SmogRank / United States / California / South San Francisco
South San Francisco, month by month
South San Francisco has a measured daily mean on 503 days since 15 November 2024. Its worst month was January 2026, at 57 US AQI (12.3 µg/m³), 911th of 2,617 cities on that month's world ranking. Its cleanest was July 2025, at 23 US AQI (4.1 µg/m³).
Every month since 2024
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
9 months
| Month | Why |
|---|---|
| Aug 2026 | 11 of 31 days measured; a month needs 22. The others: 20 with under 18 hours of readings or readings that failed our checks. |
| Jul 2026 | 11 of 31 days measured; a month needs 22. The others: 20 with under 18 hours of readings or readings that failed our checks. |
| Jun 2026 | 8 of 30 days measured; a month needs 21. The others: 22 with under 18 hours of readings or readings that failed our checks. |
| May 2026 | 11 of 31 days measured; a month needs 22. The others: 20 with under 18 hours of readings or readings that failed our checks. |
| Apr 2026 | 9 of 30 days measured; a month needs 21. The others: 21 with under 18 hours of readings or readings that failed our checks. |
| Jun 2025 | 13 of 30 days measured; a month needs 21. The others: 17 with under 18 hours of readings or readings that failed our checks. |
| May 2025 | 14 of 31 days measured; a month needs 22. The others: 17 with under 18 hours of readings or readings that failed our checks. |
| Jan 2025 | 21 of 31 days measured; a month needs 22. The others: 10 with under 18 hours of readings or readings that failed our checks. |
| Nov 2024 | 16 of 30 days measured; a month needs 21. |
A day counts once 18 hours of it are measured; a month needs 70% of its days.
South San Francisco's worst months
| Month | US AQI | World rank | Episode |
|---|---|---|---|
| January 2026 | 5712.3 µg/m³ | 911thof 2,617 | California, United States, 30 January to 6 February 2026 California and Oregon, United States, January 2026 |
| December 2025 | 5712.2 µg/m³ | 948thof 3,006 | – |
| December 2024 | 5611.6 µg/m³ | 710thof 2,616 | California and Oregon, United States, December 2024 |
| November 2025 | 519.2 µg/m³ | 1,202ndof 2,715 | – |
| February 2026 | 366.4 µg/m³ | 2,357thof 2,866 | California, United States, 30 January to 6 February 2026 |
| October 2025 | 335.9 µg/m³ | 1,643rdof 2,614 | – |
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.
South San Francisco's worst days
| Day | US AQI | Episode |
|---|---|---|
| 8 Dec 2024 | 9030.0 µg/m³ | California and Oregon, United States, December 2024 |
| 7 Dec 2024 | 8728.5 µg/m³ | California and Oregon, United States, December 2024 |
| 15 Jan 2026 | 8527.6 µg/m³ | California and Oregon, United States, January 2026 |
| 5 Feb 2026 | 8225.5 µg/m³ | California, United States, 30 January to 6 February 2026 |
| 5 Dec 2025 | 8024.9 µg/m³ | – |
| 11 Dec 2024 | 8024.8 µg/m³ | California and Oregon, United States, December 2024 |
| 20 Dec 2024 | 7823.8 µg/m³ | – |
| 31 Jan 2026 | 7723.2 µg/m³ | California, United States, 30 January to 6 February 2026 |
| 6 Dec 2025 | 7622.7 µg/m³ | – |
| 26 Jan 2026 | 7622.5 µg/m³ | – |
Daily means, the highest since 15 Nov 2024; 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, South San Francisco, United States, 15 Nov 2024 – 11 Oct 2026. Monitor data from Clarity via OpenAQ; sources listed at smogrank.com/sources. smogrank.com/us/south-san-francisco/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.
South San Francisco 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).