Bad Air Days in Chester County, Pennsylvania: 3 a Year
3 bad air days a year
About the middle of monitored US counties. Ranked 346 of 996 monitored counties, where 1 is worst.
What makes them bad: mostly ozone: summer smog (34% particle days).
When the bad days come in Chester
- J
- F
- M
- A
- M
- J
- J
- A
- S
- O
- N
- D
Most bad days here fall in June and July and May and September. Averaged over 2016 to 2025.
What actually helps here
- Bad air days are rare here. A standard furnace filter is enough; no purifier needed for outdoor air.
- On the odd bad day, check AirNow and keep windows shut.
The record for Chester
In 2000 to 2009 this county averaged 41 bad days a year; in 2016 to 2025, 3. The air has improved. The worst recent year was 2016, with 9 bad days on 365 monitored days.
| Date | AQI | Main pollutant |
|---|---|---|
| 2023-06-07 | 248 | PM2.5 |
| 2023-06-08 | 216 | PM2.5 |
| 2023-06-29 | 176 | PM2.5 |
| 2023-06-30 | 158 | PM2.5 |
| 2025-06-14 | 157 | PM2.5 |
The air on a normal day
Across 3,605 monitored days from 2016 to 2025, the air in Chester County rated Good on 60% of them, less often than the 72% typical of monitored counties, and Moderate on 39%. The median day sat at AQI 46; one day in ten went above 64.
The pollutant setting the AQI on an ordinary day here:
- fine particles (PM2.5)60%2,170 days
- ozone40%1,435 days
Year by year
| Year | Days monitored | Good | Moderate | Over 100 | Over 150 | Median AQI | Highest AQI |
|---|---|---|---|---|---|---|---|
| 2025* | 363 | 259 | 102 | 2 | 1 | 43 | 157 |
| 2024 | 363 | 264 | 96 | 3 | 0 | 41 | 143 |
| 2023 | 361 | 220 | 134 | 7 | 4 | 45 | 248 |
| 2022 | 364 | 249 | 115 | 0 | 0 | 45 | 80 |
| 2021 | 347 | 215 | 130 | 2 | 0 | 45 | 138 |
| 2020 | 355 | 233 | 121 | 1 | 0 | 44 | 143 |
| 2019 | 364 | 223 | 139 | 2 | 0 | 46 | 115 |
| 2018 | 359 | 220 | 138 | 1 | 0 | 44 | 112 |
| 2017 | 364 | 137 | 222 | 5 | 0 | 54 | 136 |
| 2016 | 365 | 158 | 198 | 9 | 0 | 52 | 143 |
| 2015 | 365 | 133 | 230 | 2 | 1 | 55 | 166 |
| 2014 | 363 | 154 | 205 | 4 | 0 | 52 | 129 |
| 2013 | 365 | 154 | 208 | 3 | 0 | 53 | 113 |
| 2012 | 355 | 170 | 166 | 19 | 3 | 51 | 151 |
| 2011 | 345 | 127 | 207 | 11 | 2 | 56 | 161 |
| 2010 | 351 | 117 | 211 | 23 | 1 | 57 | 151 |
*2025 may be a partial year in EPA's file.
Pennsylvania compared
- Allegheny County12/yr
- Philadelphia County9/yr
- Bucks County8/yr
- Warren Countythin data
- Lancaster County6/yr
- Berks County5/yr
- Delaware County4/yr
- Northampton County4/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.