Take a whiff, and you think you're breathing pure, clean, unadulterated air. Say, however, that you travel through 114.0 linear miles in one day outside of a car, completely open to the elements of road dust and asphalt flecks. About 54 square inches of skin, the front area between the end of my ankle-length sock and the beginning of my spandex shorts a couple of inches above my knee, are exposed to wind per leg.

Multiply that by two for two legs, and then by 114 miles, or 7.2 million inches, and you arrive at the conclusion that my lower legs passed through 780 million cubic inches, or 12,782 cubic meters, of air. Even if those cubic meters contain 0.5 grams of dust each (warnings are issued for West Virginia coal mines at 2.5 grams), that's 6.4 kilograms of dust my legs have passed through.

Granted, a very small percentage of that actually sticks to me. But I just wanted to do some thinking on paper to justify why my legs are now caked in visible grit.