Median AQI
37
Good
South Dakota · County air quality · 2025
Clay County, South Dakota
Median AQI 37 (Good). 82.6% of monitored days rated Good, direct from EPA Air Quality System stations.
- 37
- Median AQI
- 82.6%
- Good-air days
- 0
- Unhealthy days
- 59%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Clay County's air is good on balance, cleaner than the median US county, a median AQI of 37 with 82.6% of 167 monitored days in the Good band, driven mainly by Ozone.
- 37
- median AQI · Good
- 82.6%
- days rated Good
- Ozone
- main pollutant
- 59%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 74.
Data updated July 2026
- Good · 138 d · 82.6%
- Moderate · 29 d · 17.4%
How did Clay County's air break down in 2025?
Across 167 monitored days, 138 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 74 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 167 days monitored. Median AQI: 37 (Good).
Insights: Clay County, South Dakota. EPA monitors recorded a median Air Quality Index of 37 across 167 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 138 (83%) fell in the Good band (AQI 0–50), 29 (17%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 57, while the worst single-day peak hit 74 (Moderate). Residents averaged about 24.8 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (106 days), PM2.5 (61 days). Across the 2022–2025 trend window, good days shifted by -182 and the median AQI moved -1 points, placing the multi-year trajectory at worsening. The worst single reading over the 2022–2025 window was AQI 74, moderate but never Unhealthy. Nearby counties show comparable readings: Brookings (53.0% good, AQI 48); Brown (87.1% good, AQI 25); Codington (82.1% good, AQI 33); Custer (72.4% good, AQI 46); Hughes (94.4% good, AQI 14); Jackson (78.5% good, AQI 44).
Air Quality Summary
Good Days
82.6%
138 of 167 monitored days
7.6pp vs 75% target
Max AQI Recorded
74
Moderate
Health Grade
Good
90th-pct AQI 57
Clay County, Good days share 82.6%
National avg
Share of monitored days at AQI 0-50 (Good band).
Clay 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 167 monitored days across EPA AQI categories.
Good
138
83%
Moderate
29
17%
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 Clay County.
- Ozone · 106 days · 63.5%
- PM2.5 · 61 days · 36.5%
Health Impact Assessment
Good
Clay County's air quality is rated Good based on 82.6% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 82.6% of monitored days in 2025, with a median AQI of 37. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 106 of 167 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Clay County. Best year: 2022 (320 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2022 | 320 | 45 | 0 | 0 | 38 | 87 |
| 2023 | 244 | 103 | 15 | 3 | 43 | 173 |
| 2024 | 289 | 75 | 2 | 0 | 39 | 137 |
| 2025 | 138 | 29 | 0 | 0 | 37 | 74 |
How does Clay County compare?
How Clay County compares
Good-day share better than US avg
Clay County 82.6%
Brookings County, South Dakota 53.0%
US average 76.5%
Median AQI better than US avg
Clay County 37
Brookings County, South Dakota 48
US average 38
Unhealthy days / yr about average
Clay County 0
Brookings County, South Dakota 1
US average 0
Other Counties in South Dakota
Air quality comparison for nearby counties in South Dakota.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Clay County.
Alachua County, Florida
AQI 37 · 90.1% good
Beltrami County, Minnesota
AQI 37 · 88.8% good
Benewah County, Idaho
AQI 37 · 74.6% good
Brevard County, Florida
AQI 37 · 89.7% good
Brown County, Indiana
AQI 37 · 94.5% good
Brown County, Wisconsin
AQI 37 · 79.0% good
What Clay 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 37 means for exposure. Health guidance
- See how Clay County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Clay County with its South Dakota neighbours before drawing a conclusion about the region. All of South Dakota
Based on 167 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 Clay County, South Dakota? ▼
Clay County has a median Air Quality Index (AQI) of 37, rated "Good." 82.6% of monitored days (138 out of 167) had good air quality in 2025.
What is the main pollutant in Clay County? ▼
The primary pollutant in Clay County is Ozone (O3), which was the main AQI driver on 106 days (63% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Clay County have? ▼
In 2025, Clay County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 74.
Is the air quality in Clay County getting better or worse? ▼
Air quality in Clay County is worsening based on 5-year trends (2022-2025). Best year: 2022 with 320 good days. Worst year: 2025 with 138 good days.
What does AQI 37 mean for health? ▼
Clay County's median AQI of 37 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 Clay County compare to other counties in South Dakota? ▼
Clay County has 82.6% good air days. Nearby counties in South Dakota: Brookings (53.0% good), Brown (87.1% good), Codington (82.1% good), Custer (72.4% good), Hughes (94.4% good), Jackson (78.5% good).
What is the 90th percentile AQI in Clay County? ▼
The 90th percentile AQI for Clay County is 57, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 74 (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.