Median AQI
38
Good
Louisiana · County air quality · 2025
St. Tammany County, Louisiana
Median AQI 38 (Good). 84.9% of monitored days rated Good, direct from EPA Air Quality System stations.
- 38
- Median AQI
- 84.9%
- Good-air days
- 0
- Unhealthy days
- 66%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, St. Tammany County's air is good on balance, cleaner than the median US county, a median AQI of 38 with 84.9% of 271 monitored days in the Good band, driven mainly by Ozone.
- 38
- median AQI · Good
- 84.9%
- days rated Good
- Ozone
- main pollutant
- 66%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 105.
Data updated July 2026
- Good · 230 d · 84.9%
- Moderate · 40 d · 14.8%
- USG · 1 d · 0.4%
How did St. Tammany County's air break down in 2025?
Across 271 monitored days, 230 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 105 (USG).
EPA AQS →Air quality data from EPA Air Quality System (2025). 271 days monitored. Median AQI: 38 (Good).
Insights: St. Tammany County, Louisiana. EPA monitors recorded a median Air Quality Index of 38 across 271 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 230 (85%) fell in the Good band (AQI 0–50), 40 (15%) registered Moderate readings, and 1 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 58, while the worst single-day peak hit 105 (USG). Residents averaged about 25.5 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (271 days). Across the 2020–2025 trend window, good days shifted by -38 and the median AQI moved -1 points, placing the multi-year trajectory at improving. The worst single reading over the 2020–2025 window was AQI 105, usg but never Unhealthy. Nearby counties show comparable readings: Ascension (94.1% good, AQI 33); Bossier (86.5% good, AQI 40); Caddo (84.3% good, AQI 38); Calcasieu (61.5% good, AQI 47); East Baton Rouge (49.3% good, AQI 51); Iberville (78.4% good, AQI 41).
Air Quality Summary
Good Days
84.9%
230 of 271 monitored days
9.9pp vs 75% target
Max AQI Recorded
105
USG
Health Grade
Good
90th-pct AQI 58
St. Tammany County, Good days share 84.9%
National avg
Share of monitored days at AQI 0-50 (Good band).
St. Tammany 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 271 monitored days across EPA AQI categories.
Good
230
85%
Moderate
40
15%
USG
1
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 St. Tammany County.
- Ozone · 271 days · 100.0%
Health Impact Assessment
Good
St. Tammany County's air quality is rated Good based on 84.9% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 84.9% of monitored days in 2025, with a median AQI of 38. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 271 of 271 monitored days.
5-Year Air Quality Trend
Air quality is improving in St. Tammany County. Best year: 2024 (291 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 268 | 96 | 0 | 0 | 39 | 90 |
| 2021 | 288 | 64 | 0 | 0 | 37 | 71 |
| 2022 | 277 | 87 | 1 | 0 | 41 | 103 |
| 2023 | 248 | 116 | 1 | 0 | 42 | 105 |
| 2024 | 291 | 70 | 0 | 1 | 39 | 186 |
| 2025 | 230 | 40 | 1 | 0 | 38 | 105 |
How does St. Tammany County compare?
How St. Tammany County compares
Good-day share better than US avg
St. Tammany County 84.9%
Ascension County, Louisiana 94.1%
US average 76.5%
Median AQI about average
St. Tammany County 38
Ascension County, Louisiana 33
US average 38
Unhealthy days / yr about average
St. Tammany County 0
Ascension County, Louisiana 0
US average 0
Other Counties in Louisiana
Air quality comparison for nearby counties in Louisiana.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to St. Tammany County.
Adams County, Illinois
AQI 38 · 94.4% good
Anoka County, Minnesota
AQI 38 · 82.1% good
Atlantic County, New Jersey
AQI 38 · 79.6% good
Avery County, North Carolina
AQI 38 · 93.9% good
Barnstable County, Massachusetts
AQI 38 · 92.5% good
Bay County, Michigan
AQI 38 · 71.8% good
What St. Tammany 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 38 means for exposure. Health guidance
- See how St. Tammany County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare St. Tammany County with its Louisiana neighbours before drawing a conclusion about the region. All of Louisiana
Based on 271 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 St. Tammany County, Louisiana? ▼
St. Tammany County has a median Air Quality Index (AQI) of 38, rated "Good." 84.9% of monitored days (230 out of 271) had good air quality in 2025.
What is the main pollutant in St. Tammany County? ▼
The primary pollutant in St. Tammany County is Ozone (O3), which was the main AQI driver on 271 days (100% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does St. Tammany County have? ▼
In 2025, St. Tammany County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 105.
Is the air quality in St. Tammany County getting better or worse? ▼
Air quality in St. Tammany County is improving based on 5-year trends (2020-2025). Best year: 2024 with 291 good days. Worst year: 2025 with 230 good days.
What does AQI 38 mean for health? ▼
St. Tammany County's median AQI of 38 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 St. Tammany County compare to other counties in Louisiana? ▼
St. Tammany County has 84.9% good air days. Nearby counties in Louisiana: Ascension (94.1% good), Bossier (86.5% good), Caddo (84.3% good), Calcasieu (61.5% good), East Baton Rouge (49.3% good), Iberville (78.4% good).
What is the 90th percentile AQI in St. Tammany County? ▼
The 90th percentile AQI for St. Tammany County is 58, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 105 (USG).
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.