Those travelling by train were found to experience the lowest air pollution concentrations, as the train travelled on its own dedicated track, well removed from any road traffic. Apart from the train’s own engine, a possible additional source of carbon monoxide on board trains may have been from the exhaled air of smokers having smoked a cigarette immediately prior to boarding the train. The same applies for buses. Apart from the motorcycle, in general, the average CO exposures were found to be highest for car and bus users. The international literature explains that this is due to the fact.
