Ambient Ozone

Ambient (also called ground-level or tropospheric) ozone (O₃) is a highly reactive gas produced by a complex combination of volatile organic compounds (VOCs) and nitrogen oxides (NOx) in the presence of sunlight. VOCs are emitted primarily by mobile sources (such as automobiles) and industries using and manufacturing petroleum-based products. NOx are emitted primarily by mobile sources and fossil-fuel power plants. Ambient ozone should not be confused with stratospheric ozone, which acts beneficially to filter out harmful solar radiation and can be destroyed by chlorofluorocarbon emissions. Since the two sources of ozone do little mixing, control of ambient ozone and of stratospheric ozone are usually treated as separate issues.

What effect does ground-level ozone have?

It is clear that short-term exposure can cause short-term, reversible reductions in lung function and increases in the frequency of respiratory symptoms. Long-term exposure is suspected of causing premature aging of the lung (and increasing the risk of developing chronic respiratory disease). Ozone also has been shown to damage field and row crops, trees and ornamental plants, and organic materials such as fabrics, rubber, dyes, and paints. It may also impair visibility.

How is it regulated?

The Clean Air Act directs the EPA to set a primary ozone standard to "protect the public health with a margin of safety." The primary standard for ozone is 0.12 ppm averaged over one hour. An "area" is permitted a total of three days in three successive years with peak readings over this level before it is out of compliance. This means, in theory, that two areas with the same number of days in violation of compliance would be viewed as identical even though in one, the ozone concentrations exceeded the standard for all 24 hours, while in the other, only one hour was in violation. ("Areas" are most often individual cities, some are individual counties, and others encompass broad, multi-city geographical areas.) The act also directs EPA to set a secondary standard to "protect the public welfare against any existing or anticipated adverse effect," including effects on crops, materials, visibility, comfort, and well-being. The secondary standard is a one-hour daily maximum of 0.08 ppm.

What does compliance involve?

Areas face specific deadlines for meeting the primary standard, but not the secondary standard. States are required to develop plans to bring their nonattainment areas into compliance with the primary standard. As ozone is not emitted directly, these plans must specify how the ozone precursors (VOCs and NOx) will be controlled and what effect such control will have on ozone concentrations. States failing to meet compliance deadlines may face a variety of federal sanctions, such as loss of federal grants for highways and for the construction of sewage treatment plants.

How is ozone monitored?

Areas with suspected problems are required by EPA to set up monitors to check ozone concentrations on an hourly basis. There were 707 counties (out of 3,100) with ozone monitors operating at least one year in the 1983-1985 period. These monitors are supposed to be strategically placed at locations likely to have poor air quality. State and local governments have the responsibility for collecting air quality data and reporting it to EPA.