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Decatur, IL Air Quality

EPA monitor data through 2025 · Updated September 02, 2026 · Methods

In 2025, Decatur, IL recorded 3 days above an Air Quality Index of 100, the level at which EPA begins warning sensitive groups, out of 365 days with monitoring data. The rest of the year was 237 Good days and 125 Moderate days. Most of the bad days were driven by fine particles. Under the AQI rules that applied before 2013, the same air would have scored 2 days. Since 2000, unhealthy days are 50% 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.

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 Decatur, IL's, from EPA's long-running trend monitors: 76 ppb in 1990 to 62 ppb in 2024, 18% lower.

What 2025 looked like

Median AQI 44 · 90th-percentile AQI 64 · worst day 141. Scored under current rules, out of 365 days with an AQI.

What drove the unhealthy days

Of the 3 days above 100 in 2025, 1 were driven by ozone and 2 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. 1 of these 3 would not have counted as unhealthy under the pre-2013 rules.

Statewide context

Decatur, IL sits in Illinois.

Days in each air quality category, by year

How many days fell in each AQI category, for every year with an AQI on record. Categories are scored under EPA's current breakpoints. A year with fewer than 300 days is marked, because a short year lowers every count for reasons that have nothing to do with the air.

Good is AQI 0–50, Moderate 51–100, Unhealthy for sensitive groups 101–150. “Unhealthy or worse” combines the Unhealthy, Very Unhealthy and Hazardous categories, which are shown together because most areas record too few of each to report separately. The four columns sum to the last one by construction. These counts exclude sulphur dioxide, following EPA, which drops SO₂ from county and metro summaries because it is too localized to describe a whole area.

The full series

The same years scored two ways, with the pollutant that drove the days above 100. EPA's files record which pollutant set each day's AQI; they do not attribute days to wildfire smoke, so no column here claims to.

Sources

• EPA AirData daily monitor files, recomputed under both breakpoint tables. Methods and validation.
• Download the full dataset for every state and metro.