On Jan. 16, 2025, the Environmental Protection Agency (EPA) reclassified several counties in southeastern Wisconsin from moderate to serious nonattainment for the 2015 ozone National Ambient Air Quality Standards (NAAQS).1
Ozone pollution is an ongoing challenge for the Greater Milwaukee area that affects the environment, the region’s economy, and the public health of approximately one-third of Wisconsin’s population that calls the area home.
Michael Moran, Michigan State 2020, is a staff attorney with the Wisconsin Department of Natural Resources in Milwaukee, where he focuses on environmental and administrative law.
What is Ozone?
Ground-level ozone is a component of smog that is associated with adverse health effects and vegetation damage. Ozone found near the Earth’s surface is distinguishable from the beneficial stratospheric ozone layer that shields the Earth from the Sun’s harmful radiation.
Unlike other types of pollution, ground-level ozone is not typically emitted directly from a source. Rather, ozone is formed when its precursor compounds – nitrogen oxide (NOX) and volatile organic compounds (VOCs) – photochemically react with sunlight. Ozone pollution levels vary throughout the year, but they are usually the highest during the summer.
Although ozone precursors can originate from natural sources, most NOX and VOC emissions come from combustion sources like motor vehicles, power plants, and industrial boilers. Once ozone is formed, pollution can be carried vast distances away from where it first originated through the wind.
The Clean Air Act and the NAAQS
One of the ways the Clean Air Act regulates air quality in the United States is through the National Ambient Air Quality Standards (NAAQS). The EPA establishes NAAQS for common, widespread pollutants, known as “criteria pollutants,” at a level necessary “to protect the public health.”2
Ozone is one of these criteria pollutants. Under the Act, multiple statutory clocks begin ticking once EPA promulgates a NAAQS for a particular pollutant. The law sets deadlines for areas to achieve the NAAQS by as well as time limits for states to formulate state implementation plans (SIPs) for how they intend to achieve the standard within their borders.
The NAAQS are not intended to be set in stone once they are established. The EPA is required to periodically review and revise the standards, if necessary, based on the most recent scientific evidence regarding a pollutant’s health and welfare impacts.3 The EPA last revised the ozone NAAQS in 2015 when it lowered the standard from 75 to 70 parts per billion (ppb).
The EPA reviews ambient air concentration data from all over the country to determine which areas are meeting the federal standard.4 This review is typically done on a county-by-county basis. If data demonstrates that the ambient concentrations of a pollutant fall under the NAAQS, then EPA will consider that area to be in “attainment.”
However, if ambient concentrations in an area are exceeding the NAAQS, then the EPA will designate it as in “nonattainment.”
The Clean Air Act further breaks down ozone nonattainment into five sub-classifications of nonattainment ranging from “marginal” to “extreme.” The law imposes different requirements and deadlines to reach attainment depending on that classification. Areas classified at lower nonattainment levels face fewer and less stringent requirements than those at the higher levels.
Additionally, the law treats the regulatory requirements for ozone nonattainment levels as additive. That means, for example, that if an area in serious nonattainment does not achieve the NAAQS in time after imposing the statutory requirements for serious nonattainment, then it must comply with the requirements for both the serious
and severe nonattainment (the next highest level).
Generally, areas that fail to meet the NAAQS by the attainment deadline are “reclassified by operation of law” (or “bumped up”) to the next highest nonattainment level.5
Southeastern Wisconsin 'Bumps Up'
Multiple counties in southeastern Wisconsin – including all of Milwaukee and Ozaukee counties and portions of Washington, Waukesha, Racine, Sheboygan, and Kenosha counties (see the map on the Wisconsin DNR website) – have all persistently failed to meet the ozone NAAQS since it was last revised in 2015. As a result, the EPA has gradually reclassified these areas upward from marginal to moderate nonattainment.
In turn, Wisconsin has responded by implementing greater and greater regulatory controls in these areas, such as increased emission offset requirements and a vehicle inspection and maintenance program.
By law, these areas had until Aug. 3, 2024, to demonstrate attainment with the NAAQS before they would be reclassified to serious. Based on air monitoring data captured from the past three years, EPA determined that these areas again failed to meet the ozone NAAQS. Consequently, EPA reclassified these areas to serious nonattainment.
Nonattainment Consequences
The consequences of the bump up are real and immediate.
The Clean Air Act sets out the full list of requirements for serious ozone nonattainment areas under 42 U.S.C. § 7511a(c). Some of the regulatory changes that result from reclassification from moderate to serious nonattainment include:
A lower major source threshold for NOX/VOC emissions in air permitting from 100 to 50 tons.
A higher offset ratio for new major sources of NOX/VOC emissions, or major modifications to existing sources, from 1.15 to 1 to 1.2 to 1.
Strictly defined reasonable forward progress (RFP) requirements for states to demonstrate reductions in VOC emissions.
Southeastern Wisconsin faces variety of challenges, some geographic and meteorological and some man-made, that contribute to southeastern Wisconsin’s chronic ozone nonattainment.6 A major contributor is ozone transported from areas outside of the state. These interstate emissions present significant hurdles to Wisconsin’s ozone attainment as Wisconsin’s regulatory authority only reaches to sources within its borders.
The Clean Air Act contains a provision requiring states to prohibit emissions that “contribute significantly” to nonattainment of a NAAQS in a downwind state.7 However, the EPA’s efforts to put this provision into practice, like the
Good Neighbor Plan, have not resolved the issue of interstate ozone transport in Wisconsin so far.
Along with interstate ozone emissions, Wisconsin also has limited authority to regulate the emissions coming from motor vehicles, as the Clean Air Act leaves regulation of mobile sources almost entirely to the federal government.
Where to Find Out More
Visit
the Wisconsin DNR website and FAQsto learn more about the bump up and permitting requirements for sources in nonattainment areas in Wisconsin.
This article was originally published on the State Bar of Wisconsin’s
Environmental Law Section Blog. Visit the State Bar
sections or the
Environmental Law Section webpages to learn more about the benefits of section membership.
Endnotes
1 “Findings of Failure to Attain and Reclassification of Areas in Illinois, Indiana, Michigan, Ohio, and Wisconsin as Serious for the 2015 Ozone National Ambient Air Quality Standards,” 89 Fed. Reg. 101,901 (December 17, 2024) (EPA Docket No. EPA–R05–OAR–2024–0546).
2 42 USC §§7409(a)(1), (b)(1).
3Id. at § 7409(d).
4 EPA makes this determination based on an area’s Design Value (DV). A DV is a statistic used to compare data collected at an ambient air quality monitoring site to the NAAQS to determine compliance. The EPA provides how it calculates the DV for the 2015 ozone NAAQS in appendix U to 40 CFR part 50.
5Id. § 7513(b)(2)(A).
6See Charles O. Stainer, et al., “Overview of the Lake Michigan Ozone Study 2017,” 102
Bulletin of the American Meteorological Society 12 (2021);
See also Patricia A. Cleary, et al., “Impacts of Lake Breeze Meteorology on Ozone Gradient Observations along Lake Michigan Shorelines in Wisconsin,”
Atmospheric Environment, Vol. 269 (2022).
7 42 U.S.C. §7410(a)(2)(D)(i)(I).
