Ambient Air Quality (Air Pollution)

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Human exposures to airborne chemicals vary widely among inhalation microenvironment categories, which include workplaces, residences, outdoor ambient air, transportation, recreation areas, and public spaces. There are also wide variations in exposure within each category, depending on the number and strength of the sources of the airborne chemicals, the volume and mixing characteristics of the air within the defined microenvironment, the rate of air exchange between indoor and outdoor air, and the rate of loss to surfaces within the microenvironment.

Exposures to airborne chemicals in the workplace are extremely variable in terms of composition and concentration, depending on the materials being handled, the process design and operation, the kinds and degree of engineering controls applied to minimize releases to the air, work practices followed, and personal protection provided. Airborne chemicals in residential microenvironments are attributable to their presence in the air infiltrating from out of doors and to their release from indoor sources, such as unvented cooking stoves and space heaters, cigarettes, and consumer products, including volatile emissions from wallboard, textiles, carpets, and other materials. Indoor sources can release nitrogen dioxide (NO2), fine particle mass (FPM), and formaldehyde (HCHO) to such an extent that indoor concentrations for these chemicals can be much higher than those in ambient outdoor air.

For pollutants having National Ambient Air Quality Standards (NAAQS), such as particulate matter, NO2, carbon monoxide (CO), ozone (O2), lead (PB), and sulfur dioxide (SO2), there is an extensive network of fixed-site monitors, generally on rooftops. Although these devices generate large volumes of data, the concentrations at these sites may differ substantially from the concentrations that people breathe, especially for tailpipe pollutants such as carbon monoxide.

Transportation sources represent a significant source of exposures. Many people spend up to three hours each day in autos or buses as they go to work, to school, or shopping. Inhalation exposures to CO in vehicles and garages can represent a significant fraction of total CO exposures. Recreational exposure while exercising may also be important to total daily exposure because the increased respiratory ventilation associated with exercise can produce much more than proportional increases in delivered dose and functional responses.

Current concern regarding community air quality, is focused on particulate matter (PM) and ozone. A broad variety of processes produce suspended particulate matter (PM) in the ambient air in which we live and breathe, and there are statistically significant associations between the concentrations of airborne PM and the rates of mortality and morbidity in human populations. The PM concentrations have almost always been expressed in terms of mass. Also, in studies that reported on associations between health effects and more than one mass concentration, the strength of the association generally improves as one goes from total suspended particulate matter (TSP) to thoracic particulate matter (PM10 microns or less in aerodynamic diameter [PM10]), to fine particulate matter (PM2.5 microns or less in aerodynamic diameter [PM2.5]).

Morton Lippmann

(see also: Airborne Particles; Carbon Monoxide; Clean Air Act; Hazardous Air Pollutants; Inhalable Particles [Sulfates]; Lead; National Ambient Air Quality Standards; Smog [Air Pollution]; Sulfur-Containing Air Pollutants [Particulates]; Total Suspended Particles [TSP] )


Lippmann, M., ed. (2000). Environmental Toxicants, 2nd edition. New York: Wiley.

U.S. Environmental Protection Agency (2001). Air Quality Criteria for Particulate Matter. EPA 600/P-99/002. Washington, DC: Author.