There was a study [0] in Paris that demonstrates a signifiant positive benefit/risk ration of bicycling even in polluted air: the effect on physical and psychic health benefits largely outweighs (sometimes to x30) the risk of accidents and pollution disease.
My city attempts to promote cycling, but hasn't bothered to first take care of the problem with people heating their homes with solid fuels like coal dust and whatnot.
It's frustrating, because cycling during the heating season is not only unpleasant, but also unhealthy.
