The EPA sets national ambient air-quality standards for six air pollutants: carbon monoxide, nitrogen dioxide, ozone, lead, sulfur dioxide, and particulate matter.
Carbon monoxide is a colorless, odorless, poisonous gas produced through incomplete combustion of burning materials. Nitrogen oxides—a byproduct of burning fuels in vehicles, power plants, and boilers—are one of the main components (together with ozone and particulates) of the brownish haze or smog that forms over congested areas; they also precipitate as acid rain. Ozone in the stratosphere plays an important role, shielding the planet from ultraviolet radiation; it's far less desirable in the troposphere, where it can irritate lungs and impair breathing. Sulfur oxides come from transportation sources as well as from the burning of fossil fuels in nontransportation processes such as oil refineries, smelters, paper mills, and coal power plants; they also contribute to acid rain. Hydrocarbons, also known as volatile organic compounds (VOCs), contribute to smog; tropospheric ozone forms when oxygen in the air reacts with VOCs in the presence of sunlight. Particulates—tiny solid or liquid particles suspended as smoke or dust—come mainly from construction, agriculture, and dusty roads as well as industries and vehicles that burn fossil fuels.
Mobile sources (cars and trucks) account for more than 70 percent of the emissions that cause smog and 90 percent of the emissions that lead to higher levels of carbon monoxide. Industrial processes that burn fossil fuels also contribute to air pollution. While electric power plants in the Los Angeles basin are relatively clean, nationwide, they produce two-thirds of all sulfur dioxide, more than one-third of nitrogen oxides, and one-third of the particulate matter released into the air.