Keep one room clean on smoky days
Shut the windows and run an air purifier sized for the room.
Choosing a purifier →Air quality in San Mateo County, CA
Includes San Mateo, Daly City, Redwood City and South San Francisco · EPA monitor readings, 2021 to 2025 · 764,000 people
For today's air and tomorrow's forecast, check AirNow by ZIP code.
Bad-air days a year
<1
day with an AQI over 100
No bad-air days from 2021 to 2025. That's down from about 4 a year in the early 2000s. San Mateo County is tied with 1 other California county for the fewest bad-air days of the 52 with monitors.
0days unhealthy for everyoneAQI over 150, a year
32AQI on a typical dayGood: fine for everyone
Sep to Novworst monthsSmoke and particles in summer and fall
199worst day since 2016Sep 11, 2020, fine particles
How to read the AQIThe EPA's air score runs from 0 to 500. Higher is worse. Over 100 counts as a bad-air day.
Typical day 32 (Good)Worst day 199 (Unhealthy)
Good0 to 50Moderate51 to 100Sensitive groups101 to 150Unhealthy151 to 200Very unhealthy201 to 300Hazardous301 to 500
When bad air happens in San Mateo County
Average bad-air days in each month, from 9 years of daily readings since 2016. Most fall in September, October and November.
Jan0
Feb0
Mar0
Apr0
May0
Jun0.2
Jul0
Aug0.3
Sep1.1
Oct0.6
Nov1.3
Dec0
Fine particles (smoke, soot)Ozone (smog)Bars stack in that order, from the bottom.
See the numbers by month
| Month | Ozone | Fine particles | All |
|---|---|---|---|
| January | 0 | 0 | 0 |
| February | 0 | 0 | 0 |
| March | 0 | 0 | 0 |
| April | 0 | 0 | 0 |
| May | 0 | 0 | 0 |
| June | 0.2 | 0 | 0.2 |
| July | 0 | 0 | 0 |
| August | 0 | 0.3 | 0.3 |
| September | 0.3 | 0.8 | 1.1 |
| October | 0 | 0.6 | 0.6 |
| November | 0 | 1.3 | 1.3 |
| December | 0 | 0 | 0 |
The worst day since 2016 was September 11, 2020: AQI 199, from fine particles.
What causes it
The pollutant that pushed the AQI over 100 on each of the 32 bad-air days since 2016.
16%Ozone: smog that forms in sunlight on hot days
84%Fine particles: smoke and soot
The rare bad days here are mostly fine-particle days in summer and fall, the months when wildfire smoke drives particle pollution.
CDC estimates by month, 2001 to 2022
The CDC's model counts bad-air days at the county's worst spot, from monitors and satellites. Particles (brown) fill the inside of the ring and ozone (blue) the outside.
See the numbers by month
| Month | Fine particles | Ozone |
|---|---|---|
| January | 1.0 | 0 |
| February | 0.2 | 0 |
| March | 0 | 0 |
| April | 0 | 0 |
| May | 0 | 0 |
| June | 0.2 | 0.1 |
| July | 0.1 | 0 |
| August | 0.4 | 0.1 |
| September | 0.4 | 0.2 |
| October | 0.2 | 0 |
| November | 1.1 | 0 |
| December | 1.0 | 0 |
What helps in San Mateo County
Most bad days here are smoke and other fine particles. Filtering the air at home helps most.
Upgrade the furnace filter
Use MERV 13 if your system can take it, and run the fan on smoky days.
Furnace filters →Wear an N95 outside on smoky days
A well-fitted N95 helps. Cloth and surgical masks do little against smoke.
Wildfire smoke guide →For today's reading and tomorrow's forecast, check AirNow by ZIP code. Its fire and smoke map adds smoke plumes.
Purifiers with a verified smoke rating
For a clean room on smoky days. Each has a smoke CADR tested by AHAM's independent program and is certified by California as a mechanical cleaner, so it makes no ozone.
Levoit Vital 100S-P
143 smoke CADR
- Everyday
- up to 222 sq ft
- Heavy smoke
- up to 143 sq ft
- Filters
- about $40 a year
The cheapest unit we found with a verified smoke rating. Enough for a bedroom clean room.
$119.99 at LevoitLevoit Vital 200S-P
250 smoke CADR
- Everyday
- up to 388 sq ft
- Heavy smoke
- up to 250 sq ft
- Filters
- about $50 a year
The most verified smoke cleaning per dollar of any qualifying model: 75% more than the 100S-P for $50 more.
$169.99 at LevoitLevoit Core 600S-P
391 smoke CADR
- Everyday
- up to 606 sq ft
- Heavy smoke
- up to 391 sq ft
- Filters
- $60 to $120 a year
For a big room or a small apartment. Buy the LAP-C601S-WUS version, which CARB lists as mechanical.
$299.99 at LevoitWhy all 3 are Levoit: of the models we checked that pass both tests, these give the most verified smoke cleaning per dollar at each room size. Medify's MA-25 and MA-40 also pass but cost more for the same cleaning.
Ratings from the AHAM Verifide directory and CARB's certified list. Prices from Levoit's site, checked November 7, 2026; they change. We don't earn anything from these links. How to pick a purifier
Every year since 2000
Bad-air days each year. The striped part is days unhealthy for everyone, over AQI 150. No year from 2021 to 2025 had a bad-air day.
Unhealthy for sensitive groups (101 to 150)Unhealthy or worse (over 150)
See the numbers by year
| Year | Bad-air days | Over 150 |
|---|---|---|
| 2000 | 5 | 0 |
| 2001 | 5 | 1 |
| 2002 | 3 | 0 |
| 2003 | 3 | 0 |
| 2004 | 6 | 0 |
| 2005 | 7 | 0 |
| 2006 | 2 | 1 |
| 2007 | 5 | 0 |
| 2008 | 1 | 0 |
| 2009 | 0 | 0 |
| 2010 | 2 | 0 |
| 2011 | 1 | 0 |
| 2012 | 0 | 0 |
| 2013 | 4 | 0 |
| 2014 | 0 | 0 |
| 2015 | 1 | 0 |
| 2016 | 0 | 0 |
| 2017 | 7 | 3 |
| 2018 | 13 | 10 |
| 2019 | 2 | 0 |
| 2020 | 10 | 5 |
| 2021 | 0 | 0 |
| 2022 | 0 | 0 |
| 2023 | 0 | 0 |
| 2024 | 0 | 0 |
San Mateo County and its neighbors
Bad-air days a year, 2021 to 2025. California counties with monitors averaged about 53 bad-air days a year, weighted by population.
Questions people ask about air in San Mateo County
How often is the air bad in San Mateo County?
From 2021 to 2025, monitors in San Mateo County recorded almost no bad-air days a year with an AQI above 100, which is unhealthy for sensitive groups or worse. No year from 2021 to 2025 had a bad-air day.
What causes bad air in San Mateo County?
The rare bad days here are mostly fine-particle days in summer and fall, the months when wildfire smoke drives particle pollution. Bad days cluster in September, October and November.
Where can I see today's air quality in San Mateo County?
AirNow, run by the EPA and its partners, shows the current AQI and tomorrow's forecast for your ZIP code. Its fire and smoke map adds smoke plumes. On a typical day the AQI in San Mateo County is about 32, which is good, fine for everyone. Check it most in September, October and November, when bad days here cluster.
About these numbers
Day counts come from the EPA's county air quality summaries, which take the worst reading across a county's monitors each day. Monitors here reported on about 353 days a year from 2021 to 2025. A county is large, and your street can be cleaner or worse. CDC's model, which estimates every county from monitors and satellites, gives the worst spot in San Mateo County about 5 high-particle days and about 1 high-ozone day a year from 2018 to 2022.
Sources: EPA AirData county AQI summaries (2000 to 2025) and daily county AQI (2016 to 2025); CDC Environmental Public Health Tracking Network modeled PM2.5 and ozone (2001 to 2022). Updated November 8, 2026.