EPA’s Ground-level Ozone Standard

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Air Quality Standards

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EPA’s New Standard for Ground-level Ozone

On October 1, 2015, the U.S. Environmental Protection Agency (EPA) strengthened the National Ambient Air Quality Standards (NAAQS) for ground-level ozone, the main constituent of smog. Specifically, EPA revised the primary (public health) and secondary (public welfare) 8-hour ozone standards to 70 parts per billion (ppb). The previous standard, the 2008 standard, was 75 ppb. The 70 ppb federal 8-hour ozone standard became effective on December 28, 2015.


Primary standards provide public health protection, including protecting the health of "sensitive" populations such as asthmatics, children, and the elderly. Secondary standards provide public welfare protection, including protection against decreased visibility and damage to animals, crops, vegetation, and buildings.


The Clean Air Act requires the EPA Administrator to set primary air quality standards to protect public health with an “adequate margin of safety,” including the health of at-risk groups. In making this judgment, the Administrator considers factors such as the nature and severity of health effects, the size of the at-risk groups affected, and the degree of certainty and uncertainty in the science on ozone-related health effects. The law charges the Administrator with setting standards that are “requisite” -- neither more nor less stringent than necessary -- to accomplish this.


EPA’s panel of science advisors, the Clean Air Scientific Advisory Committee (CASAC) concluded that science indicates the 2008 standard (75 ppb) is not adequate to protect public health and that science supports a standard within a range of 70 ppb down to 60 ppb.


Based on an expanded body of scientific evidence that includes thousands of studies on the effects of ozone on health, the EPA Administrator concluded that the 2015 standard of 70 ppb is requisite to protect public health with an adequate margin of safety, as required by law.


Along with EPA’s action in October 2015 to strengthen the ozone NAAQS, EPA also revised the Air Quality Index (AQI) for ozone to reflect the new, more stringent standard. The AQI is a color-coded chart designed to inform the public about daily air pollution levels in their communities.



Air Quality Index (AQI) Breakpoints
for the 2008 and 2015 Federal 8-Hr Ozone Standards



AQI Value


8-hour (ppb)


8-hour (ppb)





0-54 0-59





55-70 60-75





71-85 76-95





86-105 96-115




201-300 106-200 116-374





201 to the Significant Harm Level*

375 to the Significant Harm Level*


*The Significant Harm Level for ozone is 600 ppb, two-hour average.


As part of the Ventura County APCD’s mission to improve Ventura County’s air quality and provide timely information to the public, the APCD uses the AQI to inform the public about the county’s air quality. Click here to see the current air quality forecast for Ventura County.


How do these changes affect Ventura County?

On June 15, 2004, EPA designated Ventura County as a moderate nonattainment area for the 1997 8-hour ozone standard, based on Ventura County’s ozone levels over the previous three years. However, on February 14, 2008, ARB formally requested that EPA reclassify Ventura County up one classification level to a serious 8-hour ozone nonattainment area. On May 20, 2008, EPA approved the reclassification request, which became effective June 19, 2008. Therefore, Ventura County is now a serious ozone nonattainment area, and must meet the 75 ppb federal 8-hour ozone standard by December 31, 2021.


EPA will finalize designations for the 2015 ozone standard in 2017. A nonattainment area will meet the standard if the fourth-highest maximum daily 8-hour ozone concentration per year, averaged over three years, is equal to or less than 70 ppb.


APCD is reporting the County’s ozone levels using the Air Quality Index for the 2015 federal 8-hour ozone standard. The use of the new AQI is more protective of the public’s health and will result in more days being shown with an increased AQI.


For more information:

EPA's website for regulatory actions related to the ozone standards is: