Median AQI
53
Moderate
Indiana · County air quality · 2025
Marion County, Indiana
Median AQI 53 (Moderate). 43.3% of monitored days rated Good, direct from EPA Air Quality System stations.
- 53
- Median AQI
- 43.3%
- Good-air days
- 2
- Unhealthy days
- 4%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Marion County's air is moderate on balance, among the most polluted counties nationally, a median AQI of 53 with 43.3% of 305 monitored days in the Good band, driven mainly by PM2.5.
- 53
- median AQI · Moderate
- 43.3%
- days rated Good
- PM2.5
- main pollutant
- 4%
- cleaner than US counties
2 days reached Unhealthy or worse; the worst single reading hit AQI 170.
Data updated July 2026
- Good · 132 d · 43.3%
- Moderate · 169 d · 55.4%
- USG · 2 d · 0.7%
- Unhealthy · 2 d · 0.7%
How did Marion County's air break down in 2025?
Across 305 monitored days, 132 fell in the Good band (AQI 0–50) and 2 reached Unhealthy or worse. The worst single day peaked at AQI 170 (Unhealthy).
EPA AQS →Air quality data from EPA Air Quality System (2025). 305 days monitored. Median AQI: 53 (Moderate).
Insights: Marion County, Indiana. EPA monitors recorded a median Air Quality Index of 53 across 305 measured days in 2025, earning a Moderate classification and a Poor health grade. Of those days, 132 (43%) fell in the Good band (AQI 0–50), 169 (55%) registered Moderate readings, and 2 (1%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 69, while the worst single-day peak hit 170 (Unhealthy). Residents averaged about 13.0 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (240 days), Ozone (63 days), PM10 (1 days), CO (1 days). Across the 2020–2025 trend window, good days shifted by +22 and the median AQI moved -2 points, placing the multi-year trajectory at improving. Residents and visitors experienced 2 unhealthy-or-worse days combined, periods when the EPA recommends sensitive groups limit prolonged outdoor exertion. Nearby counties show comparable readings: Allen (58.2% good, AQI 45); Bartholomew (69.2% good, AQI 44); Boone (86.9% good, AQI 38); Brown (94.5% good, AQI 37); Carroll (87.8% good, AQI 37); Clark (59.7% good, AQI 46).
Air Quality Summary
Good Days
43.3%
132 of 305 monitored days
-31.7pp vs 75% target
Max AQI Recorded
170
Unhealthy
Health Grade
Poor
90th-pct AQI 69
Marion County, Good days share 43.3%
National avg
Share of monitored days at AQI 0-50 (Good band).
Marion County, Unhealthy-or-worse share 0.7%
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 305 monitored days across EPA AQI categories.
Good
132
43%
Moderate
169
55%
USG
2
1%
Unhealthy
2
1%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Marion County.
- PM2.5 · 240 days · 78.7%
- Ozone · 63 days · 20.7%
- PM10 · 1 days · 0.3%
- CO · 1 days · 0.3%
Health Impact Assessment
Poor
Marion County's air quality is rated Poor based on 43.3% good air days. There were 2 unhealthy days in 2025 when outdoor activity should be limited.
Air quality is acceptable most days, but 2 days reached "Unhealthy for Sensitive Groups" (AQI 101-150) and peak AQI reached 170. Sensitive groups should monitor real-time conditions at AirNow.gov.
The dominant pollutant is PM2.5, driving AQI on 240 of 305 monitored days.
5-Year Air Quality Trend
Air quality is improving in Marion County. Best year: 2024 (134 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 110 | 250 | 6 | 0 | 55 | 144 |
| 2021 | 81 | 278 | 6 | 0 | 58 | 114 |
| 2022 | 101 | 251 | 12 | 1 | 56 | 154 |
| 2023 | 84 | 260 | 17 | 3 | 60 | 259 |
| 2024 | 134 | 228 | 4 | 0 | 54 | 138 |
| 2025 | 132 | 169 | 2 | 2 | 53 | 170 |
How does Marion County compare?
How Marion County compares
Good-day share worse than US avg
Marion County 43.3%
Allen County, Indiana 58.2%
US average 76.5%
Median AQI worse than US avg
Marion County 53
Allen County, Indiana 45
US average 38
Unhealthy days / yr worse than US avg
Marion County 2
Allen County, Indiana 0
US average 0
Other Counties in Indiana
Air quality comparison for nearby counties in Indiana.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Marion County.
Bexar County, Texas
AQI 53 · 39.8% good
Cook County, Illinois
AQI 53 · 40.5% good
Cuyahoga County, Ohio
AQI 53 · 43.6% good
Dallas County, Texas
AQI 53 · 41.0% good
Jefferson County, Alabama
AQI 53 · 36.8% good
Kings County, California
AQI 53 · 39.8% good
What Marion County's air means for you
On most days the air here is fine, but PM2.5 drives 2 unhealthy-or-worse days a year that sensitive groups should plan around.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-PM2.5 days, the guide explains what a median of 53 means for exposure. Health guidance
- See how Marion County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Marion County with its Indiana neighbours before drawing a conclusion about the region. All of Indiana
Based on 305 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 Marion County, Indiana? ▼
Marion County has a median Air Quality Index (AQI) of 53, rated "Moderate." 43.3% of monitored days (132 out of 305) had good air quality in 2025.
What is the main pollutant in Marion County? ▼
The primary pollutant in Marion County is Fine Particles (PM2.5), which was the main AQI driver on 240 days (79% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Marion County have? ▼
In 2025, Marion County recorded 2 unhealthy air days (2 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 170.
Is the air quality in Marion County getting better or worse? ▼
Air quality in Marion County is improving based on 5-year trends (2020-2025). Best year: 2024 with 134 good days. Worst year: 2021 with 81 good days.
What does AQI 53 mean for health? ▼
Marion County's median AQI of 53 falls in the "Moderate" category. See the <a href="/guides/understanding-aqi" class="underline">AQI category guide</a> for what each range means for health.
How does Marion County compare to other counties in Indiana? ▼
Marion County has 43.3% good air days. Nearby counties in Indiana: Allen (58.2% good), Bartholomew (69.2% good), Boone (86.9% good), Brown (94.5% good), Carroll (87.8% good), Clark (59.7% good).
What is the 90th percentile AQI in Marion County? ▼
The 90th percentile AQI for Marion County is 69, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 170 (Unhealthy).
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.