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  • Reducing-type air pollution, characterized by SO2 and smoke, is capable of producing deleterious human health effects.
  • Photochemical air pollution arises from a series of complex reactions in the troposphere close to the earth's surface and comprises a mixture of ozone, nitric oxides, aldehydes, peroxyacetyl nitrates, and myriad reactive hydrocarbon radicals.
  • Indoor air can be even more complex than outdoor air, and outdoor air can permeate the indoor environment in spite of the reduced air exchange in buildings.
  • Sick-Building Syndrome may occur in new, poorly ventilated, or recently refurbished office buildings due to the outgasing of combustion products, volatile chemicals, biological materials and vapors, and emissions from furnishings.


* This article has been reviewed by the National Health and Environmental Effects Research Laboratory, U.S. Environmental Protection Agency, and approved for publication. Approval does not signify that the contents necessarily reflect the views and the policies of the Agency.


Air pollution is a reality of our twenty-first century lifestyle, and unsatisfactory air quality now plagues broad geographic areas. Typically, public exposures are characterized by prolonged periods of relatively low-level exposure to complex mixtures of photochemically transformed industrial and mobile emissions with periodic moderate excursions due to weather. Previously, scientific information regarding the impacts of air pollution on human health had largely been collected on the individual components that constitute air pollution's complex mixture. In turn, this knowledge was used both to develop public health standards and to establish regulatory controls. Because of the obvious complexities of urban air pollution, research agendas have morphed to include the study of the interaction and transformation of individual pollutant components within atmospheric systems—a truer reflection of real-world exposure complexities.


Classically, air pollution has been distinguished on the basis of the chemical redox nature of its primary components. A reducing-type atmosphere has been associated with smelting and related combustion-based industries, in which SO2 and smoke from incomplete combustion of coal accumulate as a chilled, acidic fog. This acidic mix reacts with surfaces, corroding metal and eroding masonry, characteristic of reductive chemistry. In contrast, photochemical air pollution with atmospheric reaction products of automobile exhaust consists of NOx and many secondary photochemical oxidants, such as O3, aldehydes, and electron-hungry hydrocarbon radicals. Today, most metropolitan areas have atmospheres with both reducing and oxidant air pollutants.


People of most industrialized nations spend in excess of 80 percent of their time indoors at work, at school, and at home or between these places in an automobile. Generally, the time spent indoors is disproportionately higher for adults, who have relatively less time to participate in outdoor activities, especially during the day, when outdoor pollutants are usually at their highest levels. Children and outdoor workers, by contrast, are much more likely to encounter outdoor air pollution at its worst; in fact, because of the relatively high activity levels of these subgroups compared with inactive office workers, their lungs may incur a considerably larger dose ...

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