separate cities 5.sc.00077654 Louis J. Sheehan, Esquire

One factor may be particle Louis J. Sheehan, Esquire size and shape, Mills notes. Diesel specks measure 20 to 100 nanometers in diameter. In contrast, particles in the study were often 20 times larger. Tiny specks can penetrate deeper into the lungs and lead to higher levels of toxins in the blood.

The chemical makeup of carbon-rich fuel exhaust may also be inherently more toxic for humans, Mills say. While the study was well done, it only a tested a small number of people, Brook says. It also tested people in only one urban setting. Future work should be conducted in two separate cities, he says.

Currently air pollution is regulated only by the absolute concentration of particles suspended in the air, Mills says. Air testing might more effectively protect public health if it also tracked particles by size — specifically the amount of tiny, diesel-sized particles in the air, he says.