Until today, wildfires were never a problem in my city. Suddenly, it is a persistent threat, to be expected throughout the summer and successive summers.

The general advice is to refrain from exercising outdoors. There is no mention of refraining indoors. However, most buildings and gyms don't HEPA filter the ventilated air, and it is the fine particulates that cause most of the problems.

Why is it that the advice to refrain from exercising only applies to outdoors?

I know that subjectively, the air indoors seems fresher, but am not sure why, in light of the rarity of HEPA filtration in most building HVACs, which often draw air in from the outside. I am an regular hard-as-one-can indoor ellipticaller, but am now unsure whether this is wise.

1 Answer 1


It turns out that I'm right. The internal air isn't that much different from the external air unless you are recirculating internal air with HEPA filtering. The reason for remaining indoors is that it reduces physical activity and hence, breathing polluted air. That reasoning is circumvented, however, if you exercise indoors (again, unless you are HEPA filtering recirculated indoor air). Here is the source of that information.

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