One late August morning, 500 mountain bikers lined up to race in the Emerald Mountain Epic in Steamboat Springs, a two-day tour de force of cross-country cycling and trail running. However, the air quality index (AQI) measured at 175—almost two times higher than the “healthy” threshold, as dictated by the Environmental Protection Agency (EPA).

The looming smoke from wildfires had the cyclists wondering what, exactly, the air quality meant for their health. “A lot of people came up to me and asked what they should do,” recalls race director Eli Campbell. He told them that he “personally would not race under these conditions.”

After consulting with medical advisors, Campbell decided to let the race proceed, but gave participants a refund option if they didn’t feel comfortable exercising amidst the smoke. About 10 percent accepted that offer.

Wildfire smoke, you see, contains a mix of microscopic and coarse particulates, as well as methane and volatile organic compounds (toxic solvents like those in paint and glue). Recent research even found that smoke may wear down our immune system, which is particularly concerning news as the Delta variant spikes infection rates across Colorado.

Daniel Kiser, a researcher with the Desert Research Institute in Reno, Nevada, observed that the worst periods of wildfire smoke correlated with an 18 percent increase in positive COVID-19 tests. Ultrafine particles also pass from the lungs into the bloodstream, where they circulate through our vital organs and appear to raise our risk of lung and heart disease.

But the research on air quality is limited, says respiratory specialist Dr. Tod Olin. A pulmonologist and director of the Exercise Breathing Center at National Jewish Health, Olin also consults with the U.S. Olympic and Paralympic Committee to help develop air-quality protocols. “It’s hard to measure the long-term effects [of exercising in polluted air],” says Olin.

Most of the research that’s been completed so far doesn’t look at wildfire smoke specifically, but at urban air pollution. Studies have noted a correlation between higher levels of air pollution and elevated rates of human mortality. Similarly, a study on 12 healthy adults suggested that exercising in polluted air increases your absorption of the ultrafine particles that appear to be most harmful to our health. And emerging evidence indicates that pollution from wildfires may be even more harmful to our health than emissions in urban areas. In a study published in Nature, hospitalizations for respiratory issues increased by nearly 10 percent for Southern Californians during wildfires.

Negative effects become more pronounced with repeated workouts in polluted air, says Olin. “After two weeks in Asia in 2019—when the AQI stayed at around 250—the athletes on the national swim team really did notice a difference in their recovery time and performance.”

AQI is the magic number that Olin watches to monitor impacts for athletes (and everyone). Developed by the EPA, the index runs from 0 to 500, with numbers of 100 or less indicating moderate to good air quality. AQI values from 101 to 150 indicate that people with underlying health conditions might experience impacts, but the general population should be unaffected. The EPS publishes AQI data on AirNow, while sites like PurpleAir can offer readings around neighborhoods and playgrounds.

At AQI values above 151, everyone is likely to experience health effects. The EPA claims these range from respiratory irritation and reduced lung function to inflammation and increased susceptibility to infection, depending on the makeup of the pollutants.

Olin’s recommendation for outdoor athletes is simple: Identify an AQI that becomes your own personal no-go number, and develop a backup plan for indoor workouts. The thresholds that he helped to develop for USA Swimming suggest that 101-150 is the scale-back zone for athletes with underlying respiratory conditions; 151-200 is the take-it-easy zone for everyone else. Above 200, training and competition should be canceled.

The Emerald Mountain Epic had developed an amply researched mitigation plan for wildfire smoke, so that morning’s surprise AQI didn’t derail proceedings. But there’s no standard procedure, says Campbell. “It’s in flux. And all races will need a policy, because unfortunately, this is not a problem that is going away.”