This a bit surprising, but here is a quote from the latest (very small, London, England) study.

"The first study found that the lungs of bicycle commuters in London contain black carbon levels 2.3 times higher than pedestrian commuters. The report's abstract notes that inhalation of black carbon is associated with reduced lung function and heart disease."

The rest of the report is at http://www.dw-world.de/dw/article/0,,15417025,00.html

Fortunately most of mine is way off peak hours, and mostly on a bike trail.