Median AQI
51
Moderate
California · County air quality · 2025
San Luis Obispo County, California
Median AQI 51 (Moderate). 44.0% of monitored days rated Good, direct from EPA Air Quality System stations.
- 51
- Median AQI
- 44.0%
- Good-air days
- 0
- Unhealthy days
- 5%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, San Luis Obispo County's air is moderate on balance, among the most polluted counties nationally, a median AQI of 51 with 44.0% of 273 monitored days in the Good band, driven mainly by Ozone.
- 51
- median AQI · Moderate
- 44.0%
- days rated Good
- Ozone
- main pollutant
- 5%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 119.
Data updated July 2026
- Good · 120 d · 44.0%
- Moderate · 146 d · 53.5%
- USG · 7 d · 2.6%
How did San Luis Obispo County's air break down in 2025?
Across 273 monitored days, 120 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 119 (USG).
EPA AQS →Air quality data from EPA Air Quality System (2025). 273 days monitored. Median AQI: 51 (Moderate).
Insights: San Luis Obispo County, California. EPA monitors recorded a median Air Quality Index of 51 across 273 measured days in 2025, earning a Moderate classification and a Poor health grade. Of those days, 120 (44%) fell in the Good band (AQI 0–50), 146 (53%) registered Moderate readings, and 7 (3%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 69, while the worst single-day peak hit 119 (USG). Residents averaged about 13.2 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (143 days), PM2.5 (130 days). Across the 2020–2025 trend window, good days shifted by -41 and the median AQI moved -2 points, placing the multi-year trajectory at stable. The worst single reading over the 2020–2025 window was AQI 119, usg but never Unhealthy. Nearby counties show comparable readings: Amador (84.8% good, AQI 40); Butte (66.1% good, AQI 47); Calaveras (73.9% good, AQI 43); Colusa (84.9% good, AQI 39); Del Norte (81.0% good, AQI 33); El Dorado (85.8% good, AQI 39).
Air Quality Summary
Good Days
44.0%
120 of 273 monitored days
-31.0pp vs 75% target
Max AQI Recorded
119
USG
Health Grade
Poor
90th-pct AQI 69
San Luis Obispo County, Good days share 44.0%
National avg
Share of monitored days at AQI 0-50 (Good band).
San Luis Obispo 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 273 monitored days across EPA AQI categories.
Good
120
44%
Moderate
146
53%
USG
7
3%
Unhealthy
0
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in San Luis Obispo County.
- Ozone · 143 days · 52.4%
- PM2.5 · 130 days · 47.6%
Health Impact Assessment
Poor
San Luis Obispo County's air quality is rated Poor based on 44.0% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 44.0% of monitored days in 2025, with a median AQI of 51. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 143 of 273 monitored days.
5-Year Air Quality Trend
Air quality is stable in San Luis Obispo County. Best year: 2023 (207 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 161 | 174 | 21 | 7 | 53 | 334 |
| 2021 | 154 | 203 | 8 | 0 | 53 | 133 |
| 2022 | 117 | 245 | 3 | 0 | 55 | 115 |
| 2023 | 207 | 152 | 6 | 0 | 47 | 122 |
| 2024 | 193 | 163 | 10 | 0 | 49 | 122 |
| 2025 | 120 | 146 | 7 | 0 | 51 | 119 |
How does San Luis Obispo County compare?
How San Luis Obispo County compares
Good-day share worse than US avg
San Luis Obispo County 44.0%
Amador County, California 84.8%
US average 76.5%
Median AQI worse than US avg
San Luis Obispo County 51
Amador County, California 40
US average 38
Unhealthy days / yr about average
San Luis Obispo County 0
Amador County, California 0
US average 0
Other Counties in California
Air quality comparison for nearby counties in California.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to San Luis Obispo County.
Arapahoe County, Colorado
AQI 51 · 47.3% good
Bibb County, Georgia
AQI 51 · 49.5% good
East Baton Rouge County, Louisiana
AQI 51 · 49.3% good
Eddy County, New Mexico
AQI 51 · 46.9% good
Hillsborough County, Florida
AQI 51 · 49.3% good
Jefferson County, Colorado
AQI 51 · 46.5% good
What San Luis Obispo 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 51 means for exposure. Health guidance
- See how San Luis Obispo County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare San Luis Obispo County with its California neighbours before drawing a conclusion about the region. All of California
Based on 273 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 San Luis Obispo County, California? ▼
San Luis Obispo County has a median Air Quality Index (AQI) of 51, rated "Moderate." 44.0% of monitored days (120 out of 273) had good air quality in 2025.
What is the main pollutant in San Luis Obispo County? ▼
The primary pollutant in San Luis Obispo County is Ozone (O3), which was the main AQI driver on 143 days (52% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does San Luis Obispo County have? ▼
In 2025, San Luis Obispo County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 119.
Is the air quality in San Luis Obispo County getting better or worse? ▼
Air quality in San Luis Obispo County is stable based on 5-year trends (2020-2025). Best year: 2023 with 207 good days. Worst year: 2022 with 117 good days.
What does AQI 51 mean for health? ▼
San Luis Obispo County's median AQI of 51 falls in the "Moderate" category. See the <a href="/guides/understanding-aqi" class="underline">AQI category guide</a> for what each range means for health.
How does San Luis Obispo County compare to other counties in California? ▼
San Luis Obispo County has 44.0% good air days. Nearby counties in California: Amador (84.8% good), Butte (66.1% good), Calaveras (73.9% good), Colusa (84.9% good), Del Norte (81.0% good), El Dorado (85.8% good).
What is the 90th percentile AQI in San Luis Obispo County? ▼
The 90th percentile AQI for San Luis Obispo County is 69, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 119 (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.