Fire season isn’t over just yet. In fact, wildfires burning across Canada are sending yet another wave of smoke southward, which is fanning out across the eastern United States. Seven large cities, from New York to Minneapolis, are registering dips in air quality. Compared to earlier in the year, when skies above North America took on an apocalyptic hue, this smoke is mild. Still, it can present a threat to a great number of people—including the nearly 30 million Americans who suffer from asthma.
Chronic asthma is defined by tightened and inflamed airways to the lungs—often in response to certain triggers, like bad air quality. Depending on the severity of the condition, even relatively mild drops in air quality can cause an asthma attack, which is a build-up of symptoms, like airway-blocking mucus and inflammation, that makes breathing especially laborious.
As wildfires become more prominent due to climate change, millions of Americans can expect to see more days of low air quality. And because asthma can arise in somebody due to exposure to pollution, a deadly feedback loop can emerge: Worse air quality caused by pollutants such as wildfire smoke and cause asthma—and its recurrent presence can also trigger asthma attacks, including in those who only have asthma to begin with because of those pollutants. Of course, the scores of people who already have asthma—whether other environmental factors, genetics or allergies—are also among the vulnerable.
With the condition affecting around one-tenth of the US population, understanding how we measure air quality becomes crucial. The US Environmental Protection Agency relies on an Air Quality Index, which runs from zero to 500 and divides into six tiers of air quality. To come up with a reading, the index considers the concentrations of five pollutants: ground-layer ozone, fine particles, sulfur dioxide, nitrogen dioxide, and carbon monoxide. The measurements of those pollutants are synchronized, which produces the index rating.
How concerned you should be with the AQI depends on how healthy your lungs are—and your willingness to expose yourself. If you have healthy lungs, you might be able to get away with outdoor exercise in an environment with an AQI of up to 150—where sensitive groups may be vulnerable, but the general population is likely to be okay—according to one specialist interviewed by The New York Times. Any higher and someone with relatively healthy lungs may be exposing themselves to more harm than benefits. If you have asthma or are otherwise in a more-sensitive group, you’ll want to be far more conservative and talk to your doctor about a specific cutoff.
If you do decide to go outside and exercise, strenuous activity requiring extra breathing may actually be preferable to something more extended and gradual, counterintuitive as that may be. The same Times article pointed to two studies that revealed little difference in lung inflammation between serious and light exercise in polluted air. In fact, a serious and brief workout in a high AQI area might be the right choice because it would lead to less time exposed to the pollutants. All of that said, at the end of the day, the wisest place to run laps during periods of high AQI readings is probably an indoor treadmill.