Here is a BBC article that talks about a study that quantifies the issue: * http://www.bbc.com/news/health-36208003 http://www.bbc.com/news/health-36208003 Ev

Here is a BBC article that talks about a study that quantifies the issue: * http://www.bbc.com/news/health-36208003 http://www.bbc.com/news/health-36208003 Even in very polluted cities you would have to do much more cycling/walking than anyone would normally do to negate the effects of exercise (7 and 16 hours respectively)