Median AQI
39
Good
North Dakota · County air quality · 2025
Cass County, North Dakota
Median AQI 39 (Good). 73.2% of monitored days rated Good, direct from EPA Air Quality System stations.
- 39
- Median AQI
- 73.2%
- Good-air days
- 7
- Unhealthy days
- 37%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Cass County's air is good on balance, more polluted than the median US county, a median AQI of 39 with 73.2% of 239 monitored days in the Good band, driven mainly by Ozone.
- 39
- median AQI · Good
- 73.2%
- days rated Good
- Ozone
- main pollutant
- 37%
- cleaner than US counties
7 days reached Unhealthy or worse; the worst single reading hit AQI 192.
Data updated July 2026
- Good · 175 d · 73.2%
- Moderate · 53 d · 22.2%
- USG · 4 d · 1.7%
- Unhealthy · 7 d · 2.9%
How did Cass County's air break down in 2025?
Across 239 monitored days, 175 fell in the Good band (AQI 0–50) and 7 reached Unhealthy or worse. The worst single day peaked at AQI 192 (Unhealthy).
EPA AQS →Air quality data from EPA Air Quality System (2025). 239 days monitored. Median AQI: 39 (Good).
Insights: Cass County, North Dakota. EPA monitors recorded a median Air Quality Index of 39 across 239 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 175 (73%) fell in the Good band (AQI 0–50), 53 (22%) registered Moderate readings, and 4 (2%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 84, while the worst single-day peak hit 192 (Unhealthy). Residents averaged about 22.0 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (157 days), PM2.5 (82 days). Across the 2020–2025 trend window, good days shifted by -128 and the median AQI moved +2 points, placing the multi-year trajectory at worsening. Residents and visitors experienced 7 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); Dunn (78.6% good, AQI 40); McKenzie (79.0% good, AQI 41); Mercer (80.3% good, AQI 39).
Air Quality Summary
Good Days
73.2%
175 of 239 monitored days
-1.8pp vs 75% target
Max AQI Recorded
192
Unhealthy
Health Grade
Moderate
90th-pct AQI 84
Cass County, Good days share 73.2%
National avg
Share of monitored days at AQI 0-50 (Good band).
Cass County, Unhealthy-or-worse share 2.9%
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 239 monitored days across EPA AQI categories.
Good
175
73%
Moderate
53
22%
USG
4
2%
Unhealthy
7
3%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Cass County.
- Ozone · 157 days · 65.7%
- PM2.5 · 82 days · 34.3%
Health Impact Assessment
Moderate
Cass County's air quality is rated Moderate based on 73.2% good air days. There were 7 unhealthy days in 2025 when outdoor activity should be limited.
Air quality is acceptable most days, but 4 days reached "Unhealthy for Sensitive Groups" (AQI 101-150) and peak AQI reached 192. Sensitive groups should monitor real-time conditions at AirNow.gov.
The dominant pollutant is Ozone, driving AQI on 157 of 239 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Cass County. Best year: 2020 (303 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 303 | 60 | 0 | 0 | 37 | 80 |
| 2021 | 227 | 119 | 8 | 8 | 42 | 211 |
| 2022 | 285 | 79 | 1 | 0 | 40 | 111 |
| 2023 | 180 | 166 | 10 | 2 | 50 | 175 |
| 2024 | 288 | 76 | 1 | 1 | 38 | 158 |
| 2025 | 175 | 53 | 4 | 7 | 39 | 192 |
How does Cass County compare?
How Cass County compares
Good-day share worse than US avg
Cass County 73.2%
Billings County, North Dakota 80.3%
US average 76.5%
Median AQI worse than US avg
Cass County 39
Billings County, North Dakota 40
US average 38
Unhealthy days / yr worse than US avg
Cass County 7
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 Cass 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 Cass County's air means for you
On most days the air here is fine, but Ozone drives 7 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 Cass County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Cass County with its North Dakota neighbours before drawing a conclusion about the region. All of North Dakota
Based on 239 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 Cass County, North Dakota? ▼
Cass County has a median Air Quality Index (AQI) of 39, rated "Good." 73.2% of monitored days (175 out of 239) had good air quality in 2025.
What is the main pollutant in Cass County? ▼
The primary pollutant in Cass County is Ozone (O3), which was the main AQI driver on 157 days (66% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Cass County have? ▼
In 2025, Cass County recorded 7 unhealthy air days (7 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 192.
Is the air quality in Cass County getting better or worse? ▼
Air quality in Cass County is worsening based on 5-year trends (2020-2025). Best year: 2020 with 303 good days. Worst year: 2025 with 175 good days.
What does AQI 39 mean for health? ▼
Cass 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 Cass County compare to other counties in North Dakota? ▼
Cass County has 73.2% good air days. Nearby counties in North Dakota: Billings (80.3% good), Burke (72.8% good), Burleigh (77.5% good), Dunn (78.6% good), McKenzie (79.0% good), Mercer (80.3% good).
What is the 90th percentile AQI in Cass County? ▼
The 90th percentile AQI for Cass County is 84, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 192 (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.