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An overview of ground-level ozone

Ozone (O3) is a colorless, odorless gas. In the upper atmosphere, it shields us from the harmful effects of ultraviolet radiation. But in the lower atmosphere, where it is a main ingredient in smog, ozone can cause serious respiratory problems.

Unlike other emissions, ground-level ozone is not released directly into the atmosphere but is formed by complex chemical reactions of Volatile Organic Compounds (VOC) and Nitrogen Oxides (NOx) in the presence of heat and sunlight.

VOCs are emitted from a variety of sources, including motor vehicles, chemical plants, refineries, factories, consumer and commercial products, and other industrial sources. VOCs are also 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.

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.

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, it is clear that serious adverse health effects result when people are exposed to levels of ozone found today in many areas.

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Regional and global effects

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