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, the DNR monitors ground-level ozone around the state and issues air quality advisories when levels become potentially harmful. Areas 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.
Areas in eastern Wisconsin, especially along the Lake Michigan shoreline, have historically had elevated levels of ozone. However, air quality has improved significantly in recent years, and ozone levels in Wisconsin have decreased significantly over the past few decades. Learn more about Wisconsin's air quality trends.
Good (Stratospheric) Ozone
Bad (Ground-Level) Ozone
- EPA - Ground Level Ozone Pollution
- Ozone and Your Health
- El Ozono y Su Salud
- DNR - Do a Little…Save a Lot!
Air Quality Forecasts and Monitoring Data
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, the DNR can request redesignation of the county.
Overview of Ozone Standards
The federal Clean Air Act requires the 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 Setting and Reviewing Standards to Control Ozone Pollution.
History of Ozone Standards
- Oct. 26, 2015: eight-hour standard for ozone at 70 parts per billion (ppb).
- March 27, 2008: eight-hour standard for ozone at 75 ppb.
- July 18, 1997: eight-hour standard for ozone at 80 ppb.
- Feb. 8, 1979: one-hour standard for ozone at 120 ppb.
- April 30, 1971: one-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, the 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 Clean Air Technology Center.
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.
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.
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 the DNR's mobile sources page to learn more.
Pollutants can be transported great distances and may harm human health in other states downwind of a particular pollution source. Ozone, along with fine particles, can be transported hundreds of miles or more from where they were formed.
Several federal rules address pollutants that cross state boundaries. The Clean Air Interstate Rule (CAIR) applies to ozone and fine particles (often referred to as fine particulate matter, or PM2.5). 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), which also established state emission budgets for NOx and SO2.
On Sept. 7, 2016, the EPA finalized the CSAPR Update Rule. This rule addresses interstate emission transport with respect to the 2008 ozone NAAQS. Wisconsin submitted comments to the EPA regarding the proposed rule on Feb. 1, 2016.
Ozone Design Values
"Design values" are averages of pollutant concentrations. These values are used to determine compliance with federal air quality standards. An ozone design value is a three-year average of the fourth-highest ozone concentrations at a given air monitoring site each year.
2019-2021 Preliminary Design Values
The design values presented below are based on data collected during the 2021 ozone season (April 1 through October 15 for most counties, and March 1 through October 31 for Kenosha County). These data are preliminary and subject to change until the data are quality assured and certified.
Previous Design Values
These design values are final and based on quality-assured and certified data.
The U.S. Environmental Protection Agency (EPA) publishes annual design value reports at EPA Air Quality Design Values
- Preliminary 2018-2020 Wisconsin Ozone Design Values Map
- Preliminary 2020 Ozone Data to Date
- Wisconsin 8-Hour Ozone Design Values: 2017-2019 Map
- Wisconsin 8-Hour Ozone Design Values: 2016-2018 Map
- Wisconsin 8-Hour Ozone Design Values: 2015-2017 Map
- Wisconsin 8-Hour Ozone Design Values: 2014-2016 Map
- Wisconsin 8-Hour Ozone Design Values: 2013-2015 Map
2015 Ozone Standard
On October 1, 2015, EPA finalized the National Ambient Air Quality Standard (NAAQS) for ozone at 70 ppb. The final rule and related technical documents are available on EPA's 2015 National Ambient Air Quality Standards (NAAQS) for Ozone.
On December 18, 2015, DNR requested that EPA reconsider the final rule.
On June 10, 2020, EPA finalized redesignation of the Door County 2015 ozone nonattainment area to attainment/maintenance status. EPA found that the Door County ozone monitor was attaining the 2015 ozone NAAQS based on 2017-2019 data and that DNR’s redesignation request met all statutory requirements for redesignation. EPA also approved the state’s maintenance plan for the area. The rule and supporting materials can be found on EPA’s docket for the rule.
- Redesignation Request and Maintenance Plan for the Door County 2015 8-Hour Ozone Nonattainment Area (01/27/2020)
On May 24, 2021, in response to a court decision, EPA finalized revised designations for the 2015 ozone standard for Kenosha, Racine, Milwaukee, Washington, Waukesha, Ozaukee, Sheboygan, Manitowoc and Door counties. These designations will be effective 30 days after publication of the rule. For more information, see EPA's Revised Designations for the 2015 Ozone Standards webpage.
On May 1, 2018, EPA notified Wisconsin of its final designations for the 2015 ozone standard for the remaining areas of the state. These designations will be effective August 3, 2018. For more information, see EPA's Designations for the 2015 Ozone Standards.
- Fact Sheet: EPA's Final Designations for the 2015 Ozone National Ambient Air Quality Standard (AM-559)
- Maps of EPA's final 2015 ozone NAAQS area designations (5/1/18)
On February 28, 2018, DNR submitted comments and additional information in response to EPA's intended area designations.
- Response to EPA's Intended Area Designations for the 2015 Ozone National Ambient Air Quality Standards (NAAQS) (2/28/18)
In a letter dated December 20, 2017, EPA informed Wisconsin of its intended area designations for the 2015 ozone standard for the areas of the state that were not addressed under the first round of designations.
- EPA's Letter to Wisconsin Describing Intended Area Designations (Round 2) (12/20/2017)
- Maps of EPA's intended area designations (12/20/2017)
The EPA finalized an initial round of area designations for the 2015 ozone standard on November 11, 2017. In this action, EPA designated 56 Wisconsin counties "attainment/unclassifiable" because air quality in these areas meets the standard.
On April 20, 2017, DNR submitted additional information to EPA in support of the governor's recommendation dated September 21, 2016.
- Supplemental Information for 2015 Ozone National Ambient Air Quality Standard (NAAQS) Area Designations (4/20/2017)
On September 21, 2016, Governor Walker submitted a recommendation to EPA that all counties in Wisconsin be designated as attainment of the 2015 ozone NAAQS.
- Designation of 2015 Ozone National Ambient Air Quality Standards (NAAQS) Nonattainment Areas in Wisconsin (9/21/16)
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.
2008 Ozone Standard (75 ppb 8-hour)
In 2008, EPA finalized the 8-hour ozone standard at 75 ppb. For this standard, Sheboygan County and a portion of Kenosha County (essentially, the area of the county located east of I-94) were designated nonattainment. Sheboygan County has since been redesignated to attainment (see below).
On July 10, 2020, EPA finalized redesignation of the Inland Sheboygan County 2008 ozone nonattainment area to attainment/maintenance status. EPA found that the Sheboygan Haven monitor was attaining the 2008 ozone NAAQS based on 2017-2019 data and that DNR’s redesignation request met all statutory requirements for redesignation. EPA also approved the state’s maintenance plan for this area. The rule and supporting materials can be found on EPA’s docket for the rule.
- Redesignation Request and Maintenance Plan for the Inland Sheboygan County, Wisconsin 1997 and 2008 8-Hour Ozone Nonattainment Area (10/09/19)
On July 10, 2020, EPA finalized redesignation of the Shoreline Sheboygan County 2008 ozone nonattainment area to attainment/maintenance status. EPA found that the Kohler Andrae monitor was attaining the 2008 ozone NAAQS based on 2017-2019 data and that DNR’s redesignation request met all statutory requirements for redesignation. EPA also approved the state’s maintenance plan for this area. The rule and supporting materials can be found on EPA’s docket for the rule.
- Redesignation Request and Maintenance Plan for the Shoreline Sheboygan County, Wisconsin 1997 and 2008 8-Hour Ozone Nonattainment Area (02/11/20)
Reclassification to "serious" for the Kenosha County nonattainment area and one-year extension of attainment date for Sheboygan County nonattainment area
On August 23, 2019, EPA published a final rule reclassifying the partial Kenosha County nonattainment area from moderate to serious, due to the area not attaining the air quality standard by the attainment date of July 20, 2018. This reclassification does not mean ozone pollution is getting worse in Kenosha County, only that ozone levels did not meet the standard by the deadline.
EPA gave the Sheboygan County nonattainment area a one-year extension of the attainment date, to July 20, 2019; this is permitted under the Clean Air Act because the fourth highest ozone value in 2017, at the Kohler-Andrae monitor, did not exceed the standard. The rule and supporting materials can be found on EPA's 2008 Ozone National Ambient Air Quality Standards (NAAQS) Nonattainment Actions webpage
Revision of the Sheboygan County nonattainment area
On June 27, 2013, DNR submitted a request to EPA to reconsider the ozone nonattainment area boundary for Sheboygan County for the 2008 standard and set a boundary that restricts the nonattainment area to part of the county. The department later supplemented the request with additional supporting data.
On July 15, 2019, EPA finalized approval of this request by revising the designation for the Sheboygan County nonattainment area for the 1997 and 2008 ozone standards. This rule split the existing area into two distinct nonattainment areas (“Inland Sheboygan County” and “Shoreline Sheboygan County”). EPA also proposed to make a “clean data determination” for the Inland Sheboygan County area for the 2008 ozone NAAQS. This determination indicates the area has attained the standard based on 2015-2017 monitoring data. The rule and supporting materials can be found on EPA’s docket for the rule.
- Initial Reconsideration Request – Sheboygan County Ozone Nonattainment Area (6/27/13)
- DNR Response to EPA Data Requests – Correction of the Sheboygan County Ozone Nonattainment Area Boundary (1/22/14)
- Supplemental Information for the Reconsideration Request for Sheboygan County 2008 Ozone Nonattainment Area Boundary (9/25/17)
The department submitted State Implementation Plan revisions to EPA to fulfill moderate nonattainment area requirements for the 2008 standard for both the Sheboygan County and partial Kenosha County nonattainment areas. EPA has approved most elements of the State Implementation Plan submittal for the Kenosha County nonattainment area. This approval and supporting materials can be found on EPA’s docket for the rule.
- Attainment Plan for the Wisconsin Portion of the Chicago-Naperville 2008 8-hour Ozone Moderate Nonattainment Area (4/17/17)
- Attainment Plan for the Sheboygan County, Wisconsin 2008 8-Hour Ozone Nonattainment Area (9/25/17)
- Attainment Plan for the Wisconsin Portion of the Chicago-Naperville 2008 8-hour Ozone Serious Nonattainment Area (12/1/20)
1997 Ozone Standard (80 ppb eight-hour, revoked in 2015)
- 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.
- Feb. 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 eight-hour ozone standard. The former nonattainment area includes the counties of Kenosha, Milwaukee, Ozaukee, Racine, Washington and Waukesha counties.
1979 Ozone Standard (120 ppb one-hour, revoked in 2005)
- Nov. 6, 1991 – EPA designated Door, Kenosha, Kewaunee, Manitowoc, Milwaukee, Ozaukee, Racine, Sheboygan, Walworth, Washington and Waukesha counties as nonattainment.
- Aug. 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.