Air Quality and Health and Welfare

2.2 Ozone

2.2.1 Introduction
Ground-level ozone, the main ingredient in smog, is formed by complex chemical reactions of volatile organic compounds (VOC) and nitrogen oxides (NOx) in the presence of heat and sunlight. Ozone forms readily in the lower atmosphere, usually during hot summer weather. VOCs are emitted from a variety of sources, including motor vehicles, chemical plants, refineries, factories, consumer and commercial products, and other industrial sources. VOCs also are emitted by natural sources such as vegetation. NOx is emitted largely from motor vehicles, nonroad equipment, power plants, and other sources of combustion.

The science of ozone formation, transport, and accumulation is complex. Ground-level ozone is produced and destroyed in a cyclical set of chemical reactions involving NOx, VOC, heat, and sunlight. Carbon monoxide also participates in the production of ozone, albeit at a much slower rate than most VOC and NOx compounds. As a result, differences in NOx and VOC emissions and weather patterns contribute to daily, seasonal, and yearly differences in ozone concentrations and differences from city to city. Many of the chemical reactions that are part of the ozone-forming cycle are sensitive to temperature and sunlight. When ambient temperatures and sunlight levels remain high for several days and the air is relatively stagnant, ozone and its precursors can build up and produce more ozone than typically would occur on a single high temperature day. There is a growing concern that climate modification resulting from the increased buildup of greenhouse gases such as carbon dioxide may increase the amount of ozone produced from a given amount of NOx and VOCs. Further complicating matters, ozone also can be transported into an area from pollution sources found hundreds of miles upwind, resulting in elevated ozone levels even in areas with low VOC or NOx emissions.

Based on a large number of recent studies, the US EPA has identified several key health effects caused when people are exposed to levels of ozone found today in many areas (“Review of National Ambient Air Quality Standards for Ozone,” 1996 and “Air Quality Criteria for Ozone,” 2006).