Pennsylvania · County air quality · 2025
Chester County, Pennsylvania
Median AQI 43 (Good). 71.3% of monitored days rated Good, direct from EPA Air Quality System stations.
- 43
- Median AQI
- 71.3%
- Good-air days
- 1
- Unhealthy days
- 35%
- Cleaner than US counties
Chester County's 2025 annual median AQI is 43 (Good), with 71.3% Good days
According to the U.S. EPA Air Quality System, Chester County, Pennsylvania posted a 2025 median AQI of 43 (Good) across 363 monitored days, with 71.3% rated Good and 1 unhealthy or worse. This is an annual comparison, not today's advisory.
Data source EPA Air Quality System (AQS)
- Cleaner than US
- Chester County's 71.3% Good-day share places it cleaner than 35% of US counties in 2025.
- Closest median-AQI peer
- Ashtabula County (81.2% Good, median AQI 43) is the closest nationwide median-AQI peer to Chester County.
- 2020–2025 trend
- Chester County's floor-qualifying Good-day share is improving from 2020 to 2025 (65.6% → 71.3%).
- Median AQI
- Median AQI: 43
According to the 2025 EPA AQS county release on PlainAirData, Chester County, Pennsylvania ranks #631 of 951 by median AQI (43) and #617 by good-day share (71.3%). Ponce County, Puerto Rico is #1 of 951 by median AQI (3, 91.4% Good days) at good-day-share #159 ≠ largest of 7 at 100.0% Good days #1 by good-day share (100.0%, median AQI 9) at median-AQI #7.
Annual release change
What changed in the 2025 EPA annual release
- Chester County's peak single-day AQI rose from 143 to 157 between the 2024 and 2025 EPA annual releases, a change of +14 points.
- Chester County's Good-day share fell from 72.7% to 71.3% between the 2024 and 2025 EPA annual releases, a change of -1.4 percentage points.
- Chester County's Unhealthy-category days rose from 0 to 1 between the 2024 and 2025 EPA annual releases, a change of +1 days.
Data updated August 2026
- Good · 259 d · 71.3%
- Moderate · 102 d · 28.1%
- USG · 1 d · 0.3%
- Unhealthy · 1 d · 0.3%
How did Chester County's air break down in 2025?
Across 363 monitored days, 259 fell in the Good band (AQI 0–50) and 1 reached Unhealthy or worse. The worst single day peaked at AQI 157 (Unhealthy).
EPA AQS →Share of monitored days at AQI 0-50 (Good band).
Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).
Where Chester County sits nationally
COUNT-derived placement among 951 US counties with at least 100 monitored days in 2025. Chester County's Good-day share is 71.3% - cleaner than 35% of that comparison set.
Chester County Good-day share vs. every US county
Share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release
71.3% 35th percentile higher than 35% of 951 US counties
Each bar is a band; taller bars hold more US counties. The dashed line + filled bar mark this entry. Hover or tap any bar for its full count, share, and where it sits relative to this entry.
Source U.S. Environmental Protection Agency, Air Quality System (AQS) · 2025
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Chester County.
- Ozone · 227 days · 62.5%
- PM2.5 · 136 days · 37.5%
2024 daily-monitor rhythm in Chester County
Month-by-month median AQI from EPA day-level county monitors (363 reported days in 2024). Jul posted the highest typical readings (median AQI 49, peak 143); Nov was cleanest at median 35. Summer months (Jun–Aug) averaged median AQI 47 vs 36 in Dec–Feb.
Five-year seasonal record
EPA day-level county files · 2020–20241,790 daily readings show a recurring local pattern: Jul has the highest matched-month median AQI (51), while Oct is lowest (39). Each cell is that month’s median daily AQI, not a current-air forecast.
Lower AQI is better. The colors summarize EPA AQI bands; gaps mean the county had no day-level record for that month.
Health Impact Assessment
Chester County's air quality is rated Moderate based on 71.3% good air days. There were 1 unhealthy days in 2025 when outdoor activity should be limited.
Air quality is acceptable most days, but 1 days reached "Unhealthy for Sensitive Groups" (AQI 101-150) and peak AQI reached 157. Sensitive groups should monitor real-time conditions at AirNow.gov.
The dominant pollutant is Ozone, driving AQI on 227 of 363 monitored days.
5-Year Air Quality Trend
Air quality is improving in Chester County. Highest Good-day share: 2024 (72.7%, 264 of 363 monitored days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 233 | 121 | 1 | 0 | 44 | 143 |
| 2021 | 215 | 130 | 2 | 0 | 45 | 138 |
| 2022 | 249 | 115 | 0 | 0 | 45 | 80 |
| 2023 | 220 | 134 | 3 | 2 | 45 | 248 |
| 2024 | 264 | 96 | 3 | 0 | 41 | 143 |
| 2025 | 259 | 102 | 1 | 1 | 43 | 157 |
How does Chester County compare?
How Chester County compares
Data Sources
Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by County dataset, 2020–2025 (aqs.epa.gov). For current conditions (not shown here — this page is historical annual data), see EPA AirNow (airnow.gov). See our methodology for how AQI, median, and coverage figures are computed.
Informational only, not professional advice. Data from EPA AQS. See our disclaimer. Spotted a figure that looks wrong? Report a correction.
Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files for Chester County, Pennsylvania · 2020–2025
Download this county's annual AQI row as CSV , with the same EPA fields and denominator shown on this page.
Every figure on PlainAirData is rendered directly from EPA AQS air-quality monitoring data, no number is typed in by an editor. This page draws directly on EPA AQS air-quality monitoring data, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.