How Bad Is Exercising in Wildfire Smoke?

I became a runner in the spring of 2020. My run streak was my sourdough starter. Those were the Wild West days of respiratory spray warnings, when I’d get dirty looks from strangers even if I passed them while wearing my Under Armour running mask. But I wasn’t about to let a deadly pandemic — much less the wildfire smoke that descended on New York that fall — get in the way of logging my miles.

These days, I am at least a little bit older and wiser. I’ve also learned a lot about wildfire smoke in the interim — how it kills more than 20,000 people in the U.S. every year, how there’s a lot of freaky stuff in it that you don’t want in your body, and how there’s no safe threshold for exposure. But while it’s clearly a bad idea to go for a run right now if you live in Milwaukee, where the air is literally yellow due to the fires in Minnesota and Ontario, it’s maybe less clear if you’re somewhere where the AQI is still only moderate or “unhealthy for sensitive groups.” Do you really, actually need to skip your run in those conditions? Can you just go to the gym instead?

At the end of the day, everyone should make decisions based on their own risk tolerance. But John C. Quindry — a professor of integrative physiology and athletic training at the University of Montana, who described his team as “first to the party” when it comes to understanding the risks of exercising in the smoke — said his research shows that not only is exercising in the smoke hazardous to otherwise healthy individuals, there’s also a class of people for whom it might be extra dangerous, and they might not even know it.

Our conversation has been edited and condensed for clarity.

Why is exercising in the smoke worse than, say, commuting in the smoke?

If you exercise, you take more breaths per minute, and you take deeper breaths, so the total volume of air you breathe at a given time is greater.

Inhaling wildfire smoke is sometimes compared to smoking cigarettes. Some back-of-the-envelope math puts a run in Chicago this morning at the equivalent of smoking a couple of cigarettes, which sounds pretty minimal. Why take this seriously?

We don’t know for sure that when you exercise outside, it’s equivalent to, in this case, smoking a couple of cigarettes or a pack. We’ve been working for years trying to actually figure out how bad it is, and we just don’t know.

One of the things that complicates this is that not all smoke is created equal. Wildfire smoke in the West is different from wildfire smoke east of the Mississippi, where there are many different types of vegetation. And separately, if a wildfire consumes a house and all the plastics, organic solvents, and things in the roof, that’s certainly worse than just biomass burning — and that is all separate from cigarette smoke. So the chemicals that you inhale are part of it; it’s not just the particulate.

I don’t think it’s inappropriate to say we know how much particulate a filtered cigarette is going to deliver on a puff-by-puff basis and try to equate it to inhaling wood smoke or being downwind of a fire event. Those back-of-the-envelope calculations can make it one-to-one. But what is the impact on health? We don’t know.

Going back to something you said, is smoke in the West or East worse? Why?

We don’t really know, and one reason is that smoke that starts in the West goes east. There are some really good studies that look at the rates of emergency room visits and deaths downwind of fires. You can apply some pretty fancy math, and it’s clearly demonstrated by multiple research groups that whatever the source of the smoke is, there are people who are extra sensitive to it, and they show up in the emergency room more frequently. Tragically, to a smaller degree, they also occasionally die more frequently.

But if we put those data aside and say, “What does the smoke look like from Western biomass versus Eastern biomass?” We know the Western biomass, at least this time of year, is much drier. And when it’s dry, it tends to burn a little “cleaner,” which is to say, if you were to take a certain number of grams of pine wood — which burns fairly cleanly — how much PM 2.5 do you get? You’re going to get a higher PM 2.5 from wet wood. That’s the effect when you start to move to the middle or eastern part of the country. Deciduous trees, even when they’re dry, put out more PM 2.5 unit by unit.

Now, is that worse for the body in the short or long term? We really don’t know. Once you breathe in the smoke, it’s pretty easy to measure what’s in the blood: You just take a blood sample before and after the exposure. You can try to gauge how bad the smoke is by looking at what appears in the blood. Do you get oxidative stress? Do you get inflammation? Is there something else there? Has it changed metabolism?

We also look at exhaled breath condensate. It’s the immune cells, which are the first line of defense when you’re exposed to particulates, that are really sounding the alarm. In some people, that alarm gets sounded more than in others. So we’ve been trying to figure out what are these subtle but important biochemical and physiologic signals? How do they go together? They tell us how bad the smoke is, but it’s taken us years to unfold this story.

I saw in your 2025 study that half of your participants had a heightened response to physiological stress, and that you selected them for that reason. I was hoping you could tell me what a “heightened response to physiological stress” means and why it was important to include those candidates.

This is critical. Let me give you the backstory. When we would do these studies, we would put participants in the lab and burn Western locally sourced pine dried out to 15% humidity. We’d burn it carefully, measure the dose of PM 2.5, and have people breathe it in. We’d conduct studies before and after exercise, whether on a bike or a treadmill, and we varied the intensity and duration. And we didn’t find very much when we looked at the blood and exhaled breath condensate for how well the blood vessels constrict or how reactive the body’s autonomic nervous system was in controlling cardiovascular function. We’d find subtle changes, but statistically speaking, we could never really draw firm conclusions.

But we would notice that, on a person-by-person basis, there would be notable spikes in what we were looking at. We’d see, “Oh, these three or four people really seem to have this notable response, and everybody else really didn’t.” We’d publish our studies and essentially say, “Yeah, we didn’t find much.” It was honest data, and people believe it when you publish negative data that says, “We went through that much trouble, we spent $100,000, and we found nothing.” But we found these people who were a little bit different than the others.

So we’d sit around and spitball. These were normal people, by the way; we were not taking asthmatics or people with [chronic obstructive pulmonary disease]. They were not diabetic. There are people we know full well will have an exaggerated response, and we didn’t look at any of them. We looked at what are called “apparently healthy people” — the people who go to the physician and are told nothing’s wrong with them.

My doctorate was not in exercise science; it was in biomedical science, from a medical school environment. Somewhere along the line, I was exposed to something called the cold pressor test. It has been around longer than both of us. Physicians 80 years ago would have somebody come into their lab or clinic, and they’d say, “Hey, we’re going to put your hand in a bucket of ice water for two minutes and see what that does to your blood pressure.” It almost sounds like a fourth-grade sleepover prank for the first kid to fall asleep.

But we started doing the test, and we took a pretty conservative approach, meaning that if we put someone’s hand in the ice water and their systolic blood pressure — that’s the top number, 120 over 80 is sort of the high end for good blood pressure — and if their systolic number went to at least 20 millimeters of mercury, then we call them cold pressure test positive, or CPT positive.

None of these were people who were hypertensive. They had normal blood pressure. Nothing was wrong with them as far as their medical team could observe. But how many of these [CPT positive] people are sitting around? If you take the average population, 10% to 15% will be cold pressor test positive. We don’t necessarily know if this is a bad thing. It may be contextual, but these are people we can verifiably measure with something as simple as a blood pressure cuff and a bucket of ice water. You can verify their physiology is more reactive within a couple of minutes than someone else’s.

We suspected that if we took these people and then exercised them in the smoke, they would have an exaggerated response compared with people who weren’t CPT positive. And so that’s what we did, and that’s what we found.

That’s fascinating. So the practical takeaway is, unless you’ve done this test yourself, you don’t know if you’re one of these people who’s particularly reactive to wildfire smoke?

You have to always be careful that you don’t make big, broad, blanket applications from one study. We would need to confirm or refute it with additional studies, and that’s a matter of getting grant money and conducting those next-generation investigations.

But here’s something we can glean: We know that people who have an exaggerated response to a cold pressor test are more likely to have hypertension. And they’re more likely to have it earlier — instead of getting high blood pressure at 50, 60, or 70, they’re going to get it at 30 or 40. We know they’re more likely to eventually suffer from heart failure; it seems to develop more in people who have this sort of reactive response. While the cold pressor test response is not predictive in and of itself of anything — future diabetes, heart failure, heart attacks, hypertension, none of that — people with all of those conditions are more likely to be cold pressor test positive.

The way I would take it is, if you are somebody who is not in great health, isn’t fit, or if you have creeping blood pressure or a family history of heart disease, blood pressure, or metabolic derangement like diabetes, you are the kind of person who needs to be a little bit more careful [in the smoke].

Is there an air quality level at which you would tell a young, healthy person “Don’t go for your morning run?”

An educated guess is the best we can do right now. Every study is a brick in the bigger metaphorical wall of understanding this. What we can say is: If you are young and apparently healthy, the threshold would be maybe even up into the “unhealthy for sensitive groups” number. They’re probably okay. But then again, how hard are they working? How long are they outside? If you’re working very hard, your ventilatory rate is much, much higher. And how long are you staying out? What is the long-term impact year after year?

There are epidemiologic studies that can demonstrate that if you’ve lived downwind from major wildfire events for decades, it’s going to impact your health. That’s not debated. But what is the short-term impact? We don’t know.

Indoor air pollution is a major issue during these events, too. Would you advise someone to skip the gym on a particularly smoky day?

When it’s smoky outside, it makes sense to say, “Let’s just stay indoors.” But then the question is, is some of that smoke getting indoors? Sure enough, it is. Then the question is, how much? And the answer is, it depends on the air handling systems in the building; how many people are coming in and out; whether the windows open; how many doors are there; are they left open; what are they using to heat and cool the place; do they have high-volume HEPA filtration; and are they changing that filtration?

But let’s dial back. We have two ends of a spectrum. You have a leaky building. Doors are open. Windows are open. Lots of people are coming in and out. In that scenario, the air inside is only about 20% or 30% better than the air outside. And that’s not very good.

But then let’s go to the other end of the spectrum, to a building that is being kept shut. They’re controlling who can come in and out, and there are two sets of doors to help buffer that. Which is better in a smoky situation? The places with the HEPA filtration, high-volume air turnover, and where they change the filters. But in that case, you’re still going to have 25% of that particulate matter inside, even if you can’t perceive that it’s there. That’s not good news.