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Ozone Standards

Ambient air quality standards (AAQS) define clean air, and are established to protect even the most sensitive individuals in our communities. An air quality standard defines the maximum amount of a pollutant that can be present in outdoor air without harm to the public's health. Both the California Air Resources Board (ARB) and the U.S. Environmental Protection Agency (U.S. EPA) are authorized to set ambient air quality standards

Federal Standards

EPA has established standards (termed National Ambient Air Quality Standards or NAAQS), under the authority of the Federal Clean Air Act, that identify safe levels for ozone in the atmosphere to prevent and minimize adverse impacts to human health and to crops, forests, and materials.  Ozone levels measured in the atmosphere at levels equal to or lower than the standards are viewed as safe, whereas levels above the standards represent a reasonable danger to public health and welfare (non-health related damages), and thus require action to reduce emissions of ozone precursors.  EPA ozone standard is an 8-hour average of 0.08 parts per million by volume [ppm(v)] This standard is subdivided into primary standards that protect public health and secondary standards that protect public welfare (the numeric value of the standard is the same for primary and secondary standards). 

Areas in the United States where ozone levels measured in the ambient air exceed the 8-hour standard of 0.08 ppm(v) are said to be in nonattainment of this standard.  The 8-hour ozone NAAQS is based on the number of days per year with a 8-hour average concentration of 0.08 ppm(v) or greater.  An area is in compliance with the ozone NAAQS when measured 8-hour average ozone levels at any given monitoring station do not exceed 0.08 ppm(v) more than one day per year over any consecutive three-year period.  Thus an area with measured 8-hour average ozone levels greater than 0.08 ppm(v) on four or more days over a three-year period has not attained the standard, even if all of the days occurred in only one of the three years.

The severity or magnitude of a given area’s ozone nonattainment problem is given by the ozone design value, which is based on the 3-year average of the fourth-highest daily maximum 8-hour average ozone concentrations measured at a given monitoring station. The overall design value for the San Joaquin Valley Air Basin is given by the highest design value of all of the individual monitoring stations in the Basin. 

The San Joaquin Valley Air Basin, which is comprised of San Joaquin, Stanislaus, Merced, Madera, Fresno, Kings, Tulare and Kern (Valley portion only) counties, has a design value of 0.115 ppm (2005) and is classified as “serious” nonattainment for the 8-hour standard."   

1 Hour Standards

The U.S. EPA revoked the Federal 1-hour standard on April 15, 2005.  For more information, click here.

California Standards

On April 28, 2005, the California Air Resources Board (ARB) approved the nation's most health protective ozone standard with special consideration for children's health. The new 8-hour-average standard at 0.070 parts per million (ppm) will further protect California's most vulnerable population from the adverse health effects associated with ground-level ozone, or smog. The new 8-hour-average ozone standard is the first of its kind in the state.

For more information on California Ambient Air Quality Standards click here.


Concentration (parts per million)

Averaging Time



0.09* ppm


1 hour


Standard set to prevent the following short-term health effects: One-hour and multi-hour exposures - lung function decrements, and symptoms of respiratory irritation. Multi-hour exposures - airway hyperreactivity and airway inflammation.

0.070* ppm

8 hour


* Defined the highest level of ozone that can be present without adverse health effects
**These concentrations were approved by the Air Resources Board on April 28, 2005 and are expected to become effective in early 2006

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