Should You Run in Smoky Air? Probably Not.

On Tuesday, November 13, the North Face Endurance Challenge 50 Miler outside San Francisco, California was cancelled due to smoke from the devastating wildfires ravaging the state. The race organizers donated the $30,000 in prize money to charities for those affected by the fires, among other initiatives directed at fire relief.

If you can, follow the lead of The North Face and donate to fire victims. Supporting those affected by the fires any way we can is way more important than running.

But the question of whether or not to exercise outdoors when there is wildfire smoke in the air is one many athletes are facing more and more due to climate change. So what’s the science behind smoky skies and running?

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What is smoke?

In 2016, the California Air Resources Board (CARB) commissioned a report with co-authors from the CDC, EPA, NIH and other institutions looking at wildfire smoke and public health. The major public health concerns associated with smoke stem from fine particulate matter with diameters less than 2.5 micrometers (PM2.5). Smoke also contains carbon monoxide and other pollutants, most of which are more of a concern in areas immediately surrounding a fire.

Particulates from smoke are so small that they can burrow deep in the lungs and even enter the bloodstream, causing local and systemic inflammation and circulatory issues (to hop on the Magic Schoolbus and understand how it works, see this 2012 article from the Journal of Medical Toxicology).

What are the possible health effects of PM2.5 exposure?

As the CARB report states, effects of smoke inhalation “range from eye and respiratory tract irritation to more serious disorders, including reduced lung function, bronchitis, exacerbation of asthma and heart failure, and premature death.” Most of the science on PM2.5 exposure is from vulnerable subpopulations and heavily polluted areas, but even short-term exposures for healthy individuals can cause health issues. The report says: “Exposures to fine particles can also affect healthy people, causing respiratory symptoms, transient reductions in lung function, and pulmonary inflammation.” Those impacts are especially relevant for athletes relying on lung function, with up to 10 to 20 times more air inhaled during exercise than at rest.

The EPA breaks down exposure to air pollutants like PM2.5 into an Air Quality Index (AQI) number, which standardizes health risks and exposures. Over 100 is generally unhealthy for sensitive groups. When The North Face made the decision to cancel the race, the AQI was around 200, or simply “unhealthy for all individuals.”

Air quality forecasts are notoriously fickle due to the complex, sometimes non-linear interactions of weather and pollutants like smoke. However, on race day, the forecast when the race was cancelled was for air quality to remain unhealthy for sensitive groups (USG). And the public health science indicates that USG days can cause major impacts for non-athletes and athletes alike. A few days later, the air quality forecast had worsened substantially.

Are there studies that provide context?

If you’ve tried to run in smoky conditions, you probably know the itchy-eyes, burning throat feeling that can ensue. But it goes beyond discomfort and reduced performance. PM2.5 can cause major health crises.

Public health experts have an ingenious way of isolating how short-term exposure to air pollution affects population health. By cross-referencing hospital admissions with air quality data, they can determine the extent of the health crisis caused by PM2.5. A sampling:

• This 2015 review in the British Medical Journal looked at 238 articles and found increased hospital admission and mortality from PM2.5 increases, with negative health outcomes being more persistent than other pollutants, even days after initial exposure.
• This 2015 study in the Journal of the American Medicine Association found short-term PM2.5 exposure is associated with coronary events for at-risk individuals.
• This 2017 study in the International Journal of Public Health found that even relatively minor increases in PM2.5 increased respiratory and cardiovascular hospitalization and mortality.

These studies are not on smoke in particular, but it’s a similar mechanism, so they should be mostly generalizable. For healthy athletes, it’s possible that those studies could be given less weight because they may isolate vulnerable populations that are more at risk of cardiovascular or respiratory events that require hospitalization. However, other studies connect the negative effects to healthy populations, including athletes.

Short-term exposure to PM2.5 can increase blood pressure (2015 study), increase inflammatory biomarkers in athletes (2013 study), and “may inhibit the positive effect of exercise on cognition” (2014 review). Longer-term exposure in athletes may even increase bad cholesterol, decrease good cholesterol and decrease VO2 max (2018 study), along with causing endocrine disruption (2018 toxicology study).

A particularly scary study came out in 2017 in the journal Circulation. In a how-can-this-be-ethical study design, 55 healthy college students in China were split into two groups, one with sham air purifiers and one with working air purifiers, each for nine days at a time. The ambient air averaged 24.3 ug/m3 in the treated environment, and 53.1 ug/m3 in the untreated environment (for comparison, San Francisco was at 179 ug/m3 as of Wednesday, November 14). You expect some lung effects, right? That is only the start of it. Subjects in the untreated environment had significant increases in cortisol, cortisone, epinephrine and norepinephrine, along with higher blood pressure, hormones, insulin resistance and biomarkers of oxidative stress and inflammation.

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To run or not to run?

Let’s end by zooming in on two studies conducted on healthy athletes to help you make your smoke-running decisions. A 2008 study in the Journal of Strength and Conditioning Research had 15 subjects do four maximum-effort cycling tests, with the first two being in low PM2.5 conditions, and the next two being in high PM2.5 conditions. Work output was only reduced in the final trial. Therefore, even though the athletes had adequate rest of several days, and if anything should have adapted to the training stimulus, the PM2.5 exposure in trial number three diminished subsequent performance, likely indicating cardiovascular and/or pulmonary impacts.

It’s not that simple, though. A 2018 study in the Journal of Exercise Science and Fitness looked at moderate PM2.5 pollution (50 to 100 on the AQI scale) and found no effect on performance or biomarkers. Notably, that study had 16 subjects do one cycling test in both low and high PM2.5 conditions. Based on the 2008 study above, I’d be interested to know what would happen with subsequent exercise bouts in PM2.5. Would exercise performance decrease, along with an increase in inflammation?

It’s tough to know the answer to that question. But the study does present the possibility that some exercise in somewhat smoky conditions might not always be terrible in moderation, even though we know that lots of exercise in smoky conditions probably is.

Anecdotally, I tried to run on a smoky day in Colorado this September, suffering through a 10 mile, slow run. Based on the science, I thought, it likely wouldn’t cause any damage. The next morning, I was sore all over. It wasn’t until later that I put two and two together. Could that soreness have been indicative of some systemic response to smoke inhalation? As a coach, I’ll often see people complain of high stress, nausea and other symptoms during smoky periods, though I may be perceiving correlation without causation.

Sum it all up

So what’s the takeaway? The Canadian Sport Institute indicated that short, low intensity activity in a somewhat smoky environment may be okay. Meanwhile, more intense workouts (like most runs) should be modified or moved inside. In some cases, however, even indoor air quality can be affected, so be cautious in the gym, too. Respirator masks can filter out fine particles, so they may be a good option as well, especially for vulnerable populations like pregnant women, children and those with asthma.

For your own running, if the air quality outside is unhealthy due to smoke (see the EPA’s Air Now website for information on where you live), you should probably run indoors or rest. You shouldn’t do a hard workout outside (a shorter, easier run in moderate air quality may be okay, but err on the side of caution).

And you definitely shouldn’t race a 50 miler. Kudos to The North Face for their decision not to expose people to potential health risks. But the biggest kudos for donating money and resources to those in need affected by the fires.

—David Roche partners with runners of all abilities through his coaching service, Some Work, All Play. His book, Happy Runner, is co-written with his wife Megan and available for pre-order now at Amazon.