Median AQI
42
Good
Massachusetts · County air quality · 2025
Worcester County, Massachusetts
Median AQI 42 (Good). 75.0% of monitored days rated Good, direct from EPA Air Quality System stations.
- 42
- Median AQI
- 75.0%
- Good-air days
- 0
- Unhealthy days
- 41%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Worcester County's air is good on balance, more polluted than the median US county, a median AQI of 42 with 75.0% of 244 monitored days in the Good band, driven mainly by Ozone.
- 42
- median AQI · Good
- 75.0%
- days rated Good
- Ozone
- main pollutant
- 41%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 100.
Data updated July 2026
- Good · 183 d · 75.0%
- Moderate · 61 d · 25.0%
How did Worcester County's air break down in 2025?
Across 244 monitored days, 183 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 100 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 244 days monitored. Median AQI: 42 (Good).
Insights: Worcester County, Massachusetts. EPA monitors recorded a median Air Quality Index of 42 across 244 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 183 (75%) fell in the Good band (AQI 0–50), 61 (25%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 62, while the worst single-day peak hit 100 (Moderate). Residents averaged about 22.5 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (176 days), PM2.5 (66 days), PM10 (1 days), CO (1 days). Across the 2020–2025 trend window, good days shifted by -74 and the median AQI moved -1 points, placing the multi-year trajectory at improving. The worst single reading over the 2020–2025 window was AQI 100, moderate but never Unhealthy. Nearby counties show comparable readings: Barnstable (92.5% good, AQI 38); Berkshire (71.6% good, AQI 41); Bristol (77.8% good, AQI 41); Dukes (95.2% good, AQI 37); Essex (77.0% good, AQI 42); Franklin (72.8% good, AQI 40).
Air Quality Summary
Good Days
75.0%
183 of 244 monitored days
0.0pp vs 75% target
Max AQI Recorded
100
Moderate
Health Grade
Good
90th-pct AQI 62
Worcester County, Good days share 75.0%
National avg
Share of monitored days at AQI 0-50 (Good band).
Worcester County, Unhealthy-or-worse share 0.0%
National avg
Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).
AQI Day Breakdown (2025)
Distribution of 244 monitored days across EPA AQI categories.
Good
183
75%
Moderate
61
25%
USG
0
0%
Unhealthy
0
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Worcester County.
- Ozone · 176 days · 72.1%
- PM2.5 · 66 days · 27.0%
- PM10 · 1 days · 0.4%
- CO · 1 days · 0.4%
Health Impact Assessment
Good
Worcester County's air quality is rated Good based on 75.0% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 75.0% of monitored days in 2025, with a median AQI of 42. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 176 of 244 monitored days.
5-Year Air Quality Trend
Air quality is improving in Worcester County. Best year: 2024 (321 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 257 | 109 | 0 | 0 | 43 | 90 |
| 2021 | 218 | 144 | 3 | 0 | 45 | 125 |
| 2022 | 228 | 137 | 0 | 0 | 45 | 100 |
| 2023 | 268 | 93 | 4 | 0 | 39 | 148 |
| 2024 | 321 | 45 | 0 | 0 | 37 | 90 |
| 2025 | 183 | 61 | 0 | 0 | 42 | 100 |
How does Worcester County compare?
How Worcester County compares
Good-day share worse than US avg
Worcester County 75.0%
Barnstable County, Massachusetts 92.5%
US average 76.5%
Median AQI worse than US avg
Worcester County 42
Barnstable County, Massachusetts 38
US average 38
Unhealthy days / yr about average
Worcester County 0
Barnstable County, Massachusetts 0
US average 0
Other Counties in Massachusetts
Air quality comparison for nearby counties in Massachusetts.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Worcester County.
Adams County, Pennsylvania
AQI 42 · 73.8% good
Anderson County, Tennessee
AQI 42 · 91.0% good
Andrew County, Missouri
AQI 42 · 84.3% good
Armstrong County, Pennsylvania
AQI 42 · 68.1% good
Baldwin County, Alabama
AQI 42 · 72.2% good
Belmont County, Ohio
AQI 42 · 63.0% good
What Worcester County's air means for you
The air here stays in the breathable range year-round, with Ozone the main thing to watch.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-Ozone days, the guide explains what a median of 42 means for exposure. Health guidance
- See how Worcester County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Worcester County with its Massachusetts neighbours before drawing a conclusion about the region. All of Massachusetts
Based on 244 EPA-monitored days in 2025; figures reflect only days the monitors reported. Source: EPA Air Quality System annual summaries.
More Air Quality Data
Air Quality Guides
Frequently Asked Questions
What is the air quality in Worcester County, Massachusetts? ▼
Worcester County has a median Air Quality Index (AQI) of 42, rated "Good." 75.0% of monitored days (183 out of 244) had good air quality in 2025.
What is the main pollutant in Worcester County? ▼
The primary pollutant in Worcester County is Ozone (O3), which was the main AQI driver on 176 days (72% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Worcester County have? ▼
In 2025, Worcester County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 100.
Is the air quality in Worcester County getting better or worse? ▼
Air quality in Worcester County is improving based on 5-year trends (2020-2025). Best year: 2024 with 321 good days. Worst year: 2025 with 183 good days.
What does AQI 42 mean for health? ▼
Worcester County's median AQI of 42 falls in the "Good" category. See the <a href="/guides/understanding-aqi" class="underline">AQI category guide</a> for what each range means for health.
How does Worcester County compare to other counties in Massachusetts? ▼
Worcester County has 75.0% good air days. Nearby counties in Massachusetts: Barnstable (92.5% good), Berkshire (71.6% good), Bristol (77.8% good), Dukes (95.2% good), Essex (77.0% good), Franklin (72.8% good).
What is the 90th percentile AQI in Worcester County? ▼
The 90th percentile AQI for Worcester County is 62, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 100 (Moderate).
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.
Every figure on PlainAir 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.