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?
There's actually the opposite evidence; it's worse to be in a vehicle than walk or cycle, and there's a few studies that say that the benefits of cycling outweigh the pollution exposure.
Most smart people driving in London are going to leave their car's climate control on the "recirculate" setting whenever possible. That gives you a more-or-less closed environment, running cabin air back through the car's filters rather than sucking in dirty air from the street.
But cyclists and pedestrians have no such luxury - they're breathing in the filth whenever they're on, or near, a busy road.
By using the "recirculate" setting you avoid bringing in polluted air from outside the car in the first place, so it certainly does help. Just return to fresh air when away from busy roads and polluted areas.
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.
From a personal sample of one, I cycled 4 months on routes used by busses and developed a smokers cough - I've never smoked. I'd be surprised if that wasn't a lung cancer risk. I stopped after that. The worst fumes were from diesel busses though this was a few years back and they have probably fitted less polluting ones now.
I don't think it's healthy at all to ride on London buses, especially in a hot summer when the buses have all their windows open. The air seems really bad inside - you can smell it and feel it in your lungs.
http://www.breathelondon.org/south-east/project/modes-transp...