Bad Air Days in San Mateo County, California: 4 a Year
4 bad air days a year
About the middle of monitored US counties. Ranked 308 of 996 monitored counties, where 1 is worst.
What makes them bad: mostly particles: smoke and dust (84% particle days).
When the bad days come in San Mateo
- J
- F
- M
- A
- M
- J
- J
- A
- S
- O
- N
- D
Most bad days here fall in November and September and October. Averaged over 2016 to 2025.
Is a purifier worth it here?
- Particles (wildfire smoke, dust) cause enough bad days here to matter. A HEPA purifier sized to your bedroom removes them; run it with windows shut on bad days.
- Upgrade the furnace or AC filter to MERV 13 so the whole house is filtered, not just one room.
The worst days on file
In 2000 to 2009 this county averaged 4 bad days a year; in 2016 to 2025, 4. Not much has changed. The worst recent year was 2018, with 13 bad days on 365 monitored days.
| Date | AQI | Main pollutant |
|---|---|---|
| 2020-09-11 | 199 | PM2.5 |
| 2018-11-15 | 197 | PM2.5 |
| 2018-11-16 | 195 | PM2.5 |
| 2020-09-14 | 193 | PM2.5 |
| 2020-09-13 | 183 | PM2.5 |
The air on a normal day
Across 3,239 monitored days from 2016 to 2024, the air in San Mateo County rated Good on 75% of them, close to the 72% typical of monitored counties, and Moderate on 24%. The median day sat at AQI 38; one day in ten went above 57.
The pollutant setting the AQI on an ordinary day here:
- fine particles (PM2.5)56%1,813 days
- ozone40%1,285 days
- nitrogen dioxide4%126 days
Year by year
| Year | Days monitored | Good | Moderate | Over 100 | Over 150 | Median AQI | Highest AQI |
|---|---|---|---|---|---|---|---|
| 2024 | 319 | 281 | 38 | 0 | 0 | 27 | 73 |
| 2023 | 363 | 355 | 8 | 0 | 0 | 31 | 71 |
| 2022 | 365 | 267 | 98 | 0 | 0 | 36 | 85 |
| 2021 | 365 | 290 | 75 | 0 | 0 | 36 | 90 |
| 2020 | 366 | 257 | 99 | 10 | 5 | 37 | 199 |
| 2019 | 365 | 276 | 87 | 2 | 0 | 39 | 122 |
| 2018 | 365 | 236 | 116 | 13 | 10 | 42 | 197 |
| 2017 | 365 | 238 | 120 | 7 | 3 | 42 | 155 |
| 2016 | 366 | 214 | 152 | 0 | 0 | 47 | 70 |
| 2015 | 365 | 297 | 67 | 1 | 0 | 35 | 101 |
| 2014 | 365 | 278 | 87 | 0 | 0 | 38 | 99 |
| 2013 | 365 | 193 | 168 | 4 | 0 | 49 | 115 |
| 2012 | 366 | 233 | 133 | 0 | 0 | 42 | 98 |
| 2011 | 365 | 236 | 128 | 1 | 0 | 42 | 111 |
| 2010 | 362 | 250 | 110 | 2 | 0 | 42 | 122 |
Elsewhere in California
- San Bernardino County138/yr
- Riverside County132/yr
- Kern County109/yr
- Tulare County107/yr
- Los Angeles County106/yr
- Fresno County81/yr
- Kings County49/yr
- Imperial County40/yr
How this is counted
A bad air day is a day the county's daily AQI from EPA AirData went above 100, the level where EPA tells children, older adults and people with asthma or heart disease to cut back outdoors. Monitors do not run every day, so yearly counts are scaled to 365 days. This is the record, not today's air: for that, check AirNow. More on the method in how the county numbers are made, on the AQI 100 line in what counts as a bad air day.