SmogRank / United States / Illinois / Decatur
Decatur, month by month
Decatur has a measured daily mean on 1,302 days since 11 January 2023. Its worst month was June 2023, at 87 US AQI (28.4 µg/m³), 118th of 1,603 cities on that month's world ranking. Its cleanest was April 2024, at 28 US AQI (5.1 µg/m³). 2025 averaged 46 US AQI (8.3 µg/m³), against 42 US AQI (7.6 µg/m³) in 2024.
Every month since 2023
The US EPA's Air Quality Index for PM2.5 (2024 breakpoints). Show values as: µg/m³ · US AQI · India AQI · China AQI · European AQI · UK DAQI · Canada AQHI+. About the scales
Months without a mean, and why
3 months
| Month | Why |
|---|---|
| Apr 2023 | 3 of 30 days measured; a month needs 21. The others: 27 with under 18 hours of readings or readings that failed our checks. |
| Mar 2023 | 20 of 31 days measured; a month needs 22. The others: 11 with under 18 hours of readings or readings that failed our checks. |
| Jan 2023 | 7 of 31 days measured; a month needs 22. The others: 14 with under 18 hours of readings or readings that failed our checks. |
A day counts once 18 hours of it are measured; a month needs 70% of its days.
Decatur's worst months
| Month | US AQI | World rank | Episode |
|---|---|---|---|
| June 2023 | 8728.4 µg/m³ | 118thof 1,603 | Eastern United States and Canada, June and July 2023 United States and Canada, June 2023and 1 more |
| July 2026 | 6014.1 µg/m³ | 585thof 2,784 | Northern United States and Canada, July 2026 |
| July 2023 | 5813.0 µg/m³ | 284thof 1,432 | United States and Canada, July 2023 Eastern United States and Canada, June and July 2023 |
| June 2025 | 5712.3 µg/m³ | 756thof 2,508 | Northern United States and Canada, June 2025 Northern United States and Canada, May and June 2025 |
| August 2023 | 5611.6 µg/m³ | 386thof 1,980 | – |
| August 2026 | 5310.4 µg/m³ | 807thof 2,896 | – |
Monthly means over the days measured; finished months only. World rank: its place on that month's world ranking of every city with enough days. Each month opens that ranking.
Decatur's worst days
| Day | US AQI | Episode |
|---|---|---|
| 28 Jun 2023 | 289214.0 µg/m³ | Eastern United States and Canada, June and July 2023 |
| 27 Jun 2023 | 17487.7 µg/m³ | Eastern United States and Canada, June and July 2023 |
| 19 Jul 2026 | 15764.1 µg/m³ | Northern United States and Canada, July 2026 |
| 16 Jul 2023 | 15156.0 µg/m³ | United States and Canada, July 2023 |
| 1 Aug 2025 | 14252.3 µg/m³ | North-eastern United States and Canada, 29 July to 9 August 2025 |
| 15 Mar 2025 | 14151.8 µg/m³ | Eastern United States, March 2025 |
| 20 Jul 2026 | 13850.7 µg/m³ | Northern United States and Canada, July 2026 |
| 29 Jun 2023 | 10436.9 µg/m³ | Eastern United States and Canada, June and July 2023 |
| 4 Jun 2023 | 9532.8 µg/m³ | Eastern United States and Canada, May and June 2023 |
| 6 Jun 2025 | 9331.6 µg/m³ | Northern United States and Canada, May and June 2025 |
Daily means, the highest since 11 Jan 2023; the WHO's daily guideline is 15 µg/m³. Each date opens that day's world ranking, and each episode the bad-air spell it was part of.
Use this history
SmogRank, PM2.5 monthly and daily means, Decatur, United States, 11 Jan 2023 – 11 Oct 2026. Monitor data from AirNow, via OpenAQ and from the publisher directly; sources listed at smogrank.com/sources. smogrank.com/us/decatur/history (accessed 12 Oct 2026).
You're welcome to reuse a single chart, image or figure with this citation and a link. Terms. Raw monitor readings in bulk are published by OpenAQ, under each provider's licence.
Decatur now: the last 24 hours, its ranks today and the last 12 months day by day.
Updated 2026-10-12 20:07 UTC. Values are PM2.5 (fine particles), the median of each city's monitors, as the US AQI PM2.5 sub-index. Some low-cost readings: Sensor.Community, ODbL (data and licence). Some readings: Uzhydromet (Uzbekistan's Hydrometeorological Service); Department of Environment, Nepal; Vietnam's government monitors; Aurassure; Umhverfisstofnun (Environment Agency of Iceland); RMCAB (Secretaría Distrital de Ambiente, Bogotá); Love My Air (City and County of Denver), no reuse terms published (why we use them).