Air quality in Milwaukee

What is the air quality rating in Milwaukee?

Milwaukee annual air quality averages a US AQI rating of “good.” This rating indicates that Milwaukee air typically poses little to no risk to human health. Despite generally clean air, the city does suffer from a number of high pollution days and moreover exceeds the federal target for the number of permitted unhealthy ozone days.

The American Lung Association uses federal limits on air pollution to rate air quality across US counties and regions. For the 2016 to 2018 monitoring period, Milwaukee received a passing “A” rating for both annual and 24-hour PM2.5 pollution, but received a failing “F” rating for ozone.¹ Data collected in the State of the Air report reveals that Milwaukee hasn’t met ozone attainment since at least 1996. Milwaukee, however, was very close to attainment during 2013 to 2015, with a weighted average of 3.3 unhealthy ozone days. The federal standard allows no more than 3.2 unhealthy days. Since then, ozone levels have been on the rise.

Generally, Milwaukee air quality is worse in the winter months. Weather phenomena known as cool air inversions are responsible for elevated pollution levels during this season. During a cool air inversion, cold surface-level smog becomes trapped under a warmer layer of air above. While emissions may remain relatively constant, the trapping effect results in an accumulation of pollution, rising Milwaukee measured pollution levels.

December was Milwaukee’s most polluted month in both 2019 and 2018, averaging a US AQI rating of “moderate.” November, January, and February also tend to experience elevated air quality index values near or in excess of the “good” AQI threshold.

Air pollution is variable and can change on a moment’s notice. Check Milwaukee air quality ratings at the top of this page for current conditions.

How many ozone warnings occur per year in Milwaukee?

From 2016 to 2018, Milwaukee experienced a weighted average of 6 unhealthy ozone days, nearly double the federal target of 3.2 days. Milwaukee’s severe nonattainment status has resulted in an “F” rating by the American Lung Association for daily ozone.

Ozone, a key component of smog, is formed when sunlight and heat interact with pollutants in the air, such as nitrogen oxides (NOx) and volatile organic compounds (VOCs). Motor vehicle exhaust and industrial emissions are major sources of NOx and VOCs, both ozone precursor pollutants.

Milwaukee ozone problems are summer related, as winter lacks the ideal conditions for formation at dangerous levels. Highest levels often occur during the afternoon hours and around rush hour.

Heightened ozone levels can aggravate respiratory problems, especially in children, the elderly, and those with preexisting heart and lung disease. Common adverse health effects can include chest pain, coughing, shortness of breath, and throat irritation.

Studies conducted by the Lake Michigan Air Directors Consortium (LADCO) have found that weather patterns over Lake Michigan can strongly influence ozone levels in Milwaukee as well as in other locations on the lake’s shore. High pressure systems to the east create conditions including clear skies, air stagnation, and little precipitation, resulting in the accumulation of ozone over the lake in the summer. When wind blows in, Milwaukee’s ozone levels can quickly become unhealthy. According to the 2017 study, lake breezes contribute as much as 80 percent of high ozone air pollution episodes in Wisconsin. Reducing ozone precursor pollutant emissions is the best way to target and reduce unhealthy ozone.

Why is Milwaukee air hazy?

Milwaukee’s hazy air is often the result of smog, a combination of particulate and ozone air pollution. Common emission sources include vehicles, industry, and building heating.

Some periods of hazy air in Milwaukee have been the result of wildfires, sometimes blowing polluted air in from as far as Alberta, Canada.³ In 2018, the month of August experienced PM2.5 levels averaging a US AQI “moderate” rating as a result of wildfires in the northwest US and southern Canada. Polluted air was transported via a powerful jet stream thousands of miles to the Midwest.⁴ This effect is not uncommon. As the frequency and severity of wildfires is expected to increase with climate change, so too could the frequency and severity of Milwaukee’s smokey pollution haze during wildfire season.

Use the Milwaukee air pollution map to understand local variances across the city and the flow of pollution from location to location.

Is Milwaukee air pollution getting worse?

Milwaukee air quality has made drastic improvements since the city was founded in 1846. At that time, dirty coal-powered factories and steam engines billowed toxic clouds into the air. Historian John Gurda commented that this boom of economic activity hung a “permanent pall of smoke” over the city.⁵

The federal Clean Air Act of 1970 took steps towards addressing the high levels of air pollution nationally. Since the legislations passed, Milwaukee’s air quality began improving as the city transitioned from pulverized coal to cleaner coal, oil, and gas. The 1990 amendments to the Clean Air Act strengthened efforts for more stringent and dynamic regulations. At the time of the 1990 amendments, 11 Wisconsin counties were designated as “nonattainment areas,” requiring additional measures to meet federal targets. These included requiring emissions permits, limits, and the installation of emission control measures, gasoline reformulation, among others.

Milwaukee’s significant and consistent reductions in fine particle pollution indicate these regulations have been successful. It is true that overall, Milwaukee’s air quality has made huge strides. In the last three years since 2017, however, Milwaukee has experienced year over year increases in both PM2.5 and ozone. While these pollution gains have been subtle, it is concerning trend.

Shifting to cleaner transport represents one of the greatest opportunities for further reducing Milwaukee air quality index levels. More efficient, low emission vehicles can make a significant reduction in the city’s emissions. In much of the US, vehicular emissions represent as much as 50 to 60 percent of total emissions.

Is Milwaukee’s air quality worse than Madison?

Milwaukee tends to experience better air quality than Madison, Wisconsin. 80 miles of separation, local emission sources, and geographic location appear to play a role in the cities differing air quality statuses.

In 2019, Madison air pollution levels were 23.5 percent higher than those of Milwaukee. That year, Madison experienced three months of “moderate” AQI levels, while Milwaukee only experienced one.

Milwaukee’s cleaner air status may come as a surprise since the city has roughly two times the population of Madison. Its location on Lake Michigan, however, may be advantageous for the city’s particle pollution levels, as the lake represents a relatively large emission-free area.

+ Article Resources

[1] American Lung Association. (2019). State of the air – 2019.
[2] NASA. (2017). Lake Michigan ozone study - 2017.
[3] Barak R. (2019, May 31). Smoke from large wildfires in Canada causes haze in Wisconsin.
[4] WKOW. (2018, August 12). Wildfire smoke causes hazy conditions and “moderate” air quality for southern Wisconsin.
[5] Encyclopedia of Milwaukee. (2020). Pollution.