Median AQI
39
Good
North Dakota · County air quality · 2025
Mercer County, North Dakota
Median AQI 39 (Good). 80.3% of monitored days rated Good, direct from EPA Air Quality System stations.
- 39
- Median AQI
- 80.3%
- Good-air days
- 4
- Unhealthy days
- 53%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Mercer County's air is good on balance, cleaner than the median US county, a median AQI of 39 with 80.3% of 243 monitored days in the Good band, driven mainly by Ozone.
- 39
- median AQI · Good
- 80.3%
- days rated Good
- Ozone
- main pollutant
- 53%
- cleaner than US counties
4 days reached Unhealthy or worse; the worst single reading hit AQI 283.
Data updated July 2026
- Good · 195 d · 80.2%
- Moderate · 38 d · 15.6%
- USG · 6 d · 2.5%
- Unhealthy · 3 d · 1.2%
- Very Unhealthy · 1 d · 0.4%
How did Mercer County's air break down in 2025?
Across 243 monitored days, 195 fell in the Good band (AQI 0–50) and 4 reached Unhealthy or worse. The worst single day peaked at AQI 283 (Very Unhealthy).
EPA AQS →Air quality data from EPA Air Quality System (2025). 243 days monitored. Median AQI: 39 (Good).
Insights: Mercer County, North Dakota. EPA monitors recorded a median Air Quality Index of 39 across 243 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 195 (80%) fell in the Good band (AQI 0–50), 38 (16%) registered Moderate readings, and 6 (2%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 66, while the worst single-day peak hit 283 (Very Unhealthy). Residents averaged about 24.1 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (180 days), PM2.5 (63 days). Across the 2020–2025 trend window, good days shifted by -133 and the median AQI moved +3 points, placing the multi-year trajectory at worsening. Residents and visitors experienced 4 unhealthy-or-worse days combined, periods when the EPA recommends sensitive groups limit prolonged outdoor exertion. Nearby counties show comparable readings: Billings (80.3% good, AQI 40); Burke (72.8% good, AQI 40); Burleigh (77.5% good, AQI 41); Cass (73.2% good, AQI 39); Dunn (78.6% good, AQI 40); McKenzie (79.0% good, AQI 41).
Air Quality Summary
Good Days
80.3%
195 of 243 monitored days
5.3pp vs 75% target
Max AQI Recorded
283
Very Unhealthy
Health Grade
Good
90th-pct AQI 66
Mercer County, Good days share 80.3%
National avg
Share of monitored days at AQI 0-50 (Good band).
Mercer County, Unhealthy-or-worse share 1.6%
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 243 monitored days across EPA AQI categories.
Good
195
80%
Moderate
38
16%
USG
6
2%
Unhealthy
3
1%
Very Unhealthy
1
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Mercer County.
- Ozone · 180 days · 74.1%
- PM2.5 · 63 days · 25.9%
Health Impact Assessment
Good
Mercer County's air quality is rated Good based on 80.3% good air days. There were 4 unhealthy days in 2025 when outdoor activity should be limited.
Air quality reached unhealthy levels on 4 days in 2025 (3 Unhealthy, 1 Very Unhealthy). Sensitive groups, including people with asthma, COPD, cardiovascular conditions, children, and adults over 65, should review the daily breakdown below and follow local air-quality advisories on high-AQI days.
Sensitive group advisory
People with asthma, COPD, heart disease, children, pregnant women, and adults 65+ should follow EPA AQI alerts daily and limit outdoor exertion when AQI exceeds 100. Real-time conditions: airnow.gov.
The dominant pollutant is Ozone, driving AQI on 180 of 243 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Mercer County. Best year: 2020 (328 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 328 | 38 | 0 | 0 | 36 | 78 |
| 2021 | 279 | 77 | 5 | 4 | 38 | 174 |
| 2022 | 318 | 47 | 0 | 0 | 36 | 72 |
| 2023 | 252 | 97 | 10 | 5 | 41 | 322 |
| 2024 | 304 | 56 | 4 | 2 | 37 | 167 |
| 2025 | 195 | 38 | 6 | 3 | 39 | 283 |
How does Mercer County compare?
How Mercer County compares
Good-day share better than US avg
Mercer County 80.3%
Billings County, North Dakota 80.3%
US average 76.5%
Median AQI worse than US avg
Mercer County 39
Billings County, North Dakota 40
US average 38
Unhealthy days / yr worse than US avg
Mercer County 4
Billings County, North Dakota 4
US average 0
Other Counties in North Dakota
Air quality comparison for nearby counties in North Dakota.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Mercer County.
Ada County, Idaho
AQI 39 · 80.6% good
Aiken County, South Carolina
AQI 39 · 89.1% good
Albany County, New York
AQI 39 · 67.4% good
Alexander County, North Carolina
AQI 39 · 92.4% good
Ashland County, Wisconsin
AQI 39 · 85.6% good
Bennington County, Vermont
AQI 39 · 82.9% good
What Mercer County's air means for you
On most days the air here is fine, but Ozone drives 4 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-Ozone days, the guide explains what a median of 39 means for exposure. Health guidance
- See how Mercer County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Mercer County with its North Dakota neighbours before drawing a conclusion about the region. All of North Dakota
Based on 243 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 Mercer County, North Dakota? ▼
Mercer County has a median Air Quality Index (AQI) of 39, rated "Good." 80.3% of monitored days (195 out of 243) had good air quality in 2025.
What is the main pollutant in Mercer County? ▼
The primary pollutant in Mercer County is Ozone (O3), which was the main AQI driver on 180 days (74% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Mercer County have? ▼
In 2025, Mercer County recorded 4 unhealthy air days (3 Unhealthy, 1 Very Unhealthy, 0 Hazardous). The maximum AQI reached 283.
Is the air quality in Mercer County getting better or worse? ▼
Air quality in Mercer County is worsening based on 5-year trends (2020-2025). Best year: 2020 with 328 good days. Worst year: 2025 with 195 good days.
What does AQI 39 mean for health? ▼
Mercer County's median AQI of 39 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 Mercer County compare to other counties in North Dakota? ▼
Mercer County has 80.3% good air days. Nearby counties in North Dakota: Billings (80.3% good), Burke (72.8% good), Burleigh (77.5% good), Cass (73.2% good), Dunn (78.6% good), McKenzie (79.0% good).
What is the 90th percentile AQI in Mercer County? ▼
The 90th percentile AQI for Mercer County is 66, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 283 (Very 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.