Is there any research to back up your claim? What are the chances of getting cancer from this? Is there even a direct link? Is it better in aggregate to be an unfit bus rider or a fit cyclist?
http://www.breathelondon.org/south-east/project/modes-transp...
But cyclists and pedestrians have no such luxury - they're breathing in the filth whenever they're on, or near, a busy road.
And paper filters do, of course, remove dust and particulates, which are part of the problem. Just not things like NOx and volatile organics. And depending on the filter, it may not be effective against really small (eg PM2.5) particles.