San Mateo, month by month

San Mateo, California has a measured daily mean on 211 days since 19 December 2025. Its worst month was June 2026, at 16 US AQI (2.8 µg/m³), 2,449th of 2,497 cities on that month's world ranking. Its cleanest was July 2026, at 5 US AQI (0.9 µg/m³).

Every month since 2025

202619 so far202 days measured

Jan17 daysFeb12 daysMar21 daysApr17 daysMay20 daysJun16#2,449 Jul5#2,783 Aug9#2,893 Sep8 Oct29so far

2025too few days for a mean (a year needs 256)9 days measured

JanFebMarAprMayJunJulAugSepOctNovDec9 days

Each square is a month's mean PM2.5 in US AQI from San Mateo's measured days, with its place on that month's world ranking under it. A month has a mean once 70% of its days are measured, the rule the monthly rankings use; under that, the square gives the days measured. Each square opens that month's ranking with San Mateo marked; each year, the year's. A dashed square is the running month, so far.

Good0–50

Moderate51–100

Sensitive groups101–150

Unhealthy151–200

Very unhealthy201–300

Hazardous301+

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

6 months

Month	Why
May 2026	20 of 31 days measured; a month needs 22. The others: 11 with under 18 hours of readings or readings that failed our checks.
Apr 2026	17 of 30 days measured; a month needs 21. The others: 13 with under 18 hours of readings or readings that failed our checks.
Mar 2026	21 of 31 days measured; a month needs 22. The others: 10 with under 18 hours of readings or readings that failed our checks.
Feb 2026	12 of 28 days measured; a month needs 20. The others: 16 with under 18 hours of readings or readings that failed our checks.
Jan 2026	17 of 31 days measured; a month needs 22. The others: 14 with under 18 hours of readings or readings that failed our checks.
Dec 2025	9 of 31 days measured; a month needs 22. The others: 4 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.

San Mateo's worst months

Month	US AQI	Days measured	World rank
June 2026	162.8 µg/m³	22 of 30	2,449thof 2,497
August 2026	91.6 µg/m³	28 of 31	2,893rdof 2,896
September 2026	81.4 µg/m³	30 of 30	–
July 2026	50.9 µg/m³	24 of 31	2,783rdof 2,784

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.

San Mateo's worst days

Day	US AQI
18 Jan 2026	9331.6 µg/m³
14 Jan 2026	9029.8 µg/m³
15 Jan 2026	8326.4 µg/m³
21 Jan 2026	8125.2 µg/m³
22 Jan 2026	7723.0 µg/m³
19 Jan 2026	7522.2 µg/m³
13 Jan 2026	7421.7 µg/m³
20 Jan 2026	7119.6 µg/m³
17 Jan 2026	7019.4 µg/m³
12 Jan 2026	6416.2 µg/m³

Daily means, the highest since 19 Dec 2025; 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, San Mateo, California, United States, 19 Dec 2025 – 11 Oct 2026. Monitor data from national networks via OpenAQ and the publishers listed at smogrank.com/sources. smogrank.com/us/san-mateo/history (accessed 12 Oct 2026).

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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.

San Mateo now: the last 24 hours, its ranks today and the last 12 months day by day.

Updated 2026-10-12 22:07 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).