Pretty much zero. The vast majority of time there's extreme levels of air pollution outside (think 200-1000 µg/m³ of PM2.5), so it's dangerous to even open the

Pretty much zero. The vast majority of time there's extreme levels of air pollution outside (think 200-1000 µg/m³ of PM2.5), so it's dangerous to even open the window. And it's pretty difficult to do anything physically at 2000-4000 ppm of CO₂ because you feel like shit most of the time. At least that's what I was blaming my laziness on until I learned about CO₂ levels on the ISS: https://letstalkscience.ca/educational-resources/backgrounders/carbon-dioxide-on-earth-and-on-iss https://letstalkscience.ca/educational-resources/backgrounde... https://archive.thinkprogress.org/its-taking-less-co2-than-expected-to-cause-health-risks-in-astronauts-7af09e82b83/ https://archive.thinkprogress.org/its-taking-less-co2-than-e... https://www.airspacemag.com/daily-planet/why-living-space-can-be-pain-head-180951507/ https://www.airspacemag.com/daily-planet/why-living-space-ca... https://www.researchgate.net/publication/262148293_Relationship_Between_Carbon_Dioxide_Levels_and_Reported_Headaches_on_the_International_Space_Station https://www.researchgate.net/publication/262148293_Relations... So it's a bit higher than my apartment, and they still manage to exercise daily and stay in shape.