Move workouts to the morning
Ozone peaks on hot, sunny afternoons. Mornings are usually cleaner.
Ozone guide →Air quality in Stanislaus County, CA
Includes Modesto and Turlock · EPA monitor readings, 2021 to 2025 · 553,000 people
For today's air and tomorrow's forecast, check AirNow by ZIP code.
Bad-air days a year
27
days with an AQI over 100
About 1 day in 14. That's down from about 58 a year in the early 2000s. Stanislaus County has the 12th most bad-air days of the 52 California counties with monitors.
1day unhealthy for everyoneAQI over 150, a year
55AQI on a typical dayModerate: fine for most people
Jul, Aug and Decworst monthsOzone and fine particles
265worst day since 2016Nov 16, 2018, 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 55 (Moderate)Worst day 265 (Very unhealthy)
Good0 to 50Moderate51 to 100Sensitive groups101 to 150Unhealthy151 to 200Very unhealthy201 to 300Hazardous301 to 500
When bad air happens in Stanislaus County
Average bad-air days in each month, from 10 years of daily readings since 2016. Most fall in July, August and December.
Jan2.1
Feb0.9
Mar0
Apr0.2
May0.8
Jun3.1
Jul7.1
Aug6.6
Sep4.6
Oct1.7
Nov2.8
Dec4.8
Fine particles (smoke, soot)Dust (coarse particles)Ozone (smog)Bars stack in that order, from the bottom.
See the numbers by month
| Month | Ozone | Fine particles | Dust | All |
|---|---|---|---|---|
| January | 0 | 2.1 | 0 | 2.1 |
| February | 0 | 0.9 | 0 | 0.9 |
| March | 0 | 0 | 0 | 0 |
| April | 0.2 | 0 | 0 | 0.2 |
| May | 0.7 | 0 | 0.1 | 0.8 |
| June | 3.1 | 0 | 0 | 3.1 |
| July | 7.0 | 0.1 | 0 | 7.1 |
| August | 5.1 | 1.5 | 0 | 6.6 |
| September | 3.6 | 0.9 | 0.1 | 4.6 |
| October | 0.8 | 0.8 | 0.1 | 1.7 |
| November | 0 | 2.8 | 0 | 2.8 |
| December | 0 | 4.8 | 0 | 4.8 |
The worst day since 2016 was November 16, 2018: AQI 265, from fine particles.
What causes it
The pollutant that pushed the AQI over 100 on each of the 347 bad-air days since 2016.
59%Ozone: smog that forms in sunlight on hot days
40%Fine particles: smoke and soot
1%Dust: coarse particles blown up by wind
Bad days here come from both ozone on hot afternoons and fine particles such as smoke.
Winter brings its own bad days: about 11 a year from November to February are fine-particle days, when cold, still air traps smoke and exhaust near the ground (an inversion).
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 | 3.9 | 0 |
| February | 1.3 | 0 |
| March | 0 | 0 |
| April | 0 | 0.3 |
| May | 0 | 2.7 |
| June | 0.2 | 5.5 |
| July | 0.3 | 9.4 |
| August | 1.3 | 8.3 |
| September | 1.0 | 6.0 |
| October | 0.5 | 0.8 |
| November | 3.0 | 0 |
| December | 4.4 | 0 |
What helps in Stanislaus County
Bad days here come from ozone and fine particles, so it takes timing and filtering.
Keep one room clean on smoky days
Shut the windows and run an air purifier sized for the room.
Choosing a purifier →Upgrade the furnace filter
Use MERV 13 if your system can take it, and run the fan on smoky days.
Furnace filters →For today's reading and tomorrow's forecast, check AirNow by ZIP code. Its fire and smoke map adds smoke plumes.
Parents: what schools do at each AQI level, and how to plan for smoke and ozone days.
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. The worst recent year was 2021, with 47 bad-air days.
Unhealthy for sensitive groups (101 to 150)Unhealthy or worse (over 150)
See the numbers by year
| Year | Bad-air days | Over 150 |
|---|---|---|
| 2000 | 59 | 17 |
| 2001 | 47 | 16 |
| 2002 | 81 | 34 |
| 2003 | 69 | 16 |
| 2004 | 35 | 4 |
| 2005 | 40 | 8 |
| 2006 | 46 | 14 |
| 2007 | 52 | 15 |
| 2008 | 82 | 19 |
| 2009 | 67 | 12 |
| 2010 | 38 | 6 |
| 2011 | 71 | 14 |
| 2012 | 79 | 12 |
| 2013 | 65 | 11 |
| 2014 | 59 | 6 |
| 2015 | 48 | 5 |
| 2016 | 43 | 3 |
| 2017 | 63 | 8 |
| 2018 | 47 | 15 |
| 2019 | 23 | 1 |
| 2020 | 37 | 16 |
| 2021 | 47 | 5 |
| 2022 | 23 | 1 |
| 2023 | 23 | 0 |
| 2024 | 38 | 0 |
| 2025 | 3 | 0 |
Stanislaus 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.
Stanislaus County27
Tuolumne County2
Questions people ask about air in Stanislaus County
How often is the air bad in Stanislaus County?
From 2021 to 2025, monitors in Stanislaus County recorded about 27 bad-air days a year with an AQI above 100, which is unhealthy for sensitive groups or worse. The worst year in that stretch was 2021, with 47 bad-air days.
What causes bad air in Stanislaus County?
Bad days here come from both ozone on hot afternoons and fine particles such as smoke. Winter brings its own bad days: about 11 a year from November to February are fine-particle days, when cold, still air traps smoke and exhaust near the ground (an inversion). Bad days cluster in July, August and December.
Where can I see today's air quality in Stanislaus 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 Stanislaus County is about 55, which is moderate, fine for most people. Check it most in July, August and December, 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 365 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 Stanislaus County about 18 high-particle days and about 15 high-ozone days 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.