Yes, the air quality on London buses is consistently terrible on cold days. There is very little ventilation in those buses other than "open the windows", and when it gets cold people don't do that. Often it seems like you're not only breathing in everyone else's food smells and stinky breath, but road fumes and dust as well.
"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-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-informa...
The air certainly smells and feels a lot fresher and healthier on the extensively-ventilated Tube compared to on crowded buses with little or no ventilation at all.