SmogRank / United States / California / Folsom
Folsom, month by month
Folsom has a measured daily mean on 1,284 days since 4 January 2023. Its worst month was January 2025, at 52 US AQI (9.9 µg/m³), 1,012th of 2,474 cities on that month's world ranking. Its cleanest was April 2026, at 17 US AQI (3.0 µg/m³). 2025 averaged 37 US AQI (6.7 µg/m³), against 31 US AQI (5.6 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
2 months
| Month | Why |
|---|---|
| Nov 2023 | No day measured. The others: 30 with under 18 hours of readings or readings that failed our checks. |
| Oct 2023 | 15 of 31 days measured; a month needs 22. The others: 16 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.
Folsom's worst months
| Month | US AQI | World rank | Episode |
|---|---|---|---|
| January 2025 | 529.9 µg/m³ | 1,012thof 2,474 | – |
| September 2023 | 529.4 µg/m³ | 708thof 2,176 | California and Oregon, United States, September 2023 |
| December 2024 | 498.8 µg/m³ | 1,130thof 2,616 | – |
| January 2026 | 488.6 µg/m³ | 1,337thof 2,617 | – |
| December 2023 | 478.4 µg/m³ | 1,141stof 2,247 | – |
| June 2025 | 468.3 µg/m³ | 1,760thof 2,508 | – |
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.
Folsom's worst days
| Day | US AQI | Episode |
|---|---|---|
| 20 Sept 2023 | 9331.8 µg/m³ | California and Oregon, United States, September 2023 |
| 22 Sept 2023 | 8527.3 µg/m³ | California and Oregon, United States, September 2023 |
| 21 Sept 2023 | 8225.9 µg/m³ | California and Oregon, United States, September 2023 |
| 23 Sept 2023 | 7421.4 µg/m³ | California and Oregon, United States, September 2023 |
| 23 Jan 2026 | 7320.8 µg/m³ | – |
| 1 Dec 2024 | 7220.5 µg/m³ | – |
| 6 Dec 2025 | 6918.9 µg/m³ | – |
| 18 Jan 2025 | 6818.2 µg/m³ | – |
| 10 Dec 2024 | 6818.0 µg/m³ | – |
| 14 Jan 2026 | 6717.7 µg/m³ | – |
Daily means, the highest since 4 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, Folsom, United States, 4 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/folsom/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.
Folsom 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).