LRP - Licenses

LRP - Regulations

LRP - Permits

Recreation - Everyone

Recreation - Trapping

Recreation - Fishing

Recreation - Hunting

Education - Everyone

Education - Kids

Education - Educators

To sign up for updates or to access your subscriber preferences, please enter your contact information below.

current air quality in Wisconsin.
more about current air quality advisories.
Wisconsin air quality forecasts.
Contact information
For more information about ozone, contact:
Karen Walsh


Smog over Milwaukee
Smog over the city of Milwaukee.

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. 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 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 now meets the standard, the DNR can request redesignation of the county.

Overview of ozone standards

The federal Clean Air Act requires the U.S. Environmental Protection Agency (EPA) to develop two types of air quality standards for key air pollutants:

  • 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 the U.S. EPA's Ground-level Ozone Standards Designations [exit DNR].

History of ozone standards
  • April 30, 1971: 1-hour standard for total photochemical oxidants at 80 parts per billion (ppb).
  • February 8, 1979: 1-hour standard for ozone at 120 ppb.
  • July 18, 1997: 8-hour standard for ozone at 80 ppb.
  • March 27, 2008: 8-hour standard for ozone at 75 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 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 the DNR's mobile sources page to learn more.

Interstate transport

Pollutants do not respect political boundaries. Instead, they may be transported great distances by wind 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).


Ozone compliance and nonattainment areas

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.

2008-2010 Ozone Design Values
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.

1979 Ozone Standard (120 ppb 1-hour)
  • November 6, 1991 – U.S. EPA designated Door, Kenosha, Kewaunee, Manitowoc, Milwaukee, Ozaukee, Racine, Sheboygan, Walworth, Washington, and Waukesha as nonattainment.
  • August 26, 1996 – U.S. EPA redesignated Kewaunee, Sheboygan, and Walworth Counties to maintenance.
  • April 17, 2003 – U.S. EPA redesignated Door and Manitowoc Counties to maintenance.
  • April 24, 2009 – U.S. EPA made an attainment determination for Kenosha, Milwaukee, Ozaukee, Racine, Washington, and Waukesha Counties.
1997 Ozone Standard (80 ppb 8-hour)
  • April 30, 2004 – U.S. EPA designated Door, Kenosha, Kewaunee, Manitowoc, Milwaukee, Ozaukee, Racine, Sheboygan, Washington, and Waukesha as nonattainment.
  • May 21, 2008 – U.S. EPA redesignated Kewaunee County to maintenance.
  • July 12, 2010 – U.S. EPA redesignated Door and Manitowoc Counties to maintenance.
  • February 9, 2012 – U.S. 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.
2008 Ozone Standard (75 ppb 8-hour)

In 2008, the U.S. EPA issued an additional, more stringent, 8-hour ozone standard of 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.



Air quality forecasts and monitoring data
Ozone health information
Last revised: Tuesday August 25 2015