SmogRank / United States / Colorado / Thornton
Thornton, month by month
Thornton has a measured daily mean on 1,309 days since 1 January 2023. Its worst month was May 2023, at 59 US AQI (13.4 µg/m³), 192nd of 1,600 cities on that month's world ranking. Its cleanest was May 2024, at 22 US AQI (4.0 µg/m³). 2025 averaged 41 US AQI (7.4 µg/m³), against 40 US AQI (7.2 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
2 months
| Month | Why |
|---|---|
| Dec 2023 | 18 of 31 days measured; a month needs 22. The others: 13 with under 18 hours of readings or readings that failed our checks. |
| Jun 2023 | 15 of 30 days measured; a month needs 21. The others: 15 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.
Thornton's worst months
| Month | US AQI | World rank | Episode |
|---|---|---|---|
| May 2023 | 5913.4 µg/m³ | 192ndof 1,600 | North-western United States and Canada, May 2023 |
| January 2023 | 5913.3 µg/m³ | 298thof 1,261 | – |
| August 2026 | 5511.5 µg/m³ | 622ndof 2,896 | North-western United States and Canada, July and August 2026 |
| October 2024 | 5511.3 µg/m³ | 433rdof 2,020 | – |
| July 2024 | 5511.1 µg/m³ | 550thof 2,566 | North-western United States and Canada, July 2024 |
| July 2026 | 5310.0 µg/m³ | 1,357thof 2,784 | North-western United States and Canada, July and August 2026 |
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.
Thornton's worst days
| Day | US AQI | Episode |
|---|---|---|
| 19 May 2023 | 17184.3 µg/m³ | North-western United States and Canada, May 2023 |
| 20 May 2023 | 15561.3 µg/m³ | North-western United States and Canada, May 2023 |
| 23 May 2023 | 10235.8 µg/m³ | North-western United States and Canada, May 2023 |
| 23 Jul 2024 | 9432.0 µg/m³ | North-western United States and Canada, July 2024 |
| 4 Aug 2026 | 9331.8 µg/m³ | North-western United States and Canada, July and August 2026 |
| 24 Jul 2024 | 9331.7 µg/m³ | North-western United States and Canada, July 2024 |
| 4 Sept 2025 | 9230.9 µg/m³ | Colorado and Wyoming, United States, September 2025 |
| 13 Jan 2023 | 8929.6 µg/m³ | – |
| 22 May 2023 | 8828.7 µg/m³ | North-western United States and Canada, May 2023 |
| 7 Aug 2026 | 8628.0 µg/m³ | – |
Daily means, the highest since 1 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, Thornton, United States, 1 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/thornton/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.
Thornton now: the last 24 hours, its ranks today and the last 12 months day by day.
Updated 2026-10-12 19:07 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).