An study published recently in Atmospheric Environment and reported by the Times Online found that bicycle riders in urban areas "inhale tens of millions of toxic nanoparticles with every breath, at least five times more than drivers or pedestrians" and that the particles were "mostly emitted by car exhausts." The cars, especially diesels, put these dirty particulates in the air where bikers, because they're exerting themselves and breathing harder than pedestrians and automobile drivers, end up sucking down tens of millions of the pollutant particles with each breath. The nanoparticles have been linked to heart disease and respiratory problems and can get into the bloodstream, so it's not like a simple cleansing cough will eliminate the problem. One solution is to wear a mask, but this doesn't help much because the particles are so small they pass right through most any barrier. Zero-emission vehicles can't come fast enough.
[Source: Times Online via Treehugger]