Mount Vernon, IL Air Quality
EPA monitor data through 2025 · Updated August 13, 2026 · Methods
In 2025, Mount Vernon, IL recorded 0 days above an Air Quality Index of 100, the level at which EPA begins warning sensitive groups, out of 361 days with monitoring data. Most of those bad days were driven by ozone and fine particles equally. Under the AQI rules that applied before 2013, the same air would have scored 0 days. Since 2007, unhealthy days are 100% lower 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
What 2025 looked like
| Air quality category | Days | Share of monitored days |
|---|---|---|
| Good | 245 | 68% |
| Moderate | 116 | 32% |
| Unhealthy for sensitive groups | 0 | 0% |
| Unhealthy or worse | 0 | 0% |
Median AQI 42 · 90th-percentile AQI 61 · worst day 98. Scored under current rules.
Statewide context
Mount Vernon, IL sits in Illinois.
The full series
| Year | Unhealthy days, current rules | Pre-2013 rules | Days monitored |
|---|---|---|---|
| 2000 | 12 | 8 | 229 |
| 2001 | 6 | 1 | 210 |
| 2002 | 32 | 23 | 213 |
| 2003 | 15 | 5 | 212 |
| 2004 | 4 | 0 | 215 |
| 2005 | 11 | 5 | 218 |
| 2006 | 0 | 0 | 254 |
| 2007 | 11 | 5 | 316 |
| 2008 | 2 | 0 | 245 |
| 2009 | 2 | 0 | 248 |
| 2010 | 11 | 2 | 249 |
| 2011 | 5 | 3 | 214 |
| 2012 | 25 | 10 | 215 |
| 2013 | 0 | 0 | 212 |
| 2014 | 0 | 0 | 232 |
| 2015 | 1 | 0 | 261 |
| 2016 | 0 | 0 | 286 |
| 2017 | 0 | 0 | 365 |
| 2018 | 3 | 0 | 364 |
| 2019 | 1 | 0 | 344 |
| 2020 | 1 | 1 | 353 |
| 2021 | 2 | 0 | 364 |
| 2022 | 0 | 0 | 365 |
| 2023 | 11 | 3 | 365 |
| 2024 | 1 | 0 | 366 |
| 2025 | 0 | 0 | 361 |
Sources
- EPA AirData daily monitor files, recomputed under both breakpoint tables. Methods and validation.
- Download the full dataset for every state and metro.