Median AQI
49
Good
Kentucky · County air quality · 2025
Jefferson County, Kentucky
Median AQI 49 (Good). 51.5% of monitored days rated Good, direct from EPA Air Quality System stations.
- 49
- Median AQI
- 51.5%
- Good-air days
- 1
- Unhealthy days
- 7%
- Cleaner than US counties
Jefferson County's 2025 annual median AQI is 49 (Good), with 51.5% Good days
According to the U.S. EPA Air Quality System, Jefferson County, Kentucky posted a 2025 median AQI of 49 (Good) across 365 monitored days, with 51.5% rated Good and 1 unhealthy or worse. This is an annual comparison, not today's advisory.
Data source EPA Air Quality System (AQS)
- Cleaner than US
- Jefferson County's 51.5% Good-day share places it cleaner than 7% of US counties in 2025.
- Closest median-AQI peer
- Adams County (55.1% Good, median AQI 49) is the closest nationwide median-AQI peer to Jefferson County.
- 2020–2025 trend
- Jefferson County's floor-qualifying Good-day share is improving from 2020 to 2025 (41.3% → 51.5%).
- Median AQI
- Median AQI: 49
The verdict
According to EPA Air Quality System monitoring data, Jefferson County's air is good on balance, among the most polluted counties nationally, a median AQI of 49 with 51.5% of 365 monitored days in the Good band, driven mainly by PM2.5.
- 49
- median AQI · Good
- 51.5%
- days rated Good
- PM2.5
- main pollutant
- 7%
- cleaner than US counties
1 day reached Unhealthy or worse; the worst single reading hit AQI 153.
Data updated August 2026
- Good · 188 d · 51.5%
- Moderate · 167 d · 45.8%
- USG · 9 d · 2.5%
- Unhealthy · 1 d · 0.3%
How did Jefferson County's air break down in 2025?
Across 365 monitored days, 188 fell in the Good band (AQI 0–50) and 1 reached Unhealthy or worse. The worst single day peaked at AQI 153 (Unhealthy).
EPA AQS →Air quality data from EPA Air Quality System (2025). 365 days monitored. Median AQI: 49 (Good).
Insights: Jefferson County, Kentucky. EPA monitors recorded a median Air Quality Index of 49 across 365 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 188 (52%) fell in the Good band (AQI 0–50), 167 (46%) registered Moderate readings, and 9 (2%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 67, while the worst single-day peak hit 153 (Unhealthy). Residents averaged about 15.5 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (213 days), Ozone (147 days), NO2 (5 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by +10.3 points and the median AQI moved -3 points, placing the multi-year trajectory at improving. The annual record contains 1 day at Unhealthy or worse. On that day, the EPA recommends that sensitive groups limit prolonged outdoor exertion. Nearby counties show comparable readings: Bell (67.0% good, AQI 42); Boone (87.6% good, AQI 38); Boyd (80.0% good, AQI 39); Bullitt (88.4% good, AQI 39); Campbell (75.6% good, AQI 39); Carter (89.0% good, AQI 36).
Air Quality Summary
Good Days
51.5%
188 of 365 monitored days
-23.5pp vs 75% target
Max AQI Recorded
153
Unhealthy
Health Grade
Moderate
90th-pct AQI 67
Jefferson County, Good days share 51.5%
National avg
Share of monitored days at AQI 0-50 (Good band).
Jefferson County, Unhealthy-or-worse share 2.7%
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 365 monitored days across EPA AQI categories.
Good
188
52%
Moderate
167
46%
USG
9
2%
Unhealthy
1
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 · 213 days · 58.4%
- Ozone · 147 days · 40.3%
- NO₂ · 5 days · 1.4%
Health Impact Assessment
Moderate
Jefferson County's air quality is rated Moderate based on 51.5% good air days. There were 1 unhealthy days in 2025 when outdoor activity should be limited.
Air quality is acceptable most days, but 9 days reached "Unhealthy for Sensitive Groups" (AQI 101-150) and peak AQI reached 153. Sensitive groups should monitor real-time conditions at AirNow.gov.
The dominant pollutant is PM2.5, driving AQI on 213 of 365 monitored days.
5-Year Air Quality Trend
Air quality is improving in Jefferson County. Highest Good-day share: 2025 (51.5%, 188 of 365 monitored days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 151 | 208 | 7 | 0 | 52 | 115 |
| 2021 | 138 | 221 | 6 | 0 | 54 | 126 |
| 2022 | 147 | 211 | 7 | 0 | 53 | 136 |
| 2023 | 123 | 226 | 15 | 1 | 55 | 171 |
| 2024 | 186 | 168 | 11 | 1 | 50 | 156 |
| 2025 | 188 | 167 | 9 | 1 | 49 | 153 |
How does Jefferson County compare?
How Jefferson County compares
Good-day share worse than typical
Jefferson County 51.5%
Adams County, Colorado 55.1%
Typical US county 76.0%
Median AQI worse than typical
Jefferson County 49
Adams County, Colorado 49
Typical US county 39
Unhealthy days / yr about typical
Jefferson County 1
Adams County, Colorado 0
Typical US county 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Jefferson County (49), not geographic neighbors.
Adams County, Colorado
Median AQI 49 · 55.1% Good-day share
Albany County, Wyoming
Median AQI 49 · 55.6% Good-day share
Black Hawk County, Iowa
Median AQI 49 · 53.4% Good-day share
Davidson County, Tennessee
Median AQI 49 · 52.9% Good-day share
Eddy County, New Mexico
Median AQI 49 · 53.7% Good-day share
Fulton County, Georgia
Median AQI 49 · 54.3% Good-day share
Same primary pollutant: PM2.5
PM2.5 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).
Kleberg County, Texas
51.7% Good-day share · median AQI 49
Lowndes County, Georgia
51.2% Good-day share · median AQI 49
Santa Cruz County, Arizona
52.2% Good-day share · median AQI 49
Madison County, Illinois
52.3% Good-day share · median AQI 49
Tulsa County, Oklahoma
50.7% Good-day share · median AQI 50
Philadelphia County, Pennsylvania
50.4% Good-day share · median AQI 50
Kentucky counties on the map
Geographic containment only, other monitored counties in Kentucky, independent of AQI or pollutant match.
What Jefferson County's air means for you
On most days the air here is fine, but PM2.5 drives 1 unhealthy-or-worse day a year that sensitive groups should plan around.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-PM2.5 days, the guide explains what a median of 49 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 Kentucky neighbours before drawing a conclusion about the region. All of Kentucky
Based on 365 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 Jefferson County, Kentucky? ▼
Jefferson County has a median Air Quality Index (AQI) of 49, rated "Good." 51.5% of monitored days (188 out of 365) 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 213 days (58% 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 1 unhealthy air days (1 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 153.
Is the air quality in Jefferson County getting better or worse? ▼
Air quality in Jefferson County is improving across floor-qualifying years (2020-2025); years with fewer than 100 monitored days are excluded from that direction. Highest qualifying Good-day share: 2025 at 51.5% (188 of 365 days). Lowest qualifying Good-day share: 2023 at 33.7% (123 of 365 days).
What does AQI 49 mean for health? ▼
Jefferson County's median AQI of 49 falls in the "Good" category. See the AQI category guide for what each range means for health.
How does Jefferson County compare to other counties in Kentucky? ▼
Jefferson County has 51.5% good air days. Nearby counties in Kentucky: Bell (67.0% good), Boone (87.6% good), Boyd (80.0% good), Bullitt (88.4% good), Campbell (75.6% good), Carter (89.0% good).
What is the 90th percentile AQI in Jefferson County? ▼
The 90th percentile AQI for Jefferson County is 67, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 153 (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. Spotted a figure that looks wrong? Report a correction.
Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files for Jefferson County, Kentucky · 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.