When Wildfires Make Your Air Unhealthy

Joseph Allen emphasizes that retreating indoors is an incomplete solution.

Following a week marked by ominous orange skies and record-breaking air pollution caused by raging wildfires in Canada, cities along the Eastern Seaboard find themselves facing a decline in air quality that may repeat itself throughout the summer. Such events are harmful to public health—and are becoming more frequent as the climate warms. Joseph “Joe” Allen, an associate professor of exposure science at the Harvard T.H. Chan School of Public Health, has been studying air quality for more than a decade, particularly in indoor spaces where people spend the majority of their lives. As the director of Harvard’s Healthy Buildings program and co-author of Healthy Buildings: How Indoor Spaces Drive Performance and Productivity, Allen emphasizes the critical point that indoor air can be at least as harmful as outdoor air—and is often worse. In light of advice to avoid outside activities, what actions can people take? Allen shares his perspective in this interview with Harvard Magazine~Ryan Doan-Nguyen


Harvard Magazine: Can the air indoors really be as bad as air pollution outside?

Joe Allen: Not only can it be as bad as outside, but it can be a lot worse because you have a couple of things happening. First, you have outdoor air pollution that penetrates indoors, and even though buildings reduce that exposure—it could be by half, but if you have a high performing building it could be 10 times lower than the outdoor levels—we breathe much more air indoors. If you do the basic math on it, and you say, “Well, outdoor levels were 20 micrograms per cubic meter, and maybe half made it inside the building, it’s 10.” But since 90% of the breaths you take are inside, your total dose of outdoor air pollution could be five times higher from indoor breathing, which feels wildly incorrect at first glance. 

On top of that, we have a lot of indoor sources of chemicals and particles and allergens that can be two to five or even 10 times higher than outdoors. You have outdoor air pollution penetrating indoors, and you have indoor sources of many pollutants. And it’s just simple math. We spend the vast majority of our time indoors, we spend the vast majority of our life breathing indoor air, so it’s intuitive—or should be intuitive—that that indoor air has a massive impact on our health. Unfortunately, it’s usually an afterthought.

HM: On June 8, you tweeted that you’re “not prone to hyperbole.” You’re “just absolutely astounded by these levels of PM 2.5,” and you never thought you’d see this on the East Coast. 

 Joe Allen: I don’t think it takes an air quality scientist to recognize that what happened on the East Coast last week was an extraordinary event: the skies in New York City were orange.

Orange skies in New York City on June 7th, 2023
Photograph by Emma Johnson

That said, everyone in our field was talking about how surprised we all were, and when you look at the numbers, this is what’s shocking. The current health-based limit for PM 2.5 is 12 micrograms per cubic meter in the United States. If we see 20, 30, 40, or 50, we get concerned because we know that’s a level associated with adverse health effects. In New York, we saw levels that were 200, 300, 400, 500—up to 800 micrograms per cubic meter. That’s why this is astounding, and if you look across experts in our field—Dr. [Andrea] Baccarelli, former Harvard professor now chair [of the department of environmental sciences] at Columbia University, [Gamble professor of biostatistics, population, and data science] Francesca Dominici, world renowned expert in outdoor air pollution—across the board, people who study this…were astounded. I don’t think we are prone to hyperbole, but these were extraordinary levels. When levels hit 50 micrograms per cubic meter in Boston, I was tweeting about that. Those levels are really high. Then a day later it hits levels 10 times that in New York City. This is a level which we know can cause severe adverse effects, even with short term exposure. That’s where the alarm came from. 

I also think it was alarming because we hadn’t seen this on the East Coast. We have seen wildfires on the West Coast of the U.S., we have seen wildfires in Indonesia, and bad air pollution days in Beijing or Delhi. It’s not something we’ve seen on the East Coast of the United States, and it should be a massive wake-up call, because as colleagues of mine have shown, the intensity and frequency of wildfires is only going to increase.

HM: And why is that?

Joe Allen: I don’t think this will be a surprise to anybody, but the changing climate is causing a whole host of cascading environmental disasters including increases in wildfires, increases in extreme heat, increases in storm frequency and flooding. This has all been predicted, it’s all been modeled, and our studies have shown we should expect more wildfires. If you look across the world or even just across North America, you see things that we have never seen before: wildfire smoke turning the skies in New York orange; extreme temperatures in Seattle, hitting 100 degrees Fahrenheit; ice storms in Texas. These so-called 100-year events are now occurring every few months. 

HM: You just mentioned that even exposure for a bit of time can cause adverse effects. What are the main constituents of wildfire smoke, and in your opinion, what are the most significant health risks associated with exposure? 

Joe Allen: We know that smoke is a complex mix of many different components and carcinogens. We know that short term exposure to these levels of wildfire smoke increase emergency department visits for respiratory diseases, asthma exacerbation for COPD, so we will expect to see this. And at these levels, it’s not only people who have pre-existing conditions or the extremely vulnerable who are of course at higher risk, but the entire population’s at risk. My team’s own research shows impacts on cognitive performance from short term exposures, and that’s in addition to what a lot of people experience in terms of eye irritation, trouble breathing, coughing, headaches—these kinds of overt symptoms that people notice. In addition, we see subclinical effects that people don’t notice as these particles start to wreak havoc on our internal systems through inflammation, and really start causing havoc on our cardiovascular system. Again, we would expect these kinds of effects at levels 10 times lower than we saw in places on the East Coast.

HM: I think most people are curious and wondering, what can they do to protect themselves both outside and inside?

Joe Allen: There are things we can do. Some of the guidance has been good in terms of limiting time outdoors, limiting strenuous activity, using a high-quality mask when you’re outside, but the guidance around limiting exposure outdoors is incomplete because most of your exposure to outdoor pollution can occur indoors. There are also simple steps that you can take indoors. All buildings at this point should have upgraded their filtration in response to COVID. Those same filters do an excellent job of catching not only respiratory particles but also particles from wildfire smoke. You can also use portable air cleaners in places that don’t have a central HVAC system. We have evidence that this works, not only decades of peer-reviewed literature, but during this wildfire event in buildings that have upgraded filtration, we’ve been monitoring indoor particle concentrations. And even when levels hit 200, 300, 400 micrograms per cubic meter, we saw indoor levels in those buildings stay at less than 10 micrograms per cubic meter under the current health-based benchmarks. This isn’t rocket science. You put in filters that capture a higher percentage of outdoor particles, you run incoming air through those filters, and of course, you’re going to have lower particle concentrations indoors. The problem is, we haven’t always designed our buildings with these higher-grade filters. At Harvard, we have—in response to COVID. Early in the pandemic, I was advising the School Public Health, the Business School, the Kennedy School, the Graduate School of Education—and across Harvard, we upgraded to these higher-grade filters in response to COVID. And in the Harvard buildings where we’re monitoring air quality—even when particle levels were high in Boston and Cambridge—we saw very low levels in our classrooms and in our buildings. The filters work, and they’re not expensive. This is not an expensive intervention to upgrade filters in a building.

HM: When it comes to installing filters, particularly in your home office or where you don’t have formal standards or guidance, what should people keep in mind regarding air quality? You tweeted recently that a major issue with some portable air cleaners is that many are loud, and the teacher has to speak above that, which explains why, in many schools and offices, they’re set to a low fan speed. You also mentioned that people should make sure their air purifiers are the right size for the room. Are there other considerations people should note?

Joe Allen: In-room air filtration devices work really well, but there are a couple of things we have to think about. First, they have to be sized correctly for the room. It’s obvious that a small, portable air cleaner in the corner of a very large classroom isn’t going to be as effective as a larger unit in the center of the classroom or office or home. Fortunately, there are simple ways to be sure you’re sizing your air cleaner correctly. My Harvard Healthy Buildings program built a tool early on in the pandemic that’s available on the web that helps you pick an air cleaner sized correctly for your room. You enter a little bit about the size of your room, and it’ll tell you what you should look for from the manufacturer in terms of what we call the CADR, or Clean Air Delivery Rate. 

My simple rule of thumb is look for a CADR of 300 for every 500 square feet of space you want to cover. That’s important to size this correctly. You don’t need any bells and whistles, you just need a good HEPA filter and your portable air filtration device. The other point is important—that some of these devices are really loud, and when we visited schools or other buildings, a lot of times you see them turned off because they’re too loud. Going forward we need more permanent solutions—more permanent healthy building solutions. I think portable air cleaners are great stopgap measures during a crisis. So yes, during a wildfire event where the particle levels were extremely high, absolutely get a portable air cleaner, run it on high. It might be a little loud. But the trade-off there is totally worth it.  

We have to think about what strategies are available to us during a crisis where we just have to get through the event. Early on in COVID, with millions of kids out of school, people were saying they didn’t have time to fix their HVAC systems in classrooms, while there are portable air filters that can do the work relatively cheap and can be installed in minutes. That’s a great solution, but it’s not an answer for the long-term. Same with wildfire smoke. In an emergency, you have high particle levels, you have people worried about exposure. Put one of these in the short-term—the noise problem can be dealt with. As a long-term measure, what I’d like to see is that we simply improve the way we design buildings and make sure we start with higher levels of filtration and better ventilation in the first place.

HM: Regarding this long-term solution of improving building design, are there any interventions currently being put into place by governments or schools? 

Joe Allen teaching during a site visit for one of his classes.
Photograph by Anna Webster

Joe Allen: Yes, there have been some important advancements recently. I wrote about this in the Washington Post a couple of weeks ago. During COVID, I was on the Lancet COVID-19 Commission. I chaired their task force on Safe Work, Safe School, and Safe Travel, and we put out a lot of guidance—for example, a report on the first four healthy building strategies every building should pursue. Last fall, we released our recommendations for new higher health-based ventilation-filtration targets. I’ve been an advisor to the White House over the past year. We continue to work with them, CDC, and other standard-setting bodies, and just this spring, CDC and ASHRAE—the standard-setting body for ventilation—for the first time ever set new health-based targets that are in line with the recommendations from our Lancet COVID-19 Commission Report. This is important because for 40 years we haven’t had these standards, and as part of that work, the government announced that federal buildings will start to have upgraded filtration. For years we’ve used these MERV 8 filters. They capture a small percent of airborne particles. MERV 13s capture 80% or 90%. They don’t cost much, and some of these solutions are absolute no-brainers. Because even though we tend to think of these crises separately—the COVID crisis, the wildfire smoke crisis, the extreme heat crisis—they all run through our buildings. So, the solutions are the same. The same solutions we’re talking about for COVID are good for extreme heat and wildfire smoke, good for every day outdoor air pollution issues. These are really kind of simple. We know exactly what to do, and it doesn’t require inventing some new technology. All of this is readily available right now.

HM: In your book, Healthy Buildings, with John Macomber, you wrote that a recent study found that “as levels of smoke in the air increased in California and neighboring states because of the 2020 wildfires, so did COVID cases and COVID deaths, and these effects were seen up to four weeks after the fires are extinguished.” From your viewpoint, what have the similarities between the COVID-19 pandemic and combating wildfire smoke been, and what are the differences? How is fighting pollutants in the air different from infectious disease control?

Joe Allen: The work we described was led by Francesca Dominici. From my perspective as someone who thinks about control strategies and healthy building strategies, I think it’s important because for both crises, COVID and wildfire smoke, the strategies in the building stay the same, and we just modify it slightly. For example, with a respiratory pathogen, you want to lower the concentration indoors. You can do this through dilution, increasing ventilation, and through removing it from the air (filtration). With wildfire smoke, we don’t necessarily want to bring in more outdoor air, but we can use this enhanced filtration to capture those particles. So, we have to think about ventilation and filtration working in tandem, and depending on the issue—in this case, wildfire smoke—you want that enhanced filtration. You can close or lower the amount of outdoor air coming in, the amount of ventilation, while still keeping filtration high. 

Filtration becomes so important because people were saying, “Turn off ventilation. Stop bringing in outdoor air to protect against wildfire smoke.” If you do that with poor air, and don’t have good filtration, you are now increasing the concentration of respiratory particles indoors. So, you have to pair both of these strategies. The simple answer is to improve your ventilation and filtration. If you’re bringing in outdoor air, you’re helping dilute respiratory particles indoors. That can be a problem if outdoor air is bad—if you don’t have good filtration. If you have great filtration you’re going to capture that wildfire smoke even though you’re bringing in outdoor air. And as I said earlier, in the buildings that had good ventilation and good filtration, they kept their systems running during this wildfire event on the Eastern Seaboard and still had low particle levels indoors.

HM: In your opinion, what is the biggest challenge facing those who seek to improve our nation’s air quality? Would it have to do with getting building permits to implement these design measures? Or would it be more to do with combating pollutants like the smoke itself?

Joe Allen: The biggest issue here is that we have an equity issue, and that’s because of the failure to set standards. The science is really clear on what to do in our buildings. Many organizations are doing this—Harvard’s doing it, good companies are doing it, many schools are doing it—but it’s not impacting everyone. It’s not benefiting everybody. And until these healthy building strategies get codified, we’re not going to see everyone benefit. It just has to become the norm. It just has to become the way we design our buildings going forward. I’m already optimistic because the Lancet, CDC, and ASHRAE have set these new health-based targets. That’s an important step because for the first time in the past 40 years and the first time ever for CDC, the public now has targets to aim for. The first step was, “Let’s recognize there’s a problem in our buildings. Let’s set some health-based targets.” We got that, and now it’s, “How do we drive this into policy such that it doesn’t take anything special or anybody paying particular attention to do this?” So that it just becomes how architects design buildings, and how we put this into our building codes and standards. But this is this what concerns me most about the Healthy Buildings movement: the abject failure if it only benefits a select few. And it’s very frustrating as a public health professor to know that the solutions are at hand. They’re not expensive. They provide benefits across all of these climate-related disasters and also just promote good health and well-being. And we can do this without much effort. Our buildings have just been an afterthought for a long time. I think COVID has started to change that, but we have work to do to make sure this Healthy Buildings movement benefits everyone. 

HM: Which groups do you believe are being excluded from these measures? You said that they’re being implemented mostly for a select few, so who’s being left out of the equation?

Joe Allen: The pandemic relief stimulus funds provided an opportunity, for example, for schools for the first time in decades to improve their buildings. Many schools got this done. I worked with schools in some of the lowest-income parts of the state and country who got this done. Many well-resourced schools got this done. I see companies doing this, but usually it’s the companies that have the resources. Smaller companies, companies that may not have the budget for this or may not have the teams that are tracking the scientists carefully, may not be doing this. We also know that poor air quality is even worse in schools with predominantly black or Hispanic populations and/or schools with the majority of students on free or reduced lunch. So, we have the current minimum standards that are not designed for health. They slip over time because it’s a design standard, not a performance standard. And schools, for example, that are majority black or Hispanic receive lower ventilation rates. In schools with lower-income students, we see lower ventilation rates. These are the types of inequities we see across our society on every issue, so it’s disturbing, but also not surprising, that we would see these disparities when it comes to ventilation and filtration in our buildings. And this is what I mean about the need for this to become code. This has to be the way we design affordable housing. This has to be the way we design all schools everywhere. It has to be the way we design offices and restaurants. It just has to become the norm rather than something special. Good air quality and buildings being some extra or special thing—that fails on so many levels. I agree with the World Health Organization declaration that healthy indoor air quality is a fundamental human right. 

HM: What is the state of current air quality standards, and are they under review? If so, where do you see them going? 

Joe Allen: They’re currently too lax. They’ve been too lax for a long time. We worked hard during COVID to change public perception of this and change guidance around this, and we finally had some wins this past spring. CDC for the first time released a health-based ventilation target. They’ve never done this in the history of the CDC. So kudos to Dr. Rochelle Walensky and her team. ASHRAE has released new guidance for ventilation that is health-based. So there have been some wins, but it doesn’t go far enough because right now they’re not mandatory. They’re not required, not standard, not a code, not code-enforceable. It’s an important step. We should celebrate that as a win, but certainly our work isn’t over. 

HM: At the start of the interview, you mentioned a few other countries and their current air qualities. How does the rest of the world compare to the U.S.?

Joe Allen: Well, it varies. In parts of the world where energy production is still dominated by coal-fired power plants, we see how chronically bad outdoor air pollution is and how outdoor air pollution still kills millions of people a year. The EPA is re-evaluating the National Ambient Air Quality Standard for PM 2.5. My colleagues are advocating for a much lower and safer limit. But it really depends. I am optimistic because, for example, we did research that showed that the U.S. is transitioning away from coal. Over the past 10 years we see a corresponding decrease in deaths attributable to air pollution. That makes sense. Our study also shows that our work is not done on that front either because there are still plenty of sources of outdoor air pollution. But it is revealing something interesting—that one of the dominant, or one of the leading contributors to health damages from outdoor air pollution is fossil fuel combustion in our buildings. So, a lot of times we think about the pollutants generated from point source emissions—coal-fired power plants, natural gas power plants. That’s true and has been the dominant source for a long time, but as we shift away from coal in the United States, it’s revealing that the onsite fossil fuel combustion of buildings is playing a big role. So again, buildings are part of this problem, and also part of that solution. 

HM: Is it true that air quality standards in the EU, the European Union, are higher than in the US?

Joe Allen: It depends on the air pollutant. They do some things really well in terms of indoor air pollution in particular around more restrictions on toxic chemicals in building products. So, when it comes to indoor air quality, that’s important. I think Europe is ahead of us in terms of the transition to renewable energy, at least in some parts of the EU.

HM: You recently wrote that “hundreds of counties across the West will likely experience greater effects from wildfire smoke in coming decades.” What does the future look like?

Joe Allen: It depends on how quickly we get our act together. It’s unmistakable that the climate is changing. All but the totally obtuse can see that with their own eyes. They see wildfire events, they see these extreme temperature swings, increased flooding. If we think globally, parts of the world start to become unlivable. But I’m ultimately an optimist. I’m confident that the investment [associated] with the Inflation Reduction Act in the U.S. is changing the conversation quickly. I think the business community has a big role to play here in helping us transition away from dirty fuels and helping us figure out technological solutions. The federal government has played a key role, clearly, with that investment. So, I’m optimistic that we will get a handle on our fossil fuel consumption in our energy grid. The path toward addressing the climate crisis also goes through our buildings. Buildings consume 40 percent of global energy. They have to be part of the solution: the energy they consume, the energy that’s used in terms of our building materials, like cement and steel, [which represent] embodied carbon. 

We see a lot of movement towards cleaning the grid. We see many cities taking the lead on establishing laws to promote greater and stricter energy efficiency in our buildings. Boston has done this. Cambridge has done it. New York City has done it. Seattle’s done it. Washington, D.C.’s done it. Cities in Europe have done this. Things are moving in the right direction. I think the big question is, “Are they moving fast enough?” And I think the daily news stories are telling us we’re not moving fast enough.

HM: Are there any buildings on Harvard’s campus you consider to be a prime example of what we should strive for?

Joe Allen: I think a lot of buildings on our campus. And the good news here, the important point, is that it doesn’t just have to be new buildings. So absolutely. The Science and Engineering Complex is a marvel and has a lot of advancements in terms of energy efficiency, water conservation, healthy materials—and that’s important. But I don’t want to lose sight of the fact that, in most of our buildings in the U.S., and in fact, in many parts of the world, we’re going to have to address the existing building stock. There are many examples of older buildings that perform great. Take this latest wildfire issue. We’re monitoring air quality in some of the oldest buildings on campus. And because they had a little bit of attention paid to them in terms of better ventilation filtration, the indoor air quality stayed great even though things outside were really bad in terms of air pollution. I think we have examples all over campus. I work closely with the facilities team at Harvard Business School, for example, that actively monitors air quality, while also taking aggressive steps to improve energy efficiency. So, for them it’s not an either-or proposition of health or energy; they’re doing both. Same thing with facilities at the School of Public Health. I work with facilities teams around the University, I’ve worked with environmental health and safety across the University. We’re putting together new standards for Harvard sustainability plans around our buildings, so I’m happy with how Harvard’s approaching this. I think it took COVID to capture attention—everywhere—that we need to increase the level of filtration across our buildings.

HM: My next question for you has to do with exercise. Given the pollutants in the air outside, many aren’t able to go on their runs or jogs anymore. How do you advise people to continue to stay healthy—can we exercise inside if we have a HEPA filter?

Joe Allen: It’s good advice to limit your strenuous activity when outdoor air pollution is really high. And it’s only a limited time. We’re not telling people don’t exercise for the next week, month, or year—just avoid outdoors when the levels are really high. At the same time exercising indoors is great if the particle levels are low. Some of the guidance out there said, “Oh, avoid outdoor exercise and move exercise indoors.” That needed to be paired with the message that said “If the indoor air is clean.” For example, we’ve been talking about if outdoor levels are 200, and indoors it’s 100, and you’re exercising indoors and breathing deeply, you’re going to get a massive dose of particle exposure. If you have these high-grade filters like we’ve talked about your particle levels might be under 10. So absolutely, it’s great to work out indoors in that scenario. Your total dose is dependent upon the concentration, the time, and the volume that you’re breathing. So, this is pretty straightforward. Limit your time outdoors. Avoid the highest concentrations. Lower the concentration through masking when you’re outside or filters when you move inside. And avoid strenuous activities. You’re not inhaling as many of the particles. It’s pretty straightforward. “Just moving inside” is incomplete advice.

HM: Which sources or organizations do you believe provide the most reliable and comprehensive guidance on air quality? Where should people go for advice? 

Joe Allen: That’s a great question. Harvard is really fortunate that we have a School of Public Health, and in my department of environmental health, some of my colleagues are the world’s best scientists studying outdoor air pollution. So, we have a wealth of knowledge right here. But it’s incumbent upon all of us, of course, to not just use that information to inform what happens on the Harvard campus but to inform policymakers, the public, students, and everyone else—to leverage that deep expertise we have at the School of Public Health and around the University to help inform the public, and [guide] what gets done going forward. There are a lot of good resources out there. I think the AirNow website is good. It gives you a sense of what’s happening, where you are in terms of outdoor air pollution. It’s pretty straightforward and good guidance. And then, if we need to get more technical, we should tap into all of these air pollution experts. 

Outdoor air pollution has been one of the most studied topics in my field of environmental health. Out of any topic, it’s the most well-known—our understanding of particles, how they impact health, how to control them, how they move through the environment. This is really well documented, and we have some of the world’s best experts right here to tap. My interest is, “How does this influence our health in the indoor space?” Think about outdoor pollution: it moves indoors. Now what? Even when we think about outdoor air pollution, the indoor environment matters a lot.

Read more articles by: Ryan Doan-Nguyen

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