Empowered by the Clean Air Act of 1970, the U.S. Environmental Protection Agency (EPA) quickly established air quality standards to protect public health and the environment. Focusing on six “criteria” air pollutants—sulfur dioxide, nitrogen dioxide, particulate matter, carbon monoxide, ozone, and lead—the resultant National Ambient Air Quality Standards became the centerpiece of air pollution regulations and required that all U.S. states, cities, and towns maintain levels of these pollutants beneath the mandated ceilings or face substantial “nonattainment” fines and penalties. Similar standards were soon adopted by a number of other countries. The impact of the Clean Air Act and the regulation of criteria air pollutants have been significant: from 1970 to 2022, the total emissions of the six major air pollutants dropped by 78 percent in the United States, despite increases in gross domestic product, vehicle miles traveled, and population size during the same time frame. Given the serious health effects of these pollutants, it has been estimated that this air pollution success story has lengthened and improved the quality of millions of American lives.
| pollutant | common sources | maximum acceptable concentration in the atmosphere | environmental risks | human health risks |
|---|---|---|---|---|
| Source: U.S. Environmental Protection Agency | ||||
| carbon monoxide (CO) | automobile emissions, fires, industrial processes | 35 ppm (1-hour period); 9 ppm (8-hour period) | contributes to smog formation | exacerbates symptoms of heart disease, such as chest pain; may cause vision problems and reduce physical and mental capabilities in healthy people |
| nitrogen oxides (NO and NO2) | automobile emissions, electricity generation, industrial processes | 0.053 ppm (1-year period) | damage to foliage; contributes to smog formation | inflammation and irritation of breathing passages |
| sulfur dioxide (SO2) | electricity generation, fossil-fuel combustion, industrial processes, automobile emissions | 0.03 ppm (1-year period); 0.14 ppm (24-hour period) | major cause of haze; contributes to acid rain formation, which subsequently damages foliage, buildings, and monuments; reacts to form particulate matter | breathing difficulties, particularly for people with asthma and heart disease |
| ozone (O3) | nitrogen oxides (NOx) and volatile organic compounds (VOCs) from industrial and automobile emissions, gasoline vapours, chemical solvents, and electrical utilities | 0.075 ppm (8-hour period) | interferes with the ability of certain plants to respire, leading to increased susceptibility to other environmental stressors (e.g., disease, harsh weather) | reduced lung function; irritation and inflammation of breathing passages |
| particulate matter | sources of primary particles include fires, smokestacks, construction sites, and unpaved roads; sources of secondary particles include reactions between gaseous chemicals emitted by power plants and automobiles | 150 μg/m3 (24-hour period for particles <10 μm); 35 μg/m3 (24-hour period for particles <2.5 μm) | contributes to formation of haze as well as acid rain, which changes the pH balance of waterways and damages foliage, buildings, and monuments | irritation of breathing passages, aggravation of asthma, irregular heartbeat |
| lead (Pb) | metal processing, waste incineration, fossil-fuel combustion | 0.15 μg/m3 (rolling three-month average); 1.5 μg/m3 (quarterly average) | loss of biodiversity, decreased reproduction, neurological problems in vertebrates | adverse effects upon multiple bodily systems; may contribute to learning disabilities when young children are exposed; cardiovascular effects in adults |