The consensus view tends to be that the health benefits of the aerobic exercise far outweigh the problems with breathing in the freshly polluted air. Many car drivers, not to mention pedestrians, are breathing the same air you are, so is there really a healthier alternative?

Occasionally, I do see people riding in D.C. with dust masks on. But this is D.C., city of paranoia: those people might be worried about SARS or Marburg more than lung cancer.