While long-haul trucks account for less than 6% of the vehicle miles traveled over U.S. highways, they account for about 40% of the emissions of air-polluting particulate matter (PM 2.5) and about 55% of nitrogen oxides -- the precursor to ozone in the atmosphere.
Freight trucks primarily use diesel engines, which emit fine-particulate exhaust, which poses a cancer risk 7.5 times larger than all other air toxins. Diesel exhaust is classified as a Group 1 (highest level) carcinogenic, according to the World Health Organization's International Agency for Research on Cancer.