Bad Air Days in Shasta County, California: 11 a Year
11 bad air days a year
Among the worst tenth of monitored US counties. Ranked 79 of 996 monitored counties, where 1 is worst.
What makes them bad: a mix of particles and ozone (49% particle days).
The bad-air season here
- J
- F
- M
- A
- M
- J
- J
- A
- S
- O
- N
- D
Most bad days here fall in August and July and September. Averaged over 2016 to 2025.
What to buy, and what not to
- 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.
- Ozone drives many bad days here. Purifiers and filters do not remove ozone, so do not buy one for that. Ozone peaks on hot afternoons: exercise outside in the morning.
The worst days on file
In 2000 to 2009 this county averaged 23 bad days a year; in 2016 to 2025, 11. The air has improved. The worst recent year was 2021, with 32 bad days on 365 monitored days.
| Date | AQI | Main pollutant |
|---|---|---|
| 2023-11-02 | 1435 | PM2.5 |
| 2021-08-16 | 240 | PM2.5 |
| 2024-07-26 | 232 | PM2.5 |
| 2021-08-07 | 226 | PM2.5 |
| 2021-08-15 | 226 | PM2.5 |
The air on a normal day
Across 3,652 monitored days from 2016 to 2025, the air in Shasta County rated Good on 74% of them, close to the 72% typical of monitored counties, and Moderate on 24%. The median day sat at AQI 43; one day in ten went above 72.
The pollutant setting the AQI on an ordinary day here:
- ozone86%3,138 days
- fine particles (PM2.5)14%511 days
Shasta County by year
| Year | Days monitored | Good | Moderate | Over 100 | Over 150 | Median AQI | Highest AQI |
|---|---|---|---|---|---|---|---|
| 2025* | 365 | 276 | 88 | 1 | 1 | 44 | 154 |
| 2024 | 366 | 268 | 94 | 4 | 1 | 44 | 232 |
| 2023 | 365 | 247 | 112 | 6 | 1 | 45 | 1435 |
| 2022 | 365 | 253 | 106 | 6 | 0 | 45 | 119 |
| 2021 | 365 | 222 | 111 | 32 | 17 | 46 | 240 |
| 2020 | 366 | 286 | 72 | 8 | 2 | 41 | 171 |
| 2019 | 364 | 306 | 58 | 0 | 0 | 42 | 100 |
| 2018 | 365 | 272 | 72 | 21 | 6 | 43 | 206 |
| 2017 | 365 | 260 | 91 | 14 | 2 | 44 | 159 |
| 2016 | 366 | 296 | 56 | 14 | 0 | 41 | 140 |
| 2015 | 365 | 252 | 100 | 13 | 1 | 44 | 157 |
| 2014 | 365 | 272 | 87 | 6 | 0 | 43 | 126 |
| 2013 | 365 | 294 | 69 | 2 | 0 | 42 | 105 |
| 2012 | 366 | 256 | 104 | 6 | 1 | 44 | 180 |
| 2011 | 365 | 263 | 96 | 6 | 1 | 44 | 153 |
| 2010 | 365 | 269 | 81 | 15 | 0 | 43 | 119 |
*2025 may be a partial year in EPA's file.
Nearby counties on file
- 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, and on why a purifier does not touch ozone in smoke vs ozone.