Believe it or not, a recent study found that drivers of cars and busses suffer breathing problems from air pollution during their commutes but bicyclists do not.
According to the study, subjects were given breathing tests after driving in cars, busses, and riding their bikes. The bus and car drivers exhaled less volumes of air within six hours of driving in traffic. The bicyclists were not affected.
Air pollution has shown to increase chances of asthma in children and also increases chances of cardiovascular problems. More on air and noise pollution here.