Milwaukee-Waukesha-West Allis, WI Air Quality
EPA monitor data through 2025 · Updated August 13, 2026 · Methods
In 2025, Milwaukee-Waukesha-West Allis, WI recorded 18 days above an Air Quality Index of 100, the level at which EPA begins warning sensitive groups, out of 365 days with monitoring data. Most of those bad days were driven by fine particles. Under the AQI rules that applied before 2013, the same air would have scored 11 days. Since 2000, unhealthy days are 10% higher on a consistent definition.
Unhealthy days by year, scored two ways
EPA changed what an AQI of 100 means in 2013 and again in 2015, so a count published before those changes is not comparable to one published after. Each panel below applies a single fixed rulebook to every year, so each is internally consistent. Why this matters.
Scored by today's rules
Scored by the pre-2013 rules
Ozone since 1990
The index counts above only start in 2000, because nationwide fine-particle monitoring did not exist before 1999 and an index without it would measure the monitoring network rather than the air. Ozone does have a 35-year record. This is Milwaukee-Waukesha-West Allis, WI's, from EPA's long-running trend monitors: 96 ppb in 1990 to 73 ppb in 2024, 24% lower.
Peak ozone, 1990 to 2024
What 2025 looked like
| Air quality category | Days | Share of monitored days |
|---|---|---|
| Good | 219 | 60% |
| Moderate | 128 | 35% |
| Unhealthy for sensitive groups | 15 | 4% |
| Unhealthy or worse | 3 | 1% |
Median AQI 45 · 90th-percentile AQI 78 · worst day 175. Scored under current rules.
What drove the unhealthy days
Of the 18 days above 100 in 2025, 8 were driven by ozone and 10 by fine particles. The two behave differently. Ozone peaks on hot sunny afternoons; fine particles now spike mainly with wildfire smoke. In the eastern United States, ozone has stopped responding to emission cuts: nitrogen oxide emissions kept falling after 2014 and ambient ozone barely moved.
The unhealthy days in 2025
Every day the index passed 100, what it reached, and which pollutant drove it. 7 of these 18 would not have counted as unhealthy under the pre-2013 rules.
| Date | Peak AQI | Driven by | Pre-2013 score |
|---|---|---|---|
| Jun 4 | 140 | Fine particles | 122 |
| Jun 5 | 126 | Fine particles | 111 |
| Jun 11 | 115 | Ozone | 100 (not unhealthy) |
| Jun 16 | 115 | Ozone | 100 (not unhealthy) |
| Jun 29 | 129 | Ozone | 109 |
| Jul 13 | 134 | Fine particles | 118 |
| Jul 15 | 126 | Ozone | 106 |
| Jul 30 | 125 | Fine particles | 111 |
| Jul 31 | 175 | Fine particles | 165 |
| Aug 1 | 112 | Fine particles | 99 (not unhealthy) |
| Aug 2 | 152 | Fine particles | 133 |
| Aug 3 | 160 | Fine particles | 152 |
| Aug 4 | 106 | Fine particles | 94 (not unhealthy) |
| Aug 5 | 135 | Fine particles | 118 |
| Aug 7 | 126 | Ozone | 106 |
| Aug 15 | 105 | Ozone | 90 (not unhealthy) |
| Oct 3 | 112 | Ozone | 97 (not unhealthy) |
| Oct 4 | 105 | Ozone | 90 (not unhealthy) |
Statewide context
Milwaukee-Waukesha-West Allis, WI sits in Wisconsin.
The full series
| Year | Unhealthy days, current rules | Pre-2013 rules | Days monitored |
|---|---|---|---|
| 2000 | 17 | 10 | 366 |
| 2001 | 38 | 28 | 365 |
| 2002 | 37 | 24 | 365 |
| 2003 | 29 | 19 | 365 |
| 2004 | 16 | 7 | 366 |
| 2005 | 40 | 26 | 365 |
| 2006 | 12 | 6 | 365 |
| 2007 | 22 | 14 | 365 |
| 2008 | 6 | 4 | 366 |
| 2009 | 10 | 5 | 365 |
| 2010 | 14 | 8 | 365 |
| 2011 | 11 | 5 | 365 |
| 2012 | 32 | 24 | 366 |
| 2013 | 4 | 0 | 365 |
| 2014 | 9 | 3 | 365 |
| 2015 | 5 | 2 | 365 |
| 2016 | 9 | 7 | 366 |
| 2017 | 8 | 3 | 365 |
| 2018 | 9 | 4 | 365 |
| 2019 | 3 | 0 | 365 |
| 2020 | 7 | 4 | 366 |
| 2021 | 12 | 2 | 365 |
| 2022 | 6 | 4 | 365 |
| 2023 | 28 | 17 | 365 |
| 2024 | 11 | 4 | 366 |
| 2025 | 18 | 11 | 365 |
Sources
- EPA AirData daily monitor files, recomputed under both breakpoint tables. Methods and validation.
- Download the full dataset for every state and metro.