Median AQI
40
Good
Missouri · County air quality · 2025
Jefferson County, Missouri
Median AQI 40 (Good). 79.0% of monitored days rated Good, direct from EPA Air Quality System stations.
- 40
- Median AQI
- 79.0%
- Good-air days
- 0
- Unhealthy days
- 49%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Jefferson County's air is good on balance, more polluted than the median US county, a median AQI of 40 with 79.0% of 148 monitored days in the Good band, driven mainly by PM2.5.
- 40
- median AQI · Good
- 79.0%
- days rated Good
- PM2.5
- main pollutant
- 49%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 77.
Data updated July 2026
- Good · 117 d · 79.1%
- Moderate · 31 d · 20.9%
How did Jefferson County's air break down in 2025?
Across 148 monitored days, 117 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 77 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 148 days monitored. Median AQI: 40 (Good).
Insights: Jefferson County, Missouri. EPA monitors recorded a median Air Quality Index of 40 across 148 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 117 (79%) fell in the Good band (AQI 0–50), 31 (21%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 56, while the worst single-day peak hit 77 (Moderate). Residents averaged about 23.7 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (89 days), Ozone (55 days), PM10 (4 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 improving. The worst single reading over the 2020–2025 window was AQI 77, moderate but never Unhealthy. Nearby counties show comparable readings: Andrew (84.3% good, AQI 42); Boone (89.1% good, AQI 41); Buchanan (64.2% good, AQI 39); Callaway (85.6% good, AQI 42); Cass (82.2% good, AQI 41); Cedar (81.8% good, AQI 40).
Air Quality Summary
Good Days
79.0%
117 of 148 monitored days
4.0pp vs 75% target
Max AQI Recorded
77
Moderate
Health Grade
Good
90th-pct AQI 56
Jefferson County, Good days share 79.0%
National avg
Share of monitored days at AQI 0-50 (Good band).
Jefferson 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 148 monitored days across EPA AQI categories.
Good
117
79%
Moderate
31
21%
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 Jefferson County.
- PM2.5 · 89 days · 60.1%
- Ozone · 55 days · 37.2%
- PM10 · 4 days · 2.7%
Health Impact Assessment
Good
Jefferson County's air quality is rated Good based on 79.0% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 79.0% 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 PM2.5, driving AQI on 89 of 148 monitored days.
5-Year Air Quality Trend
Air quality is improving in Jefferson County. Best year: 2024 (292 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 250 | 113 | 2 | 0 | 43 | 141 |
| 2021 | 244 | 116 | 4 | 1 | 44 | 164 |
| 2022 | 236 | 121 | 3 | 0 | 44 | 108 |
| 2023 | 225 | 121 | 14 | 2 | 44 | 185 |
| 2024 | 292 | 67 | 1 | 0 | 40 | 105 |
| 2025 | 117 | 31 | 0 | 0 | 40 | 77 |
How does Jefferson County compare?
How Jefferson County compares
Good-day share better than US avg
Jefferson County 79.0%
Andrew County, Missouri 84.3%
US average 76.5%
Median AQI worse than US avg
Jefferson County 40
Andrew County, Missouri 42
US average 38
Unhealthy days / yr about average
Jefferson County 0
Andrew County, Missouri 0
US average 0
Other Counties in Missouri
Air quality comparison for nearby counties in Missouri.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Jefferson County.
Amador County, California
AQI 40 · 84.8% good
Anne Arundel County, Maryland
AQI 40 · 87.2% good
Arlington County, Virginia
AQI 40 · 84.8% good
Bergen County, New Jersey
AQI 40 · 71.1% good
Berkeley County, West Virginia
AQI 40 · 81.1% good
Billings County, North Dakota
AQI 40 · 80.3% good
What Jefferson 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 40 means for exposure. Health guidance
- See how Jefferson County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Jefferson County with its Missouri neighbours before drawing a conclusion about the region. All of Missouri
Based on 148 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 Jefferson County, Missouri? ▼
Jefferson County has a median Air Quality Index (AQI) of 40, rated "Good." 79.0% of monitored days (117 out of 148) had good air quality in 2025.
What is the main pollutant in Jefferson County? ▼
The primary pollutant in Jefferson County is Fine Particles (PM2.5), which was the main AQI driver on 89 days (60% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Jefferson County have? ▼
In 2025, Jefferson County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 77.
Is the air quality in Jefferson County getting better or worse? ▼
Air quality in Jefferson County is improving based on 5-year trends (2020-2025). Best year: 2024 with 292 good days. Worst year: 2025 with 117 good days.
What does AQI 40 mean for health? ▼
Jefferson County's median AQI of 40 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 Jefferson County compare to other counties in Missouri? ▼
Jefferson County has 79.0% good air days. Nearby counties in Missouri: Andrew (84.3% good), Boone (89.1% good), Buchanan (64.2% good), Callaway (85.6% good), Cass (82.2% good), Cedar (81.8% good).
What is the 90th percentile AQI in Jefferson County? ▼
The 90th percentile AQI for Jefferson County is 56, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 77 (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.