It can enter the air from automobile exhaust, burning of certain industrial substances, and from reaction of nitrogen in air with other atmospheric chemicals.

In the 1950s the source of photochemical smog was established as automobile exhaust emissions.

Automobile exhaust gas capture has also been seen as economical but would require extensive design changes or retrofitting.

Rail cars in cities accumulate grime from building and automobile exhaust, while cars in deserts may be subjected to sandstorms which etch or strip paint.

By the late 1980s, automobile exhaust pollution had been noticeably improved, but more stringent regulations forced car manufacturers to adopt the use of electronically controlled fuel injection and hypereutectic pistons.