Median AQI
40
Good
Ohio · County air quality · 2025
Miami County, Ohio
Median AQI 40 (Good). 84.2% of monitored days rated Good, direct from EPA Air Quality System stations.
- 40
- Median AQI
- 84.2%
- Good-air days
- 0
- Unhealthy days
- 67%
- Cleaner than US counties
Miami County's 2025 annual median AQI is 40 (Good), with 84.2% Good days
According to the U.S. EPA Air Quality System, Miami County, Ohio posted a 2025 median AQI of 40 (Good) across 240 monitored days, with 84.2% rated Good and 0 unhealthy or worse. This is an annual comparison, not today's advisory.
Data source EPA Air Quality System (AQS)
- Cleaner than US
- Miami County's 84.2% Good-day share places it cleaner than 67% of US counties in 2025.
- Closest median-AQI peer
- Anderson County (91.3% Good, median AQI 40) is the closest nationwide median-AQI peer to Miami County.
- 2020–2025 trend
- Miami County's floor-qualifying Good-day share is worsening from 2020 to 2025 (94.8% → 84.2%).
- Median AQI
- Median AQI: 40
The verdict
According to EPA Air Quality System monitoring data, Miami County's air is good on balance, cleaner than the median US county, a median AQI of 40 with 84.2% of 240 monitored days in the Good band, driven mainly by Ozone.
- 40
- median AQI · Good
- 84.2%
- days rated Good
- Ozone
- main pollutant
- 67%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 101.
Annual release change
What changed in the 2025 EPA annual release
- Miami County's Good-day share rose from 80.8% to 84.2% between the 2024 and 2025 EPA annual releases, a change of +3.4 percentage points.
- Miami County's median AQI fell from 43 to 40 between the 2024 and 2025 EPA annual releases, a change of -3 points.
Data updated August 2026
- Good · 202 d · 84.2%
- Moderate · 37 d · 15.4%
- USG · 1 d · 0.4%
How did Miami County's air break down in 2025?
Across 240 monitored days, 202 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 101 (USG).
EPA AQS →Air quality data from EPA Air Quality System (2025). 240 days monitored. Median AQI: 40 (Good).
Insights: Miami County, Ohio. EPA monitors recorded a median Air Quality Index of 40 across 240 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 202 (84%) fell in the Good band (AQI 0–50), 37 (15%) registered Moderate readings, and 1 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 61, while the worst single-day peak hit 101 (USG). Residents averaged about 25.3 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (240 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by -10.6 points and the median AQI moved +1 points, placing the multi-year trajectory at worsening. The worst single reading over the 2020–2025 window was AQI 101, usg but never Unhealthy. Nearby counties show comparable readings: Allen (67.0% good, AQI 42); Ashtabula (81.2% good, AQI 43); Athens (83.4% good, AQI 31); Belmont (63.0% good, AQI 42); Butler (33.1% good, AQI 55); Clark (61.4% good, AQI 45).
Air Quality Summary
Good Days
84.2%
202 of 240 monitored days
9.2pp vs 75% target
Max AQI Recorded
101
USG
Health Grade
Good
90th-pct AQI 61
Miami County, Good days share 84.2%
National avg
Share of monitored days at AQI 0-50 (Good band).
Miami 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 240 monitored days across EPA AQI categories.
Good
202
84%
Moderate
37
15%
USG
1
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 Miami County.
- Ozone · 240 days · 100.0%
Health Impact Assessment
Good
Miami County's air quality is rated Good based on 84.2% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 84.2% of monitored days in 2025, with a median AQI of 40. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 240 of 240 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Miami County. Highest Good-day share: 2020 (94.8%, 217 of 229 monitored days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 217 | 12 | 0 | 0 | 39 | 93 |
| 2021 | 215 | 30 | 0 | 0 | 40 | 87 |
| 2022 | 214 | 29 | 2 | 0 | 39 | 122 |
| 2023 | 193 | 48 | 3 | 0 | 43 | 112 |
| 2024 | 198 | 46 | 1 | 0 | 43 | 101 |
| 2025 | 202 | 37 | 1 | 0 | 40 | 101 |
How does Miami County compare?
How Miami County compares
Good-day share better than typical
Miami County 84.2%
Anderson County, South Carolina 91.3%
Typical US county 76.0%
Median AQI worse than typical
Miami County 40
Anderson County, South Carolina 40
Typical US county 39
Unhealthy days / yr better than typical
Miami County 0
Anderson County, South Carolina 0
Typical US county 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Miami County (40), not geographic neighbors.
Anderson County, South Carolina
Median AQI 40 · 91.3% Good-day share
Andrew County, Missouri
Median AQI 40 · 86.9% Good-day share
Anne Arundel County, Maryland
Median AQI 40 · 87.6% Good-day share
Bergen County, New Jersey
Median AQI 40 · 71.1% Good-day share
Blair County, Pennsylvania
Median AQI 40 · 76.9% Good-day share
Brazos County, Texas
Median AQI 40 · 68.5% Good-day share
Same primary pollutant: Ozone
Ozone led the AQI on the most monitored days here. These peers share that driver and the closest Good-day share (a second relationship type beyond median-AQI neighbors).
Penobscot County, Maine
84.2% Good-day share · median AQI 33
Delaware County, Ohio
84.3% Good-day share · median AQI 40
Kent County, Maryland
84.0% Good-day share · median AQI 38
Morris County, New Jersey
84.4% Good-day share · median AQI 39
Norfolk County, Massachusetts
84.4% Good-day share · median AQI 39
Plymouth County, Massachusetts
84.4% Good-day share · median AQI 37
Ohio counties on the map
Geographic containment only, other monitored counties in Ohio, independent of AQI or pollutant match.
What Miami 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 40 means for exposure. Health guidance
- See how Miami County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Miami County with its Ohio neighbours before drawing a conclusion about the region. All of Ohio
Based on 240 EPA-monitored days in 2025; figures reflect only days the monitors reported. Source: EPA Air Quality System annual summaries.
Keep exploring the annual AQI record
Air-quality reading list
Frequently Asked Questions
What is the air quality in Miami County, Ohio? ▼
Miami County has a median Air Quality Index (AQI) of 40, rated "Good." 84.2% of monitored days (202 out of 240) had good air quality in 2025.
What is the main pollutant in Miami County? ▼
The primary pollutant in Miami County is Ozone (O3), which was the main AQI driver on 240 days (100% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Miami County have? ▼
In 2025, Miami County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 101.
Is the air quality in Miami County getting better or worse? ▼
Air quality in Miami County is worsening across floor-qualifying years (2020-2025); years with fewer than 100 monitored days are excluded from that direction. Highest qualifying Good-day share: 2020 at 94.8% (217 of 229 days). Lowest qualifying Good-day share: 2023 at 79.1% (193 of 244 days).
What does AQI 40 mean for health? ▼
Miami County's median AQI of 40 falls in the "Good" category. See the AQI category guide for what each range means for health.
How does Miami County compare to other counties in Ohio? ▼
Miami County has 84.2% good air days. Nearby counties in Ohio: Allen (67.0% good), Ashtabula (81.2% good), Athens (83.4% good), Belmont (63.0% good), Butler (33.1% good), Clark (61.4% good).
What is the 90th percentile AQI in Miami County? ▼
The 90th percentile AQI for Miami County is 61, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 101 (USG).
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 Miami County, Ohio · 2020–2025
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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.