Median AQI
42
Good
Indiana · County air quality · 2025
Spencer County, Indiana
Median AQI 42 (Good). 64.8% of monitored days rated Good, direct from EPA Air Quality System stations.
- 42
- Median AQI
- 64.8%
- Good-air days
- 0
- Unhealthy days
- 23%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Spencer County's air is good on balance, among the more polluted 10% of US counties, a median AQI of 42 with 64.8% of 273 monitored days in the Good band, driven mainly by PM2.5.
- 42
- median AQI · Good
- 64.8%
- days rated Good
- PM2.5
- main pollutant
- 23%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 88.
Data updated July 2026
- Good · 177 d · 64.8%
- Moderate · 96 d · 35.2%
How did Spencer County's air break down in 2025?
Across 273 monitored days, 177 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 88 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 273 days monitored. Median AQI: 42 (Good).
Insights: Spencer County, Indiana. EPA monitors recorded a median Air Quality Index of 42 across 273 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 177 (65%) fell in the Good band (AQI 0–50), 96 (35%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 59, while the worst single-day peak hit 88 (Moderate). Residents averaged about 19.5 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (273 days). Across the 2020–2025 trend window, good days shifted by +97 and the median AQI moved +0 points, placing the multi-year trajectory at stable. The worst single reading over the 2020–2025 window was AQI 88, moderate but never Unhealthy. 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
64.8%
177 of 273 monitored days
-10.2pp vs 75% target
Max AQI Recorded
88
Moderate
Health Grade
Moderate
90th-pct AQI 59
Spencer County, Good days share 64.8%
National avg
Share of monitored days at AQI 0-50 (Good band).
Spencer 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 273 monitored days across EPA AQI categories.
Good
177
65%
Moderate
96
35%
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 Spencer County.
- PM2.5 · 273 days · 100.0%
Health Impact Assessment
Moderate
Spencer County's air quality is rated Moderate based on 64.8% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 64.8% 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 PM2.5, driving AQI on 273 of 273 monitored days.
5-Year Air Quality Trend
Air quality is stable in Spencer County. Best year: 2025 (177 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 80 | 42 | 0 | 0 | 42 | 67 |
| 2021 | 78 | 42 | 0 | 0 | 42 | 89 |
| 2022 | 75 | 36 | 0 | 0 | 42 | 79 |
| 2023 | 80 | 39 | 0 | 1 | 43 | 170 |
| 2024 | 132 | 37 | 0 | 0 | 34 | 68 |
| 2025 | 177 | 96 | 0 | 0 | 42 | 88 |
How does Spencer County compare?
How Spencer County compares
Good-day share worse than US avg
Spencer County 64.8%
Allen County, Indiana 58.2%
US average 76.5%
Median AQI worse than US avg
Spencer County 42
Allen County, Indiana 45
US average 38
Unhealthy days / yr about average
Spencer County 0
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 Spencer 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 Spencer County's air means for you
The air here stays in the breathable range year-round, with PM2.5 the main thing to watch.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-PM2.5 days, the guide explains what a median of 42 means for exposure. Health guidance
- See how Spencer County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Spencer County with its Indiana neighbours before drawing a conclusion about the region. All of Indiana
Based on 273 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 Spencer County, Indiana? ▼
Spencer County has a median Air Quality Index (AQI) of 42, rated "Good." 64.8% of monitored days (177 out of 273) had good air quality in 2025.
What is the main pollutant in Spencer County? ▼
The primary pollutant in Spencer County is Fine Particles (PM2.5), which was the main AQI driver on 273 days (100% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Spencer County have? ▼
In 2025, Spencer County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 88.
Is the air quality in Spencer County getting better or worse? ▼
Air quality in Spencer County is stable based on 5-year trends (2020-2025). Best year: 2025 with 177 good days. Worst year: 2022 with 75 good days.
What does AQI 42 mean for health? ▼
Spencer 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 Spencer County compare to other counties in Indiana? ▼
Spencer County has 64.8% 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 Spencer County? ▼
The 90th percentile AQI for Spencer County is 59, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 88 (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.