July 16, 2026
You may have noticed the sky appears sepia-toned, butthat beautiful color can bring with it terrible air quality. Smoke from Canadian wildfires is scattering and refracting the light and degrading our air quality across the region, affecting the way we breathe and reducing our visibility
Dr. Joe Allen, professor at the Harvard T.H. Chan School of Public Health, director of Harvard’s Healthy Buildings Program, and co-author of“Healthy Buildings: How Indoor Spaces Can Make Us Sick or Keep Us Well,”joined GBH’s All Things Considered host Arun Rath to discuss why this is happening and what we can do to stay safe. What follows is a lightly edited transcript of their conversation
Arun Rath: I mentioned the Canadian wildfires causing this, but can you explain how and why that affects our air quality all the way down here?
Dr Joe Allen: Yeah, sure. So, you know, we have wildfire smoke and pollution. It doesn’t just stay local. It travels, as we’re seeing here, it’ll travel all the way across the country, in fact. And you can see it if you look at the air pollution data. You see these really high concentrations up in Minnesota carrying across Toronto into New York and Boston. And at these levels, you know, the health effects are real. So just to put some numbers on it, we’re talking about air pollution outdoors in Boston. It’s about 50 micrograms per cubic meter. Typically it’s about 10. So it’s about, you know, four times over or five times higher than typical. And if you look at some of the health effects data, we know that for every 10-fold increase, the heart attack risk increases by about 2.5%. So putting that together, said better: the increase in heart attack risk with air pollution levels we’re seeing now is about 10%. Short-term exposure to this level increases heart attack risk by 10%. We also see impacts on kids and asthma and visits to the emergency room for kids when, when we see air pollution like this.
Rath: So that, that sounds really like terrible air quality. How do we best take care about that aside from limiting time outside?
Dr Allen:Yeah, so it’s good to go indoors, but I think that’s half of the correct guidance. So you want to come indoors, but the reality is, and this will be shocking to everybody listening, you breathe most of the air pollution you breathe happens indoors. So most of the outdoor air pollution you breathe happens indoors. That means we can’t just move indoors. We have to do some things inside. And what we need to do is just improve the level of filtration. So if you have a central air conditioning system, you want to increase the filter to what’s called a MERV 13 filter. Or if you have a portable air cleaner or portable air purifier with a HEPA filter, these can be really effective. Make sure they’re running indoors and you can kind of knock down some of these particle levels as it gets into your house, your school, or your office.
Rath:And what recommendations do you have for people who already are suffering from respiratory illness?
Dr Allen:Well, you know, some of the stats I gave you about heart attack risk, asthma attack, it’s exacerbated for people who are older, people with respiratory conditions or heart conditions. So the official guidance — and I agree with this — is that you should limit your amount of time outdoors. You should limit strenuous activity so that … your body’s not working as hard, you’re not breathing as much of the air pollution. And I think it’s vital that we think about this improved filtration indoors. I highly recommend — I have several in my house — these air purifiers with the HEPA filter. They don’t cost all that much. Certainly, you want one in your bedroom. I mean, you spend a third of your life in one box on this planet, and we want to be sure it has good, clean air. And these can be very effective. You’ve got to size them right for the room. So don’t get a little one for a big room, obviously. We have some tools on the Harvard Healthy Buildings website to help you size it correctly if you’re not quite sure how to do it.
Rath: And when you do have to go outside, does any kind of masking provide any level of protection?
Dr Allen: Yeah, for sure. And this will be familiar to people who were paying attention during COVID, just like the filter conversation and MERV-13s and HEPA filters, getting well-fitted N95 mask or a KN95 mask. If, as long as it’s flush along your face, these do a really good job as implied in the name, it captures about 95% of the particles at a minimum. So they can be very effective for those who have to be outside or those who have a respiratory condition or a heart condition and you wanted some extra protection. So a mask can be very effective, yes.
Rath: Because smoke is big, nasty particles as far as they go
Dr Allen: Yeah, that’s right. We’re talking about some basics here. The majority of air pollution we’re thinking about from smoke and why that sepia tone sky is happening, is the particles are they’re just physical particles. You can think about it as airborne dust. So what is a filter doing? It’s just trapping that dust. Same with a HEPA filter. You’re just blowing air across a really good filter that’s taking that, those dust particles out of the air. The reason they’re a health hazard is because they’re a size that’s small enough to get to the deepest part of our lungs, where we really don’t have any defenses. And from there, they wreak a lot of havoc across all of our body systems. So best thing we do is step inside and make sure you have good filtration either on your face or, in your house with a good portable air cleaner or a MERV 13 filter in your central air system.
