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current air quality in Wisconsin.
more about current air quality advisories.
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Contact information
For more information about ozone, contact:
Angie Dickens


Ozone is a gas that occurs in both Earth's upper atmosphere and at ground level. When ozone is at ground level, it is a major component of smog and can cause a number of health problems for humans and other living things. Because of this serious health risk, DNR monitors ground-level ozone around the state and issues air quality advisories when levels become potentially harmful. Counties that do not meet federal clean air standards for ozone require businesses and residents in those areas to take special measures to reduce ground-level ozone.

Basic info

Basic information

Good (stratospheric) ozone
Bad (ground-level) ozone

Air regulations

Federal ozone standards

Ground-level ozone's harmful effects on human health and the environment prompted the federal government to set National Ambient Air Quality Standards (NAAQS) for the amount of ozone in the air over certain time periods. Counties that do not meet these standards receive a nonattainment designation that triggers increased pollution control requirements for businesses in the area along with other efforts to reduce ozone levels. State Implementation Plans (SIPs) are then prepared and approved by the U.S. Environmental Protection Agency (EPA). These plans include regulations for controls on emissions that are needed to reduce the air pollution and meet the standard. If air quality improves based on monitored values and the county meets the standard, DNR can request redesignation of the county.

Overview of ozone standards

The federal Clean Air Act requires EPA to develop two types of air quality standards for certain air pollutants, including ozone.

  • Primary standards protect public health, including the health of groups especially affected by air pollution, such as asthmatics, children and the elderly.
  • Secondary standards protect public welfare and the environment, including protection against damage to animals, crops and buildings.

For more information, visit EPA's Ground-level Ozone Standards Designations [exit DNR].

History of ozone standards
  • October 26, 2015: 8-hour standard for ozone at 70 parts per billion (ppb).
  • March 27, 2008: 8-hour standard for ozone at 75 ppb.
  • July 18, 1997: 8-hour standard for ozone at 80 ppb.
  • February 8, 1979: 1-hour standard for ozone at 120 ppb.
  • April 30, 1971: 1-hour standard for total photochemical oxidants at 80 ppb.

Control strategies and regulations

For areas in Wisconsin that are not in attainment with federal air quality standards for ozone, or in danger of being designated as nonattainment areas, DNR and local partners have implemented several programs to reduce ground-level ozone.

Reasonably Available Control Technology (RACT) standards

Reasonably Available Control Technology (RACT) standards is a requirement of the federal Clean Air Act. For ozone control, all major sources of volatile organic compounds (VOCs) or nitrogen oxide compounds (NOx) in nonattainment areas must apply control technologies that are available and have reasonable costs. More information on RACT can be found on EPA's Technology Transfer Network [exit DNR].

In Wisconsin, RACT rules cover VOC sources. In general, the rules are designed to limit VOC emissions associated with the use, storage and handling of these ozone-forming compounds. However, control technologies and geographic coverage of these rules varies depending on the source types and when the applicable rule was created. More information on RACT rules can be found in chs. NR 419-425, Wis. Adm. Code [exit DNR].

In Wisconsin, NOx RACT rules cover major stationary sources in Kenosha, Milwaukee, Ozaukee, Racine, Sheboygan, Washington and Waukesha counties. The rules are designed to limit NOx emissions from stationary combustion sources such as boilers, heat treating and engines. More information on RACT rules can be found in ch. NR 428, Wis. Adm. Code [PDF exit DNR].

Mobile sources

There are several state and federal pollution-control programs aimed at mobile sources including highway vehicles such as cars, and off-road equipment such as all-terrain vehicles (ATVs). The federal government also regulates the fuels these vehicles and other equipment use, with a goal of having cleaner-burning fuels that do not contribute as much air pollution. Regulations include motor vehicle emission standards, vehicle inspection and maintenance, the required use of reformulated gasoline, ultra-low sulfur diesel fuel, low sulfur gasoline and gasoline vapor-recovery programs. Visit DNR's mobile sources page to learn more.

Interstate transport

Pollutants may be transported great distances and may harm human health in other states downwind of a particular pollution source. Ozone, along with fine particles and mercury, can be transported hundreds of miles or more from where they were formed.

Several federal rules deal with the problem of pollutants that cross state boundaries. The Clean Air Interstate Rule (CAIR) applies to ozone and fine particles (often referred to as particulate matter, or PM). The CAIR created an emissions "budget" of nitrogen oxides (NOx) and sulfur dioxide (SO2). This rule was replaced by the Cross-State Air Pollution Rule (CSAPR) [exit DNR], which also established state emission budgets for NOx and SO2.

On December 3, 2016, EPA proposed the CSAPR Update Rule [exit DNR]. This rule addresses interstate emission transport with respect to the 2008 ozone NAAQS. Wisconsin submitted comments to EPA regarding the proposal on February 1, 2016.

Design values

Ozone design values

"Design values" are averages of the pollutant levels and used to determine compliance with federal air quality standards. The ozone design value is the average over a three-consecutive-year period of the fourth-highest ozone concentrations at a given air monitoring site during each of those three years.

2012-2014 Ozone Design Values

Area designations

Ozone nonattainment history

Over the past several decades, counties in eastern Wisconsin have had unacceptable levels of ozone, but air quality has improved significantly in recent years. Learn more about Wisconsin's air quality trends.

2015 ozone standard (70 ppb 8-hour)

On October 1, 2015, EPA finalized the 2015 8-hour ozone standard at 70 ppb. Area designations for this standard are not expected until October 2017. See the "2015 ozone standard" tab for more information.

2008 ozone standard (75 ppb 8-hour)

In 2008, EPA finalized the 8-hour standard at 75 ppb. For the 2008 8-hour ozone standard, Sheboygan County and a portion of Kenosha County (essentially east of I-94) are considered nonattainment. In these areas, all ozone nonattainment requirements still apply.

1997 ozone standard (80 ppb 8-hour)
  • April 30, 2004 – EPA designated Door, Kenosha, Kewaunee, Manitowoc, Milwaukee, Ozaukee, Racine, Sheboygan, Washington and Waukesha counties as nonattainment.
  • May 21, 2008 – EPA redesignated Kewaunee County to maintenance.
  • July 12, 2010 – EPA redesignated Door and Manitowoc counties to maintenance.
  • February 9, 2012 – EPA proposed to redesignate Kenosha, Milwaukee, Ozaukee, Racine, Sheboygan, Washington and Waukesha counties to maintenance.
  • July 31, 2012 - EPA formally announced that the "Milwaukee-Racine Nonattainment Area" had been reclassified from nonattainment to attainment for the 1997 8-hour ozone standard. The former nonattainment area includes the counties of Kenosha, Milwaukee, Ozaukee, Racine, Washington and Waukesha counties.
1979 ozone standard (120 ppb 1-hour)
  • November 6, 1991 – EPA designated Door, Kenosha, Kewaunee, Manitowoc, Milwaukee, Ozaukee, Racine, Sheboygan, Walworth, Washington and Waukesha counties as nonattainment.
  • August 26, 1996 – EPA redesignated Kewaunee, Sheboygan and Walworth counties to maintenance.
  • April 17, 2003 – EPA redesignated Door and Manitowoc counties to maintenance.
  • April 24, 2009 – EPA made an attainment determination for Kenosha, Milwaukee, Ozaukee, Racine, Washington and Waukesha counties.

2015 ozone standard

2015 ozone standard

On October 1, 2015, EPA strengthened the National Ambient Air Quality Standards (NAAQS) for ozone to 70 ppb. The final rule and related technical documents are available on EPA's Ground Level Ozone Regulatory Actions [exit DNR].

On December 18, 2015, DNR requested that EPA reconsider the final rule.

Implications for Wisconsin

Nonattainment designations for the 2015 standard will likely be made based on 2014-2016 monitoring data. The Air Program will develop a State Implementation Plan (SIP) to bring any nonattainment areas into attainment of the standard as required by federal law.

The maps provided below illustrate potential area designations for the 2015 ozone standard based on design values from recent years. The map based on the most recent data (2013-2015) likely represents a "best case" scenario given the cooler temperatures and less ozone formation during those years. The map based on 2012-2014 data may be closer to a "worst case" scenario given the extremely hot 2012 summer. The areas of the state ultimately designated nonattainment in October 2017 will depend greatly on meteorological conditions in 2016.

For simplicity, the maps use counties and metropolitan statistical areas as default boundaries; note that these will not necessarily correlate to actual future nonattainment areas. The DNR will use the latest monitoring data and other information about ozone formation in the state to recommend specific nonattainment area boundaries to EPA as part of the designation process.

Proposed rule

The EPA proposed revisions to the ozone standard on November 25, 2014. The agency proposed to set the standard within the range of 65 to 70 ppb, and accepted comments on a range of 60 to 75 ppb.

The DNR submitted comments to EPA regarding the proposed rule on March 17, 2015.



Air quality forecasts and monitoring data
Ozone health information
Last revised: Tuesday November 29 2016