Bay City, MI Air Quality
EPA monitor data through 2025 · Updated August 13, 2026 · Methods
In 2025, Bay City, MI recorded 3 days above an Air Quality Index of 100, the level at which EPA begins warning sensitive groups, out of 358 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 3 days.
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
What 2025 looked like
| Air quality category | Days | Share of monitored days |
|---|---|---|
| Good | 252 | 70% |
| Moderate | 103 | 29% |
| Unhealthy for sensitive groups | 3 | 1% |
| Unhealthy or worse | 0 | 0% |
Median AQI 38 · 90th-percentile AQI 60 · worst day 139. Scored under current rules.
What drove the unhealthy days
Of the 3 days above 100 in 2025, 0 were driven by ozone and 3 by fine particles. The two behave differently. Ozone peaks on hot sunny afternoons; fine particles now spike mainly with wildfire smoke.
The unhealthy days in 2025
Every day the index passed 100, what it reached, and which pollutant drove it.
| Date | Peak AQI | Driven by | Pre-2013 score |
|---|---|---|---|
| Jul 31 | 135 | Fine particles | 118 |
| Aug 3 | 139 | Fine particles | 121 |
| Aug 4 | 139 | Fine particles | 121 |
Statewide context
Bay City, MI sits in Michigan.
The full series
| Year | Unhealthy days, current rules | Pre-2013 rules | Days monitored |
|---|---|---|---|
| 2000 | 0 | 0 | 39 |
| 2001 | 2 | 1 | 119 |
| 2002 | 0 | 0 | 105 |
| 2003 | 0 | 0 | 113 |
| 2004 | 0 | 0 | 120 |
| 2005 | 5 | 3 | 117 |
| 2006 | 1 | 1 | 74 |
| 2007 | 0 | 0 | 112 |
| 2008 | 0 | 0 | 114 |
| 2009 | 0 | 0 | 116 |
| 2010 | 1 | 0 | 121 |
| 2011 | 0 | 0 | 118 |
| 2012 | 0 | 0 | 119 |
| 2013 | 0 | 0 | 114 |
| 2014 | 0 | 0 | 348 |
| 2015 | 0 | 0 | 361 |
| 2016 | 0 | 0 | 355 |
| 2017 | 0 | 0 | 362 |
| 2018 | 0 | 0 | 356 |
| 2019 | 0 | 0 | 316 |
| 2020 | 0 | 0 | 365 |
| 2021 | 0 | 0 | 363 |
| 2022 | 0 | 0 | 354 |
| 2023 | 6 | 5 | 363 |
| 2024 | 0 | 0 | 363 |
| 2025 | 3 | 3 | 358 |
Sources
- EPA AirData daily monitor files, recomputed under both breakpoint tables. Methods and validation.
- Download the full dataset for every state and metro.