This is a question relevant to the Western US with all the wildfires we’ve been getting lately. ve always wondered if I’m doing more harm than good if I exercise outdoors on a day when the air quality is worse than moderate. An interesting study done in Taiwan answers this question. There are parts of Taiwan where the air quality is often poor because of pollution, like a particulate matter count (pm 2.5) around 160. The authors followed recruited close to a million participants in the study, with medical records available from 2001-2016. They were followed an additional three years until 2019, so almost 20 years of data were available. The authors were able to correlate all-cause mortality risk with air pollution exposure and levels of exercise. Even when consistently exposed to bad air, those who consistently did a high amount of exercise had significantly less risk of dying than inactive people. Of course, the lowest risk of all is breathing cleaner air and exercising more, but it is good to know that even in bad air, overall exercise helps rather than harms.
There is another option of course, and that is to exercise outdoors when the air is cleaner and stick to indoors when it is bad. But after a few days of being stuck indoors, when my stationary bike starts to resemble a Gerbil wheel, it’s nice to know I’m not doing harm if I work out outdoors even if the air is marginal.