are the substances considered to be atmospheric pollutants because the more reactive hydrocarbons (e.g., aromatics) undergo a photochemical reaction with nitrogen oxides (NOx) to form oxidants, components of smog that can cause eye irritation and respiratory problems. Motor vehicles account for about one-third of man-made hydrocarbon emissions, although automotive emission controls are reducing this amount. The greatest portion of total atmospheric hydrocarbons is from natural sources, such as pine trees.

Also see catalytic converter, emissions (automotive), pollutants.