enough to compete with whatever the air change rate is. For a gas phase reaction, okay, and there are certain gas phase reactions that are fast enough to compete with even higher change rates. But you definitely help yourself two ways with ventilation. You dilute the products with ventilation and there's less time for the chemistry to occur as the ventilation rate goes up.

Simon: 32:19

Conversely, I guess if it's too slow, you give more time for that chemistry to occur. Sweet spot between letting the ozone decrease and less chemistry taking place and moving air more quickly and bringing more and more ozone in to react. So I'm guessing the higher the air change rates, the more faster reactive byproducts you'll have. The slower the air change rate, the more slower byproducts you'll have. The slower the air change rate, the more slower byproducts you'll have. So that I guess it depends what byproducts you're producing. And that's where ozone is really interesting, because it it is a bit naughty, it does produce some rather naughty byproducts, doesn't it from from?

Charles: 33:20

an air quality perspective and we can. And I'm glad you mentioned that because maybe five minutes ago I was saying the hypothetical. Let's pretend that it's ozone, only ozone that has adverse health effects. But I think more and more we're appreciating that some of those products are going to contribute to adverse health effects. The big question is how much? But think about some of those products Formaldehyde People know formaldehyde is bad Ozone. Any organic molecule that has what's called a terminal double bond. That means at the very end of the molecule it's C double bond, c. When ozone reacts with that terminal double bond, one of the products will be formaldehyde, acetaldehyde Acetaldehyde is also not a very good chemical, toxic chemical, and you can make acetaldehyde from ozone. You make more complicated compounds. I don't want to lose your listeners with complex chemical names, but forgive me just to mention some of the things that scare me.

Simon: 34:36

Half the fun of air chemistry is the names that people go. You know the what now? So go on, go for it because I think that's the fun bit because there's some great names okay, so these are.

Charles: 34:48

these are charlie's, charlie's chemicals of concern dicarbonyls, organic peroxides, diol peroxides, organic nitrates, secondary organic aerosols. We can say more about those hydropyroxy radicals, alkyl pyroxy radicals, secondary ozonides, organic hydroperoxides those last two compounds I mentioned, they're thermally stable. They hang around if the air isn't too moist, but if we inhale them we've got a moist respiratory tract. They react to make these reactive oxygen species, ros, bad guys. So anyway, those are some of the products of concern. I ask your listeners to forgive me for the chemical names, but the larger important point is not all ozone products are innocuous. Some of the products are a concern. Unfortunately, we don't know enough about the toxicity of many of these products.

Simon: 36:02

Yeah, or have the epidemiological evidence to understand their impact at a population level. But we do, certainly for some of them, some of the aldehydes we have quite a bit of information on, particularly formaldehyde. You mentioned that it produces formaldehyde, um, it seems, relatively easily. Does it also make the existing formaldehyde problem worse? Because one of the problems we have with formaldehyde in the indoor environment is is a byproduct of a lot of the building materials and furniture materials we have in the space. Is ozone having a part to play in that story in some way as well?

Charles: 36:45

To be fair, simon, if you have a significant indoor source of formaldehyde imagine you have the resin in the plywood is a source of formaldehyde. In most cases that's going to be emitting formaldehyde at a substantially larger rate than you get formaldehyde made by the ozone-initiated chemistry. If you have no meaningful indoor sources of formaldehyde, that chemistry will certainly elevate the indoor formaldehyde. But if you have a strong indoor source of formaldehyde already the extra formaldehyde from the ozone chemistry it's just a small addition to what you already have.

Simon: 37:37

Okay, interesting. And before we forget, you said secondary something, something, twice. Secondary something something first time and secondary something, something the second time.

Charles: 37:47

Two different things that I can't remember either let's just talk about the first one, we won't. We won't bother with this with the second one. The first one is the more important, I think secondary organic aerosols. It's a fancy name for a class of particulate matter, a class of airborne particles. Airborne particles that are generated as a consequence of chemistry, chemistry involving organic compounds. They're referred to as secondary organic aerosols. In urban areas there's a great deal of interest in secondary organic aerosols. These aren't particles that are directly emitted into the air. They're formed as a consequence of atmospheric chemistry, typically photochemistry.

Charles: 38:35

Now I mentioned a short while ago the evidence for the toxicity of ozone. We have even stronger evidence for the toxicity of PM. You had Rich Corsi Professor Rich Corsi on your show a couple weeks ago Excellent show and he spoke at length about PM 2.5 and the National Academy of Sciences report that has just come out that he chaired and he explained to your listeners just how toxic PM2.5 is and the very strong evidence, mechanistic evidence behind the epidemiology. So let's compare the toxicity of PM2.5 to the toxicity of ozone. Pm2.5 is substantially more toxic, maybe a factor of 20. It depends on what you're talking about Now.

Charles: 39:39

Ozone can react with certain compounds to make PM secondary organic aerosols. So this ozone chemistry is a way to get to PM. I'll give you a good example. When you peel an orange, you emit limonene. A classic classroom demonstration of this chemistry is to have something like a bell jar with a UV lamp and you take the peel of an orange or a lemon and you squeeze it in the bell jar. You get a cloud. The ozone in that bell jar reacts with those terpenes to make this cloud of secondary organic aerosols. It's a very cool demonstration.

Simon: 40:27

Oh, brilliant.

Simon: 40:28

Yeah, those are secondary organic aerosols and we are concerned that those secondary organic aerosols, when inhaled, have substantial health effects. It sounds phenomenally complicated to second guess what's going to happen with those. I mean, you understand the chemistry, but you're, and you may even understand let's. Let's just say, for example, we don't have any internal sources of ozone, that we're just dealing with what we're bringing into the indoor environment. We're bringing into the indoor environment, but it sounds like there's so much within the indoor environment, so many varied products that can react to ozone.

Simon: 41:11

It's very difficult to tell. Just because you've got low ozone in a space doesn't mean ozone didn't come in and hasn't already reacted and turned into something else. You know that there's that, it's it's. That's the beauty of chemistry, is it's this, this constant moving feast of reaction? So, unlike a pollutant, that is what it is and just floats around in the air and tends to cause you harm. It's about filtering it or removing it or stopping it coming in basic engineering controls. You throw a good dose of chemistry in here. It makes it far more complex, doesn't it, to try to figure out what you're likely to see in any given environment.

Charles: 41:54

Certainly true, simon. I want to re-emphasize something you just said. You nailed a really important point and you mind if I just say it again when we measure indoor ozone at 5 ppb, we're measuring a residual concentration. Okay, it's the ozone remaining after that. Outdoor ozone has reacted on indoor surfaces and in indoor air. That difference, I think a really informative number, is the difference between outdoor ozone and indoor ozone at any building site. Imagine you're really interested in ozone and you're going to different buildings. If you had an instrument with you and you measure outdoor ozone and you measure indoor ozone, you look at the difference.

Simon: 42:46

That tells you something about the oxidation products in that building, in that building so you're talking about when you're talking about sinks, it's basically the thing that is reacting with ozone and dropping ozone in a space, and it's just a question of what it's become like. That. That's what you're trying to then figure out. Okay, what are the what is, and is that measurable? I mean, can you walk into a space and figure out where that extra 30 parts per billion of ozone has gone in the air? Are there some known knowns? Do we know certain building materials and certain products? Are there certain very inert interactions and things we can put into a space that are going to sink ozone and wrap it up into stuff that we don't care about? And can that totally outweigh the negative? How do we figure out what to do?

Charles: 43:45

Let's pause, sorry.

Simon: 43:46

Yeah, sorry, my brain's gone off on one.

Charles: 43:48

You've already touched on three additional important points.

Simon: 43:52

Okay.

Charles: 43:54

And let me see if I can remember them. The first question that I heard was can we identify what these products are? Well, it's only relatively recently, using very expensive, state-of-the-art instruments, where ozone and its products have been measured in a home or in an office. Alan Goldstein's group at UC Berkeley has done some beautiful studies in actual homes. I think it's up to three homes. Now They've taken their PTRMS. We won't go into what that instrument does, Just take my word for it. It's very good at detecting organic compounds at very low concentrations. It can measure concentrations down to 10 parts per trillion not billion, but per trillion and it does this in real time. Well, he's, but it's expensive. It's $700,000, $800,000. Okay, it's not cheap, and outdoor atmospheric chemists have used this technology for a number of years.

Charles: 45:16

Alan has this type of instrument. He's brought it indoors. These studies were funded by the Sloan Foundation as part of their indoor chemistry program. His first study was in a home in Oakland, California, for eight weeks during the summer. For eight weeks he's measuring outdoor ozone and indoor ozone continuously. He's measuring a slew of organic compounds in the air continuously. He's measuring them outdoors, in the kitchen and in one of the bedrooms.

Charles: 45:49

In this study you can actually see. When the air exchange rate changes. You see changes in the ratio of indoor to outdoor ozone. You typically see the highest indoor ozone, as you would expect, in the mid-afternoon, on those days when the air exchange rate was higher and the outdoor ozone was higher. When the indoor ozone goes up, we see products of ozone chemistry going up, just like they should. Some of those products are products formed when ozone reacts with the skin oils of the occupants. Our skin oils contain some chemicals that react rapidly with ozone and we've learned what the products of ozone skin oil chemistry are. And we've learned what the products of ozone skin oil chemistry are and some of these products are very they're good markers of ozone skin oil chemistry because they're not formed from other sources. So an example is 4-OPA. When ozone reacts with squalene, which is about 10% of our skin oil by weight, you get 4-OPA. So if you see 4-OPA rising indoors, that's telling you ozone skin oil chemistry has occurred.

Charles: 47:04

A simpler molecule, decanal. That's just 10 carbons with an OH at the end. Decanal is formed when ozone reacts with sapienic acid, a constituent of our skin oil, and triglycerides that have the double bond in just the right spot. Well, in that study I mentioned in this home in Oakland. When ozone increased, decanal would go up. When ozone went down, decanal would go down. They did something else really cool the occupants went on vacation for a week during the study. So what do you think happened when the occupants went on vacation? What happened to the 4-OPA or the decanal? Did they go to zero as soon as the occupants walked out the door? Zero as soon as the occupants walked out the door? No, it took several days for the concentrations of 4-O-P-A or decanal to decrease. Why? Because when we occupy a building, we leave our skin oil everywhere. It's on surfaces we touch. Think of fingerprints. Right, Fingerprints are just skin oil on surfaces. Ozone reacts with fingerprints. It gives you 4-OPA and Decanal.

Simon: 48:24

So I'm assuming it's bedsheets, clothing, anything that you touch anything that's in contact with you is picking up those oils and then reacting Right.

Charles: 48:34

And so the occupants leave and you stop the recharging process. When the occupants go out the door, that source of indoor skin oil goes with them. But they've left behind their skin flakes that contain skin oils, their surfaces. They've touched the bedding, the clothing You're so right, simon, to mention the bedding and the clothing. So it takes time to see this ozone, skin oil chemistry decay. It's occurring even when the building is unoccupied. Eventually it'll be hard to see Things get chewed up. So that's the kind of thing we can see with these state-of-the-art instruments making measurements continuously in real time and seeing what changes when ozone changes. And we can do accounting in those kinds of studies. We can talk about what fraction of the ozone has been consumed, what fraction has created product X or product Y or product Z. We can calculate yields of ozone oxidation products.

Charles: 49:52

I should call out Brandon Boer's group at Purdue. He's done analogous studies in an office building I think it's a zero energy office building. I'd have to double check that. He has two papers out now, but it's the same idea. He's taken one of these very expensive instruments. He's put it in the office. He's operated for an extended period of time. He's put it in the office. He's operated for an extended period of time In his case I think it's about a month and measured not just the organics but outdoor and indoor ozone and air change rates and temperature and relative humidity other relevant parameters so valuable in terms of testing our understanding.

Charles: 50:37

Would we have predicted what we wind up seeing? Are we seeing things that surprise us? How well does our understanding, how well does our modeling capture what we're seeing in these buildings? To my knowledge, these studies have not yet been conducted in schools. I think I'd love to see a study like this conducted Now. I take that back as I'm talking. Alan Goldstein's group did operate their PTRMS in a classroom at UC Berkeley and the first author on that paper is Tang T-A-N-G and I think that was around 216 or 217.

Simon: 51:22

I'd love to see the risk assessment of. I'm taking my 800,000 quid machine into a classroom of preschoolers and now you squared that one away with the buses.

Charles: 51:35

Yeah, I'm not sure where Alan hit his machine for those studies and to be fair, I should call out Paul Zeman's group Jose Jimenez's group at UC Boulder, because they also made measurements in a classroom, a university classroom. So off the top of my head, I can think of two studies that have made measurements in university classrooms one at Berkeley, one at UC Boulder, one from Goldstein's group, the other from Zeman-Jimenez group, and then the studies in homes by Goldstein and the studies in offices by Brandon Boer. But the very fact that, talking to you now, I can name the studies that have been done, that is just a handful of studies.

Charles: 52:23

We need so many more of these studies We've learned so much already from them and there's a lot more to be learned from them. Now. I said that your question had three elements to it. I forget the other two elements. Do you remember them?

Simon: 52:39

yeah, well, I suppose if I was to summarize the other two elements, it's to to condense what we've just talked about, which is clearly there's some fascinating stuff going on when it comes to reactions in the indoor environment with ozone. But as a practitioner, the first question that I asked is do we care? Are those reactions of interest to us or not? And of the ones that are of interest to us, what can we do about them? So it kind of goes to my question is are we aware of certain sinks and reactions that are going on in typical indoor environments that we should be concerned about or be positive about and can either enhance or try and reduce?

Simon: 53:30

So are there building materials that we can introduce into buildings that have positive reactions with ozone? That limits the amount of ozone that can react with stuff we don't want? Is there stuff that we do indoors that creates things that has negative interactions with ozone? Because ultimately because I imagine the chemistry of this stuff there's, there's content for days on, but ultimately it's the. There's content for days on, but ultimately it's the what's causing us harm. How can we understand it and have an outcome, improve our outcomes with it?

Charles: 54:03

Take me back to the do we care? Question. But right now, right now I want to answer the second part, the mitigation question, and you asked it initially in a way that suggested perhaps there's certain surfaces that would be good to have indoors, and indeed that's the case. Let's go back to Professor Rich Corsi. Rich has, together with Glenn Morrison from University of North Carolina and colleagues, they've studied materials that react with ozone without generating bad stuff. So imagine you had a sheet, a bed sheet, that contained lots of activated carbon and you just put that bed sheet on a wall. Some of the ozone will strike that bed sheet, it'll be removed, but you won't get bad stuff. There's a Japanese company. I forget the name of the company, but I remember the name of the product. The product is called Semia, s-e-m-i-a, and Semia is something like that hypothetical bed sheet I just described.

Simon: 55:18

I think I've heard of that. I've seen people hanging it in big open spaces and you just allow the air to kind of waft through it and it reacts yeah.

Charles: 55:27

We did studies at a museum outside of Copenhagen. It's a museum in a suburb of Copenhagen, outside of Copenhagen. It's a museum in a suburb of Copenhagen and they have these storerooms where they keep their cultural artifacts and they're concerned about ozone in the storeroom because it's bad for the cultural artifacts. And they hung simia on the walls of the storeroom and it made a substantial difference in ozone levels in the storeroom and it made a substantial difference in ozone levels in the storeroom. It was a relatively inexpensive solution. I use that word cautiously. I don't know the lifetime of simia. How long does it keep doing its thing, I don't know. But when I was involved in this study it was impressive. It worked. So let's go back to what which Corsi and and Glenn Morrison did. They looked at materials like that that have the potential to remove ozone without creating bad stuff. One of the things that they looked at was taking a ceiling fan I'm pointing up because I have a ceiling fan here and they put strips of activated carbon cloth on the blades of the fan and just circulated that fan and that was taking some of the ozone out of the room. They also looked at certain types of paint and they found that some clay-based paints were good at removing ozone without creating products, and Rich actually came to Technical University of Denmark, where I've been working as a visiting professor since the early 2000s. We have some wonderful chambers at Technical University of Denmark, at DTU, and in one of those chambers we deliberately generated ozone and looked at ozone concentrations with a surface that had not been painted with one of these paints and then after it had been painted with one of these paints, and indeed the ozone levels were lower. You know, using this particular paint, again, we don't know how long the paint is efficacious. So this is an attractive area to passively remove ozone. To passively remove ozone Now it's nowhere near as impactful as removing ozone actively. Ozone is actively removed as we speak, in a number of commercial buildings.

Charles: 58:21

If you're making semiconductors, you're probably taking ozone out of the air of that space where you're fabricating the semiconductors. I got involved in this to some extent when I was still working at Bell Labs, or actually one of its subsequent institutions, and we looked at different charcoal filters and in this case we looked at the lifetime At these charcoal filters. We looked at how efficient they were taking ozone out of the air. Now, these filters used a large amount of charcoal. As I recall it was about 45 pounds of charcoal. As I recall it was about 45 pounds of charcoal for what was about a two foot by two foot filter, so about two thirds of a meter by two thirds of a meter. And I'm not good at converting. Well, 45 pounds to kilograms, but you can make a rough estimate. It's a lot of charcoal to kilograms, but you can make a rough estimate. It's a lot of charcoal.

Charles: 59:20

But the filters lasted a very long time. We kept those studies up for eight years for our primary project and those filters initially were taking out 95% of the ozone in the air and this is back when New Jersey had much higher levels of ozone. And eight years later they were still taking out 65% of the ozone in the air. We did another study. It only lasted three years but we looked at how we could protect the filters, the activated carbon, from particles and whether that impacted the efficiency. And if we had a pretty good particle filter upstream of the activated carbon filter, it remained very effective for the entire three years. The soiling by the particles was over time impeding the effectiveness of the activated carbon, but if that carbon was protected from soiling by particles, it remained effective.

Charles: 1:00:24

Say something else about activated carbon filters they have a relatively short lifetime for removing odors. Activated carbon, those pores that capture organics. Those pores fill up. Maybe a higher molecular weight organic will displace a lower molecular weight organic from the pore. Ozone. It's a totally different removal mechanism. It's not the pores. Ozone is reacting with dangling carbon bonds on the activated carbon and activated carbon has a lot of dangling carbon bonds and it's going to take a very, very long time for ozone to chew up all the dangling carbon bonds.

Charles: 1:01:10

And you keep making new bonds, dangling bonds, because what you're making breaks off. You might get some CO2. You might get some carbon monoxide Not enough to be of concern.

Simon: 1:01:35

But when that happens, you in into the residential settings that have activated carbon in them. These air cleaners that, as part of the filtration layers, may have a carbon filter perspective of removing some gases and odors. Actually, it's probably still doing a very good job when it comes to ozone removal, even perhaps if it's not as effective downstream.

Charles: 1:02:04

That would be correct, Simon. That would be correct, Assuming that the filter contained enough carbon in the first place. In some of these filters I've seen on standalone air cleaners, there's a very small amount of activated carbon. We did studies at DTU with what was called a combination filter. At the time it was being sold by Cambridge, I think, and these studies go back to 2003, 2004. And we operated. We compared different types of filters, but this combination filter was appealing. Are you familiar with EU7 filters, the terminology EU7?

Simon: 1:02:51

Don't think so no.

Charles: 1:02:52

EU7 refers to removal efficiency, and it's a pretty good removal efficiency for PM, an EU7 filter. I think it's something equivalent to maybe a MERV 13 filter. Many of your listeners know better than I do, oh, sorry, yeah, I know them as F7 filters. Okay, F7. Yeah.

Simon: 1:03:12

I think they've changed them now. They're now under new European stands. They're called EPM1 and EPM 2.5. So I think they now directly relate to the particle size, whereas previously they were called F7 or G4. Filters would be the pre-filters, so people would know those names.

Charles: 1:03:33

So at the time Cambridge had a filter, an F7 filter, and they put enough activated carbon in it that it also removed ozone. As I said, they called it a combination filter and when we evaluated it at DTU it effectively removed ozone and one of the benefits was, over time the air downstream of that filter was not as obnoxious to a sensory panel as air coming through a standard F7 filter. You know how a standard F7 filter over time it can affect air quality Well, and we had these panels at DTU that were assessing the air quality downstream of the filters and, yeah, these filters. So I can envision, imagine if you want to build something like a Corsi Rosenthal box, but if you could build it with this F7 filter that can contain enough activated carbon, I think it'd be very interesting to see what kind of job it did on indoor ozone conversation.

Simon: 1:04:53

what I was asking was do we care? And what we've just discussed is the the positive side of that um equation, the what products could we put in a space or how could we build our indoor environments to have a positive impact on ozone? What have those studies that have done the expensive monitoring shown when it comes to the kind of activities or the products that are in our indoor spaces that are causing negative reactions, reactions that are harmful, the formaldehydes and the other aldehydes, for example? Are there things that we should know about or understand or be changing in our indoor environment to limit the impact, because ozone is always going to be there? Right, the question is is are we, are we creating the ingredients to bake the cake? You know what? What do we need to be concerned about?

Charles: 1:05:47

those studies that I mentioned to a large extent confirmed our understanding of the ozone chemistry that occurs indoors. Before those studies were conducted, we had done chamber studies at DTU looking at ozone when human beings were present and we identified 6-MHO and 4-OPA and Decanal and other products of ozone human chemistry. There was a large project at DTU called iCHEAR that focused on the emissions that result when humans are exposed to ozone. They had a 28 cubic meter chamber. They had different panels of four volunteers I think there were altogether maybe 12 different panels of four volunteers and they exposed them to ozone whose steady state concentration when the occupants were in the chamber was 35 ppb. And they've published some very good papers showing the products that are observed. These are products primarily of ozone reacting with humans and if you compare the results of those chamber studies to what was observed in these homes that were studied by Alan Goldstein or the office studied by Brandon Bohr or the classroom studied by Paul Zeman and Jimenez, things fit together nicely. Okay. We're seeing what we should be seeing, based on the chamber studies. So now we're back to what we were talking about maybe 45 minutes ago. We don't have.

Charles: 1:08:00

Humans were an important part of the indoor ozone equation when we did studies at DTU in their simulated Boeing 767. 767. One of the chambers at DTU is five or six aisles of a Boeing 767. And Boeing built that chamber there. To recall I'll help you recall the 767 is a two-aisle plane, it's a widebody, okay. So if you have five or six rows of the 767 in economy you've got room for a fair number of volunteers. And DTU did a lot of simulated flights with volunteers, looking at how temperature and relative humidity affected the perception of air quality on the part of these volunteers. And at some point we got permissions, ethical approval, to add ozone to those flights and we did it just as you would. The ozone levels were typically lower than would actually be encountered in commercial flights if the aircraft wasn't deliberately taking ozone out of the ventilation air, and we were just struck by what we saw. That's the first time we really understood that ozone was reacting with our skin oils. When I had done studies before that, I had avoided occupied environments. Occupants were a nuisance and I was being dumb. I didn't realize how important the occupants were to the chemistry. So this was.

Charles: 1:09:48

We started these ozone aircraft experiments in 2004. And Armin Visthaler, who was at University of Innsbruck at the time. He's now in Norway and a full professor. Armin brought his state-of-the-art PTRMS from Innsbruck to DTU and we set it up so we could make the measurements. And Armin's looking at the products and he's saying I know this, I know what I'm seeing. And he says we're looking at products of ozone, squalling chemistry. Armin had the knowledge of the literature to recognize that when he saw it and we spent a lot of time looking at that and it was one of those exciting periods. Armin would stay there till three in the morning working over the results and we'd get ready for the experiments the next day and Armin would have his pot of coffee next to him and we'd start all over again with a new set of volunteers and we'd change conditions and measure yields. No-transcript Okay, so I'm making this too long.

Charles: 1:11:25

We saw 4-OPA and 4-OPA is a dicarbonyl and 4-OPA sent oh, it made bells ring in my head, alarm bells. It's a dicarbonyl and dicarbonyls are dangerous and a classic example is diacetyl, which is used to give butter taste to microwave popcorn. And perhaps you've read about the workers who actually stirred those huge pots and put in the diacetyl. They had serious lung damage. Some of the lung damage was so bad that those workers required lung transplants. That's diacetyl.

Charles: 1:12:03

So 4-OPA, this chemical we were seeing, it's another dicarbonyl and I asked a friend at NIOSH do you know anything about the toxicity of 4-OPA? I tried to find something. I couldn't find anything. My friend at NIOSH, ray Wells, he had much better sources he couldn't find anything. He talked to a toxicologist there, stacey Anderson. She couldn't find anything. But she thought the question was important enough that she wound up doing her own toxicity tests. She's published three papers now on the toxicity of 4-OPA.

Charles: 1:12:39

It is toxic. It's not as toxic as diacetyl, but if the concentrations get high enough it will certainly cause problems. It'll cause irritation. Fortunately in most indoor environments the concentration doesn't get high enough, but it's an example of a compound I think we want to pay attention to. We don't want to see the concentrations of 4-OPA get up to 50 ppb. Typically they're like 2 ppb or less. Simon, just to put it in perspective. I don't want to scare the listeners, but this is just one product and we have all these other products. I mentioned some of these products that scare me Diol epoxides. I'd like to see some toxicity tests on some of these diol epoxides that are being produced.

Charles: 1:13:32

I worry about highly reactive, short-term compounds that are being produced. We worry about reactive oxygen species that are generated when we inhale PM2.5 or when we inhale ozone. What happens when we inhale reactive oxygen species? We inhale reactive oxygen species. What happens when ozone reacts with limonene, that lemon scent? One of your products is hydrogen peroxide. We have a sense of how toxic hydrogen peroxide is not terribly toxic. We use it to clean wounds and what have you. But if the concentration gets high enough it could cause problems. But what about some of these hydropyroxy radical or an alkyl pyroxy radical? How damaging is it when we inhale something like this? And then the secondary ozonides that's the other secondary that you heard me mention. That's an example of a compound that is perfectly happy, content to sit on a surface if things are dry and it will last for a fairly long time days and months, time, days and months. But if that secondary ozonide has been formed on the surface of a particle and you inhale that particle, it's going to encounter the moisture and you've got a reactive oxygen species wherever the reaction happens. So we don't know the toxicity species. But I'm worried about the potential toxicity of some of these products and that's why I care.

Charles: 1:15:18

Yeah, let's speak more generally. We've been talking about a lot of chemistry driven by ozone. I told you about in an earlier conversation we had, about the National Academy's study why Indoor Chemistry Matters, and that study came out about two years ago. I love the title of the report why Indoor Chemistry Matters. I mean the title is telling us that this is something we should pay attention to. And here's this National Academy panel 12 experts from different fields, including toxicology, and a lot of the chemistry that they're looking at in this report is ozone-initiated chemistry and they're saying that we should pay attention to this.

Charles: 1:16:10

This matters that funding agencies should fund studies on indoor chemistry and its consequences, that it's not just academic interest. It'd be nice to see in the US the National Institutes of Health, the NIEHS, the National Institutes of Environmental Health Sciences. It would be good to see them funding studies of the consequences of indoor chemistry, the potential health effects, and I think we're going to see those kind of studies going forward. But we haven't had we really haven't had them large-scale health studies examining different types of indoor chemistry. We haven't had those kind of states yeah, I'll tell you a funny story.

Simon: 1:17:02

I'm involved in um, the international energy agency's annex on air quality and ventilation, the aivc, which you probably know about.

Simon: 1:17:14

A lot of your colleagues in the states would have been involved in it over the years and recently there's been an annex that's looked at.

Simon: 1:17:22

Among other things it's been the start of the work by Ben Jones and Max Sherman on the harm intensities of certain pollutants, right, but one of the pieces of work within that annex it's called Annex 86 86.

Simon: 1:17:37

I'll share a link along with all the other stuff we've been talking about today um, they look at material science and materials chemistry, talking about much of the stuff we were talking about a minute ago, about the kind of materials and chemistry that you can use to have an impact on air quality, and it looks like a real frontier of building science. But if you look around the room when that part of the conversation is happening and that room is mostly filled with academics who are from an engineering, a building, physics, fluid dynamics kind of arena you see this fear and a white look come over their face as the chemistry starts getting discussed, because it's instantly something that they start to hit their glass ceiling on really quickly, and the reason I mention that is that it sounds like air chemistry does matter. There's lots going on in that space and it's incredibly complicated, um, but probably struggles to get the traction and the funding that it deserves I think.

Charles: 1:18:47

I think you're right there. Uh, you've noticed that I've authored quite a few papers with Bill Nazaroff and I bring up Bill because he got his PhD with Glenn Cass at Caltech and as part of his PhD he developed a complex model, a very good model, for indoor chemistry and he published a paper in 1986 on modeling indoor chemistry. It's a beautiful paper and it captures. It was really ahead of its time. So this is, this is 1986 and it's taken a long time for, I think, a lot of researchers to catch up with what was in that paper in 1986. And certainly in terms of funding, trying to find funding to examine indoor chemistry prior to the Sloan Foundation, good luck.

Charles: 1:19:55

I was fortunate enough to be at Bell Labs for 25 years and I was in the physical science division. They were willing to support the work I was doing because it did have implications for telephone switching equipment. Bill's PhD work at Caltech in the mid-80s that was supported by the Getty Foundation. Why? Because of the impact of indoor chemistry on works of art, on cultural artifacts. So my work at the time, bill's work at the time neither of us were being funded because of health concerns. It was apparent that indoor chemistry was adversely affecting equipment electronic equipment. It was apparent that indoor chemistry was affecting artwork. It's not a great leap to think that indoor chemistry affects us, but good luck in those in that period trying to get funding to study the health effects of indoor chemistry now.

Simon: 1:21:05

So how did you, how did you find yourself in this particular field and space?

Charles: 1:21:11

okay, um, my phd. I did my phd in. I'm not a chemical engineer, I'm a chemist. My PhD in chemistry was at the University of Chicago and it was electron transfer chemistry, henry Tauby type chemistry far removed from indoor air, indoor chemistry. I did a postdoc at Northwestern with Fred Basolo and we were looking at synthetic oxygen carriers, again far removed from indoor chemistry. I got hired by Bell Labs in 1975. It was a dream place to work. You had enormous freedom in terms of what you wanted to pursue.

Charles: 1:21:53

I was, as I said, in the physical sciences area. I was in specifically a chemistry group and at the time there were these interesting observations that switching offices in New York City had failures that were different from switching offices in Los Angeles. Now, these are cookie cutter offices. It's the same type of office, it's the same type of equipment. Why are we seeing one set of failures in LA and another set of failures in New York? And that was my introduction to indoor chemistry. Now, to be clear, I didn't do those studies myself. Those studies were conducted by my supervisor at the time, charles Russell and Harold Hermance, who was someone he worked with. But that was my first experience of indoor chemistry. I just found it compelling. I'd had a high school science teacher who got us interested in chemistry by telling us about the chemistry that occurred all around us, the chemistry that affected us. I've always been interested in the chemistry that affects me and that work at Bell Labs. It caught my interest and I said to Bud I'd like to work in this area and he said go for it. And so it was in 1975 that I started doing indoor chemistry.

Charles: 1:23:25

So then, what got me interested in ozone? We fast forward to, oh, middle 80s, maybe 87, and there was a office in Atlanta and they had neoprene insulated power cable and this neoprene insulation was cracking, cracking at a high rate, and it was a safety concern and they were replacing it much more frequently than they should have had to replace it and that's expensive. And it sure looked like it was classic ozone rubber chemistry. But people said, no, this is inside, we don't have that kind of ozone indoors. They were saying it's negligible. Forget about indoor ozone, it's something else. Then they thought, oh, maybe something's going on in the office that's making ozone.

Charles: 1:24:22

Well, we wound up making measurements and we made outdoor measurements and indoor measurements and the indoor ozone wasn't negligible. The indoor ozone was going up when outdoor ozone went up, going down when outdoor ozone went down, the office was ventilated at a high rate. It was an economizer system and when the conditions were right it was a fairly high air exchange them and when the conditions were right it was a fairly high air exchange. I told you about these detailed measurements we did at a switching office in Burbank and it was just striking. There were times when the outdoor ozone was over 100 and the indoor ozone was 50% of the outdoor ozone, again because of the relatively high air change rates. So we were seeing 50, 60 ppp ozone in that office. And one thing led to another you started to get interested in what happens when you have 50 or 60 ppp of ozone indoors. So that's how I got interested in the first place, simon.

Simon: 1:25:24

I'm still very interested in it.

Charles: 1:25:26

By the sounds of it, I mean it's uh because, because, as you said, it's so complicated it's, it's a very rewarding area to look at, because it keeps on giving and, yeah, and the better we understand, ah, in a way, the more interesting it becomes okay, well, let's, let's just play a game then of baking this down then to as simpler terms as we possibly can within a built environment.

Simon: 1:25:53

It's something we understand how to manage risk in the built environment, and that is the hierarchies of control. So, when it comes to ozone, we can't eliminate that risk. Ozone is always going to be present. We could perhaps, as you're saying, filter it out of the indoor space. We could create an in an ozone free environment, or as little ozone as possible. So is that a possible solution? When we look at the hierarchies of control, one way of removing the chemistry here is to remove the ozone from that space. Do you think that's a practical solution?

Charles: 1:26:33

I do think it is.

Charles: 1:26:36

You're familiar with ASHRAE, yeah, and ASHRAE funded a study by Rich Corsi and one of his PhD students, allred, and I forget they published two papers came out of this study.

Charles: 1:26:52

But basically ASHRAE gave the money to look at the cost benefit of removing ozone from buildings and one of the papers was published in Indoor Air.

Charles: 1:27:05

I'm sorry I don't remember the date, but I think they looked at different climate zones in the United States and different scenarios and for most of the climate zones and most of the scenarios they found that it was cost effective to take ozone out of the ventilation air in these mechanically ventilated buildings.

Charles: 1:27:31

And that study was using the scenarios were built around what existed at the time they did this paper the kind of filters that were available to take ozone out of there and the kind of mechanical systems that were being used. And they accounted for the added pressure drop. If you're using activated carbon filters in a system and in some cases you might not have a fan with enough power for that pressure drop and you might have the additive expense of upgrading the fan and maybe you don't have enough room in your air handling system to put in the new fan, so that's an additional expense you have to consider. Anyway, they did what I thought was a careful job and found that in most of these scenarios they examined, it was cost effective to take ozone out of the ventilation there now and you can.

Simon: 1:28:34

I don't want to, I don't want to lose my train of thought here, but one thing that did pop into my head when you said, um, that was I wonder if there's something in the lessons we've learned with the cosy rosenthal box that it's less about single pass efficiency and about just passing it past something that's activated many times in a space. So maybe the pressure drops and the efficiencies of the hevax systems aren't so important. It's about maybe something simpler with activated carbon in it, that you just get the air across enough that ozone is going to be attracted to and be pulled out of the air, getting it down low enough that there's not a lot of chemistry being created, that you don't necessarily have to have an unbelievable active carbon filter doing absolutely everything. That you just need to get the the air in the space past something that's going to capture ozone what extent recirculation was playing a role in their calculated results.

Charles: 1:29:38

I'm assuming that they did have buildings with recirculated air and perhaps I just don't don't remember whether they looked at both recirculated air and perhaps I just don't remember whether they looked at both recirculation and single pass. But certainly what you're saying is true. If you do have recirculation, that does improve the effective efficiency. Right of the charcoal filter.

Simon: 1:30:07

Now I'm probably having the health and safety people shouting down the screen at me that that's not elimination, that's engineering control. So we're probably skipping ahead a little bit. One of the challenges with the air chemistry that we're talking about is that it seems, from what you've been saying, that a significant part of the air chemistry that's going on is the, the human generated air chemistry from the skin, oils and things. Now, we can't eliminate humans from the space. We build buildings for people, so that's a that's a factor in this equation that's difficult for us to have an impact on, I'm guessing.

Charles: 1:30:49

You keep taking me places. I'm glad you're taking me. I'm sorry I didn't get here earlier. I get so excited about the ozone human chemistry that I forget about the other types of chemistry that's occurring. The humans are very important because in any occupied environment, as you just said, we've got humans. So humans, they add generality to this discussion. But we can do so much by trying to reduce the use of certain chemicals indoors ozone reactive chemicals. So I'll give you an example.

Charles: 1:31:33

You've heard me mention limonene several times. So limonene is this terpene that has, depending on whether you're talking about a mix of isomers or just one isomer or the other isomer smells something like fruit or lemons. So when you get that fruity scent or that lemon scent, that's limonene, and limonene and ozone react very fast and you get secondary organic aerosols from that reaction and you get formaldehyde from that reaction and you get other compounds that are oxidized and exist both in the air and on surfaces and attached to particles and attached to dust. Now limonene is a compound that you will measure in most indoor environments. We use it so many different ways. It's not at all unusual to measure a limonene concentration of 5 ppb indoors, and it's everything from the scented air fresheners to different cleaning products that are scented, to scented laundry detergents or dishwashing detergents.

Charles: 1:32:49

Limonene is just used in a lot of places and limonene is cheap, which is one of the reasons it's used as a scenting agent so much. Not all scents react with ozone the way limonene reacts with ozone. There's other terpenes that have pleasant scents, but they react with ozone slower, or the products that are formed are not of as much concern as are the products of ozone-limonene chemistry. So if I'm making air fresheners imagine I'm making plug-in air fresheners Well, I can deflect criticism that might come my way because of ozone chemistry if I use a terpene or terpene alcohol that reacts with ozone slowly and makes products that are not of great concern. So so you can actually formulate a product by design.

Simon: 1:33:54

So which then leads me to ask the question could you then start to develop labeling where you would have a an ozone reaction score for certain products, so that you could? I like that, I like that idea, you know I like that idea very much.

Charles: 1:34:13

You, you certainly could. We know enough to do it now politically. Could you do it?

Simon: 1:34:21

I don't know.

Charles: 1:34:22

Good luck, yeah, but but we, we, we know enough to do it, we could do that.

Simon: 1:34:27

Yeah, no, that's interesting because you're really talking, then, about the whole elimination, substitution debate, which is often based around source control. It's about are we creating the products and the environments that generate that harm in the first place, and is there activities that we can do that that would minimize that? Um, something I I made a note of earlier. We've been talking a lot about the reaction of skin oils. Are there cooking oils and other oil type things in the built environment that are having similar reactions? Because you've got the potential to generate far more of those, particularly in frying. And you know, we know, there's a lot of conversation around particulate matter, particularly ultrafine particulate matter, and the, the vaporization of fat oils, and chemistry from food and products.

Charles: 1:35:21

It is that is that chemistry going on as well with ozone that we should be absolutely yeah, um and there's a bit of an irony here um, we're encouraged by nutritionists to use unsaturated cooking oils, and when they say unsaturated they mean in terms of carbon-carbon bonds. They want carbon-carbon double bonds in the cooking oil. So you think of olive oil, and olive oil is rich in carbon-carbon double bonds. If you think of something like an oil that doesn't smoke when you cook, something like a corn oil, a highly saturated oil, very few double bonds. Ok. So let's compare olive oil with a lot of double bonds and corn oil with very few double bonds. Well, olive oil might be better for our heart than corn oil, but in terms of indoor chemistry, olive oil is worse than corn oil. But in terms of indoor chemistry, olive oil is worse than corn. Great ozone, ozone loves olive oil. Ozone gobbles up olive oil, making, um, making all sorts of products, and you don't. So the question is, does it?

Simon: 1:36:40

does that reaction cause products that we're concerned about? Or is it just reactive and it's producing near stuff Like is there, because I guess there's that in that that? That question as well? Is it? Are we concerned about the interaction with cooking activities in the same way that we might be concerned with skin oils, for example?

Charles: 1:37:01

Yeah, Glenn Morrison from university of North Carolina did a series of studies where he had a flow reactor with an open side. He could put it on different indoor surfaces and he could pass ozone through that flow reactor. An open face of that flow reactor was on a surface, so you might put that flow reactor on a carpet or on a wall or on linoleum flooring. So when he put that flow reactor on kitchen counters and passed ozone through the flow reactor he saw all sorts of aldehydes. So presumably he was seeing exactly what you're talking about. He was seeing these unsaturated cooking oils accumulating on the surface of the kitchen counter and then when he passed ozone over that surface he made lots of aldehydes. And we know the major aldehyde he saw was non-anal and non-anal is an irritating aldehyde. He saw was nonanal and nonanal is an irritating aldehyde when, when indoor concentrations of nonanal get high enough, people complain about mucous membrane irritation and eye irritation. So you're absolutely right, simon, these cooking oils can be an important source of ozone reactive compounds.

Simon: 1:38:28

Okay, so we then move on to my favorite subject, which is the engineering controls, because I'm more an engineer than a chemist. I'm neither of either, I should say, but I'm more an engineer than a chemist, I can tell you that. But engineering controls, controls nonetheless. I often talk about the fact that we've it's quite simple in the, particularly with ventilation. There's only a few levers we can pull. One of them is the background rate of ventilation in a property.

Simon: 1:38:57

So the amount of air renewal, the amount of pollutants we generally remove by diluting it with outdoor air and we've discussed that a little bit that there's a balance probably between high flow rates and more ozone coming in and lower flow rates and not removing pollutants. We also have local exhaust ventilation. So our ability to remove pollutants, like showering, which I'm guessing is also going to produce products with soaps and chemicals and you know, scents and things like that but also cooking, all of these things we can engineer out of the building rapidly by design. So there's lots of things we can do physically to remove some of the agents that would react with ozone. I think it's fair to say so. If we could do that well, we would limit the potential for that chemistry to occur.

Charles: 1:39:54

Yes, yes, let me mention a point there, simon. It is much better if it's feasible to take ozone out of indoor air before any chemistry has occurred. So I'll give you a concrete example of contrasting situations here. If I'm talking about an office building in the United States with a central air handling unit, if you put the right kind of activated charcoal filter in that air handling unit, if you put the right kind of activated charcoal filter in that air handling unit, you can take ozone out of the ventilation air before it ever enters the space, before any chemistry has occurred.

Charles: 1:40:37

I've done a lot of work in China since 2010, and they tend to use a lot of split units in China. Central air handling units are much less common in China. So with a split unit, you're cooling air, if you will, or heating air in individual spaces. Right, you've got your heat pump outside and you're doing your thing indoors, but you're really not doing anything with that split unit in terms of filtration. In China, if you want to remove pollutants, you use standalone air cleaners, and standalone air cleaners have become more and more popular in China, amplified by the pandemic and the ability of those cleaners to remove pathogens. Well, if you're talking about taking ozone out of an office in China with a standalone air cleaner, you're helping. You're reducing the indoor chemistry, but it's not as effective as it is for an office in the US, where you're taking ozone out before it ever enters the space. In China, the ozone is entering the space You're making products before you have a chance to address the ozone itself and the products.

Simon: 1:42:05

so don't get me wrong it's still worthwhile using a standalone air cleaner that takes out ozone, but it's much better to take it out before any chemistry occurs is that just because, objectively, we're introducing so much more ozone from atmospheric ozone that we are, generally speaking, in most environments, creating it indoors with electronic equipment and other things like?

Charles: 1:42:30

most of the ozone.

Charles: 1:42:32

That's not what I was saying, simon okay so I think I think I wasn't clear enough. In both scenarios there's no indoor source of ozone in these two hypothetical scenarios. Let's say, in both scenarios the outdoor ozone is 100 ppb. Okay, let's say it's Burbank in the old days and it's Beijing today 100 ppb outdoors, and in Burbank we've got an office with a central air handling unit, office with a central air handling unit, and perhaps the outdoor air has its own filtration potential, or maybe it's only the mixed air that gets filtered. But we're taking the ozone out of that air in Burbank before it enters the office and does its thing In Beijing with 100 ppb. We're taking ozone out of the air with a standalone air cleaner. The ozone has entered with ventilation air in Beijing and now, after it's entered, with the ventilation air. Now you take it out. But there's been time for chemistry to occur. So now you have to worry about not just taking out the ozone but you have to worry about taking out some of the products too.

Simon: 1:43:58

That's the point I was trying to make yeah, no, very true, and then perhaps my mistaken understanding of it is something that we have mentioned earlier. So far, all of our discussions have been based around the fact that we're not producing any ozone in spaces environment where we are generating significant enough amounts of ozone for it to be a problem. Maybe we'll come on to air cleaning type products that might do that, but, like general office equipment, the electronic componentry lighting are, is our life in the modern built environment creating a significant enough amount of ozone for us to be concerned about? Or, as a proportion, is it still mostly outdoor ozone?

Charles: 1:44:54

Okay, let's talk about indoor sources. Anytime you have a corona discharge you're going to make some ozone. So photocopiers historically have been a concern. Now the people who make photocopiers. They know that they also generate some ozone and any decent photocopier contains a good filter. Photocopiers are designed so that they don't actually emit ozone to the room. They might make ozone but they don't emit ozone to the space around them.

Charles: 1:45:34

Now you've probably used a photocopier I certainly have where you smell ozone. When you're using a photocopier and you smell ozone, it's time to call the maintenance person. It's time to do something about that photocopier. However, they're getting rid of the ozone. That has to be addressed. So although photocopiers can be a source of ozone, it's a malfunctioning photocopier that's a source of ozone. Functioning photocopier that's a source of ozone and I think that's most frequently encountered. Where you're doing high flow put photocopying. You might have a small room that's photocopying a huge number of documents in a small period of time and there you might see ozone building up simply because the machines haven't been maintained as they should have been. Uv lamps can generate ozone. I'll come back to that, but let's continue on ozone from corona discharge. There are people who sell ozone generators and claim all sorts of benefits from indoor ozone. Unfortunately, normally elevating indoor ozone is not a good idea and during the pandemic we saw more of these devices being sold. We saw more of these devices being sold.

Charles: 1:47:10

It takes a very high concentration of ozone to kill a virus, something like the coronavirus and when you get to the levels that are necessary to kill the coronavirus, you're at levels that you do not want to inhale. The chamber studies tell us those levels are bad for human beings. So let's go back to the UV issue. Currently there's a lot of interest in something called far UV for killing pathogens, and it's an area that's still under investigation. And it's an area that's still under investigation and it's taking advantage of special lights that emit UV at 222 nanometers, and these are, yeah, krypton chloride lamps, lasers. At any rate, the emission is at 222 nanometers and at this wavelength it appears that it doesn't harm the skin and it doesn't harm the eyes. It will kill pathogens, but it produces some ozone where you're willing to accept a little bit of ozone if you can kill the pathogen. Now we're in a really interesting situation in terms of how do you limit the ozone chemistry If you want to use RUV to control the risk of being exposed to the coronavirus, to kill the coronavirus.

Charles: 1:48:46

Well, if you're in that setting, you really want to pay attention to ozone reactive compounds. There's a situation, I think, where you can avoid and be explicit about it Avoid the use of products that emit terpenes or terpene alcohols and make sure you're ventilating at a high rate, because that high ventilation rate is going to reduce the concentration of the products from the chemistry and it's going to mean less time for the gas-based chemistry. So I think, when we're talking about the use of UV to control pathogens, it's certainly a risk-benefit situation and you have to consider both. But I can see situations where you're willing to risk the exposures to ozone and the products of ozone because of the benefits of controlling virus. But let's keep that chemistry as small as possible and we have ways we can do that you've got to love that risk conversation.

Simon: 1:49:58

Yeah, because it is. That's why I'm so fascinated by risk, because it's so nuanced that it's always in balance and people's tolerance of risk are different. People's susceptibility to certain pollutants are going to be different. There's so much at play, you know. So, notwithstanding bipolar ionization, plasma UV, far UV there's all of this technology that clearly there are pros and cons to all of it.

Simon: 1:50:30

It's found itself in a world of incredibly complex air chemistry and my concern is is that I don't think we're mature enough in the science and the standards and regulation to have that nuanced conversation yet, that there's a lot of development of product and sales of systems where a grown-up conversation has to happen about risk. But that's not happening. That it's that there's either fear-mongering or there's sales pitches or there's academic rigor. It's two opposite ends of the spectrum in the conversation and it hasn't reached a nuanced point in the middle. I know you were going to Scotland in a couple of weeks to have those kind of conversations about far UV and risk and all sorts of stuff. So it's happening. But we've seen great turmoil in the market with massive influx of air cleaners and products all that potentially generate ozone, all with lots of claims that they don't produce ozone, but are we measuring the byproducts that they've created when they're not creating? He quotes, unquotes ozone in the air Like it's. It's complex man Like and I'm just not sure we're ready yet.

Simon: 1:51:58

That's that's. My worry is that I don't think we're quite in that position to have that balanced conversation about it yet.

Charles: 1:52:08

You're making a strong argument to fund research in indoor chemistry yeah, aren't I just yeah and and this this issue of of devices that are devices that add things to the air, devices that themselves use chemistry or increase the chemistry that occurs indoors. We need to be very careful about using such devices. And you're right, too many of these devices have found their way onto the market with insufficient testing and we, the people who buy them, we're the test subjects for these products. That's not how we would like to see it, and I think there's more to be done in Europe and in the United States.

Charles: 1:53:09

In Europe and in the United States I'm not so sure about China right now in terms of regulating devices before they're evaluating devices before they're actually out there to be used by people. In Europe, you have REACH, and a new chemical has to be evaluated for toxicity before it can be used in a commercial product in Europe, and REACH has been very effective and it's really reduced some horror stories regarding new chemicals that occurred in the past. In the United States, we have the new TSCA T-S-C-A. Unfortunately, its implementation has been slow, but if it's implemented as intended, new chemicals will have to be evaluated for safety before they can actually be used in products, and I'd like to see something similar when it comes to so-called air cleaners that can be sold.

Simon: 1:54:10

Where do you think the opportunities are then for research, charles, when you look at what next for air chemistry and ozone in particular, where does this go next?

Charles: 1:54:21

The instruments have made huge advances. There have been these amazing improvements in our ability to detect indoor chemicals and I think those improvements are going to continue. I think there's opportunities for instruments that are even more sensitive than the instruments today, and I'd like to think that these instruments will become less expensive. Right now, only a handful of people can afford these instruments. It would make a huge difference if an instrument that today is 700,000 US dollars were available for 50,000 US dollars, and I can envision a point in the future where that's the case and when these very powerful instruments become less expensive, we're going to see them used in more and more studies of indoor environments, so that's a real opportunity. I've been impressed at the advances that have been made in instruments that anyone can use, instruments that are part of citizen science. I have behind me right now purple air on the wall and I have another purple air outdoors and those cost me about $200 each and they're monitoring PM 2.5 continuously. And there's also an official air station in New Jersey and I can see what. I can compare my outdoor measurement to that official air station in New Jersey and that air station is using an instrument maybe $10,000. It's really impressive how well this $200 instrument will do compared to the $10,000 instrument, and that's a relatively new situation. I mean 10 years ago that wasn't the case. We didn't have inexpensive but good PM2.5 instruments 10 years ago and I'd love to see that kind of instrument available for ozone. Right now we do not have inexpensive ways of measuring ozone at typical indoor concentrations. Most of the inexpensive ways of measuring ozone have sensitivities that are too high. You might look at an inexpensive ozone monitor and it might tell you that it's sensitive down to 25 ppb. That's not going to cut it for indoors and we'd like to know indoors so we can do the difference between outdoors and indoors and get a sense of the products. We can't get a sense of the products if we don't know reliably that difference between outdoor and indoor ozone. I think that's an opportunity and I talk to people who feel that, oh, there are possibilities there in terms of better, inexpensive but sensitive ozone monitors.

Charles: 1:57:42

I talked about biomarkers a little bit, monitors I talked about biomarkers a little bit and we didn't get into a study we did where we used biomarkers to try and distinguish between ozone exposures and ozone product exposures. We did do that. We did do such a study. It's a preliminary study and published it last year and we saw a difference. It looked like we could use certain biomarkers to disentangle the impacts of ozone from the impacts of the products. Well, biomarkers is another area that's just exploded in terms of capabilities and sensitivities. So there's more biomarkers for urine samples, more biomarkers for blood samples. I think there's opportunities here and to the extent that we can develop biomarkers that say, to agents that already contain reactive oxygen species Now, what happens when we inhale something that contains ROS already, rather than making the ROS endogenously, I think there's opportunities there, opportunities there. So those are, those are. I think there are a number of opportunities that make this it's continuing for me to be an exciting field.

Simon: 1:59:15

Absolutely, and if ever there was a podcast that needed a part two. I think we've just heard it. We've just swallowed up two hours without even trying. Charles, um, it's been absolutely fascinating talking to you as much because it's chemistry so fascinating and it's an area that a lot of people in the built environment just aren't exposed to enough, and and it's clearly an exploding field of research and science and learning and I'd encourage anybody that's coming into this field that's got an interest in chemistry to have a look at this indoor environmental chemistry, because it sounds absolutely phenomenal, charles, listen. Thanks so much for spending the time talking to me this evening. It's been really great having you on the podcast. Thank you.

Charles: 2:00:08

Thank you, Simon, for your interest, and thank you whoever winds up listening to this.

Simon: 2:00:15

Thanks, william, thank you.