Raggytash asks Has anyone ever measured the volume of CO2 emitted by people exercising?

Naturally, they have, Raggytash. There's been a couple of studies. In general, the finding is that sitting in a car in traffic is worse for your lungs than cycling past it - its one of the reasons that cyclists go to the front of traffic and wait in the bicycle box and take off instantly when the lights change. In my case, it's the reason why I'll usually find a way through quieter streets when possible.

Benefits of cycling and walking 'outweigh air pollution risk' in cities https://www.theguardian.com/environment/2016/may/05/benefits-cycling-walking-outweigh-air-pollution-risk-cities
Cyclists and fumes https://www.theguardian.com/environment/bike-blog/2014/feb/20/air-pollution-cyclists-bike-blog
Cyclists exposed to five times less air pollution than those in cars, experiment suggests http://www.cyclingweekly.com/news/latest-news/cyclists-exposed-five-times-less-air-pollution-cars-experiment-suggests-133129#JoWRmJqpBJAspqdl.99