It's also possible that air quality on the London Tube is much worse than the bus, at least for particulate matter: "Researchers found that a person travelling

It's also possible that air quality on the London Tube is much worse than the bus, at least for particulate matter: "Researchers found that a person travelling on a bus for two and a half hours was exposed to a third of what they would be travelling on the Tube for just an hour." https://www.independent.co.uk/news/uk/home-news/air-pollution-london-underground-tube-worse-than-above-ground-a8721586.html https://www.independent.co.uk/news/uk/home-news/air-pollutio... EDIT: Regarding CO2 levels, there's a 2018 FOI request which seems to imply it wasn't monitored back then, but was within regulatory limits though I'm not sure I'd want to travel on the tube more often than necessary? "The regulatory limits for CO2 are 5000 parts per million (ppm) for 8 hours time weighted equivalent or a short term exposure limit (15 minutes) of 15000ppm." https://tfl.gov.uk/corporate/transparency/freedom-of-information/foi-request-detail?referenceId=FOI-0540-1819 https://tfl.gov.uk/corporate/transparency/freedom-of-informa...