Ain't possible to breathe except thro' mouth when working hard.

Don't worry about pollution - up to 3 times worse inside motor vehicles than outside. Also, breathing fairly hard clears lungs, whereas shallow breathing doesn't clear, for instance, carbon monoxide from bloodstream. Again. drivers and passengers have higher levels of latter in blood stream.

Regular riders (min. 4 miles/day) are 20 times more likely to live longer because of increased fitness (than to die prematurely because of accident. So pollution doesn't seem to affect cyclists so much, it would seem.