Pollution

Ambient Air Pollutants

Pollutants are released to the ambient air from an array of stationary or point sources (e.g., power plants, industrial sites), area sources (e.g., forest fires), and mobile sources (e.g., motor vehicles, boats, lawn mowers). Indoor sources are discussed separately below. Human exposure generally involves complex mixtures rather than individual pollutants.

The U.S. Environmental Protection Agency (EPA) is required by the Clean Air Act to set national ambient air quality standards for the protection of public health and welfare. The six major pollutants for which these standards are set, known as ‘criteria’ pollutants, are carbon monoxide (CO), sulfur dioxide [Page 819](SO2), nitrogen dioxide (NO2), ozone, airborne particulates, and lead.

• CO is formed by incomplete combustion of carbon-based fuels such as gasoline or natural gas. The primary outdoor source is motor vehicles, while indoor sources include gas appliances and environmental tobacco smoke. CO has a high affinity for hemoglobin and interferes with oxygen transport. It causes acute poisoning at high levels; for typical environmental levels, associations with cardiovascular endpoints have been observed.
• SO2 is formed by the combustion of fossil fuels containing sulfur (primarily coal). Levels were very high in urban areas of the United States and Europe in the early to mid-20th century and have declined since the 1970s. However, high levels are now observed in other regions such as China, where coal use is currently high. SO2 is associated with decreased lung function and respiratory symptoms, especially in asthmatics, and contributes to the formation of acid rain.
• Primary sources of NO2 include motor vehicle and power plant emissions and burning of fossil fuels. Local levels vary with the density of traffic. NO2 is associated with lung irritation and lowered resistance to respiratory infection. In the presence of sunlight, NO2 contributes to the formation of ozone.
• Ozone is a secondary pollutant formed by solar radiation and other pollutants such as nitrogen oxides and volatile organic chemicals (VOCs) from sources such as gasoline vapors, solvents, and consumer products. Due to the role of solar radiation in its formation, ozone levels are higher in summertime and in sunnier areas. Concentrations are higher downwind of urban centers than in cities themselves because of the time needed for these photochemical reactions to occur. Ozone in our ground-level air is considered a pollutant that contributes to formation of urban smog and is associated with reduced lung function and sensitization to other irritants; however, depletion of stratospheric ozone is associated with global warming and decreased protection from UV exposure.
• Particulate matter (PM) is a heterogeneous mixture of small particles and liquid droplets. Components include acids, organic chemicals, metals, soil, and dust particles from sources, including fuel combustion, high temperature industrial processes, atmospheric reactions of other pollutants, and mechanical processes such as demolition or road wear. PM is classified according to the diameter of the particles, which affects how far they can penetrate into the respiratory system: coarse PM (PM10) is <10 µm, fine (PM2.5) is <2.5 µm, and ultrafine particles are <0.1 µm. Respiratory and cardiovascular outcomes and visibility impairment have been associated with PM.
• Lead is a naturally occurring metal. For much of the last century, motor vehicles were the principal source of lead in air, and ambient concentrations have decreased markedly in the United States since the phase-out of lead from gasoline. Current sources for lead pollution in air include metal processing and waste incinerators. Lead exposure can have adverse effects on many of the body's organs and systems. It is considered of particular concern for infants and young children as one of the primary targets is the nervous system and exposure can lead to impaired neurodevelopment and reduced IQ.

Numerous air pollutants in addition to these six are known or suspected to pose health threats. The Clean Air Act amendments of 1990 designated 188 of these as ‘hazardous air pollutants.’ Chemicals in this category have been associated with cancer or other adverse health effects, including neurological, reproductive, developmental, immune, and respiratory outcomes. Examples include benzene, formaldehyde, perchloroethylene (used in dry cleaning), polycyclic organic matter (produced as combustion byproducts), and compounds of metals such as mercury and cadmium. The World Health Organization (WHO) and European Union (EU) also set air quality guidelines.

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