Athletes at the Australian Open are breathing in poor-quality air polluted by bushfire smoke, making it nearly impossible for them to do their jobs. Dalila Jakupovic, ranked 210 in the world by the Women’s Tennis Association, withdrew from her qualifying match yesterday after smoke sent her into a coughing fit. Maria Sharapova’s match was abandoned after two hours of play in smoky air, and Novak Djokovic said before the start of the tournament that delaying the competition until the air cleared might be necessary, albeit as a last resort.

Questions over appropriate air quality conditions for professional sports have been ongoing since at least the 2008 Beijing Olympics, which had the highest levels of air pollution of any measured games. Bad air can impair athletic performance and impact athletes’ health. As fire seasons grow in length and intensity, leagues and athletic governing bodies will keep being confronted with the problem.

Breathing in polluted air over a short period of time can exacerbate respiratory and heart conditions. Over longer terms, it can increase the risks of heart disease and some cancers.

Physical activity increases the amount of air someone takes in per minute, so someone who is playing a sport in bad air would inhale more pollution than someone who is just sitting outside. “Because they work so hard and breathe so much, athletes actually turn out to be a sensitive subgroup to pollutants,” says Ed Avol, a professor of clinical preventive medicine and air pollution expert at the University of Southern California.

Air pollutants can decrease lung function and reduce blood flow — both of which are key for athletic performance. At the professional level, where athletes are only separated by slim margins, any hit to physical function can have a major impact. One study of the German professional soccer league, Bundesliga, showed that polluted air was associated with worse athlete productivity on the field.

The air quality in Melbourne, where the Australian Open is held, is currently some of the worst in the world, thanks to an intense bushfire season that’s burned millions of acres across the country. Practice sessions at the Open on Monday were halted because of the smog, and matches were delayed two hours on Wednesday. Tournament organizers said that they were monitoring the air quality conditions.

“With the wide ranging smoke and fire issues, you have high concentrations of particles and gases in the air,” Avol says. “Everyone should be taking some personal protections and minimizing exercise. Breathing in high amounts gives you a much higher dose.”

The Women’s Tennis Association has rules on the books for modifications to matches during extreme heat, as do the Australian Open and some other major tournaments, but they’re making decisions around air quality on the fly. Other sports leagues in Australia are doing the same: Football Federation Australia said in an email to The Verge that it was still in the process of formalizing an air quality policy for its men’s and women’s leagues, the A-League and W-League. In practice, though, the leagues are postponing games if the Air Quality Index (AQI), a composite measure of air pollution, is above 200, a spokesperson told The Verge. If the AQI is between 151 and 200, the league consults with team doctors to decide if games should move forward. A W-league game was postponed due to poor air quality at the start of January, while an A-league match scheduled for the same day (but played in an area with better conditions) went forward.

During the 2018 California wildfire season, US leagues had to make similar decisions. That year, the football game between the University of California, Berkley and Stanford University was postponed due to poor air quality. The National Collegiate Athletic Association recommends that schools should consider rescheduling games or moving them indoors if the AQI, is above 200.

Some professional sports teams, on the other hand, kept playing through the smoky air during the 2018 fire season. Outdoor games in the National Women’s Soccer League went on, despite poor air quality. The league added hydration breaks to the games and had oxygen on hand at the sidelines, but it faced criticism for not responding to the issue quickly enough. The San Francisco 49ers and the New York Giants played outdoors in San Francisco in November 2018, despite unhealthy air quality. The league said it would relocate the game if the AQI was above 200, but it hovered around 156, which is still considered unhealthy by the Environmental Protection Agency.

While leagues are setting their own limits, it’s hard to say objectively what the air quality limit should be, from a health perspective, for sporting events. Groups like the World Health Organization and the Environmental Protection Agency have air quality standards, but those are based on how air pollution would affect the average person, and everyone responds differently to exposure. “As a rough rule of thumb, if you can smell the smoke and see the smoke, it’s probably not a good environment where you should be exercising hard,” Avol says. But leagues and tournaments also consider the financial impact of scheduling decisions. “They’re going to make business-based decisions, and they’re going to operate at air quality levels you or I might choose not to.”

Northern California, western Oregon, and the Great Plains are projected to have over 50 percent more multiday periods with high levels of wildfire-generated air pollution as climate change progresses. In Australia, climate change means the intense bushfire season seen this year could happen more often. That’s going to impact human and animal health, across the board, but it’s also going to change sports, exercise, and recreation. “Science says exposure to these contaminants have negative health effects,” Avol says. Integrating that awareness into conversations around sports is only going to become more of an issue. “Clearly, we’re headed for a collision course.”

Update January 16th, 9:45 ET: This story has been updated to include information from Football Federation Australia.