Median AQI
48
Good
California · County air quality · 2024
Contra Costa County, California
Median AQI 48 (Good). 53.5% of monitored days rated Good, direct from EPA Air Quality System stations.
- 48
- Median AQI
- 53.5%
- Good-air days
- 0
- Unhealthy days
- 7%
- Cleaner than US counties
Contra Costa County's 2024 annual median AQI is 48 (Good), with 53.5% Good days
According to the U.S. EPA Air Quality System, Contra Costa County, California posted a 2024 median AQI of 48 (Good) across 361 monitored days, with 53.5% rated Good and 0 unhealthy or worse. This is an annual comparison, not today's advisory.
Data source EPA Air Quality System (AQS)
- Cleaner than US
- Contra Costa County's 53.5% Good-day share places it cleaner than 7% of US counties in 2024.
- Closest median-AQI peer
- Alameda County (53.0% Good, median AQI 49) is the closest nationwide median-AQI peer to Contra Costa County.
- 2020–2024 trend
- Contra Costa County's floor-qualifying Good-day share is stable from 2020 to 2024 (52.6% → 53.5%).
- Median AQI
- Median AQI: 48
The verdict
According to EPA Air Quality System monitoring data, Contra Costa County's air is good on balance, among the most polluted counties nationally, a median AQI of 48 with 53.5% of 361 monitored days in the Good band, driven mainly by PM2.5.
- 48
- median AQI · Good
- 53.5%
- days rated Good
- PM2.5
- main pollutant
- 7%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 94.
Annual release change
What changed in the 2024 EPA annual release
- Contra Costa County's Good-day share rose from 47.4% to 53.5% between the 2023 and 2024 EPA annual releases, a change of +6.1 percentage points.
- Contra Costa County's peak single-day AQI fell from 110 to 94 between the 2023 and 2024 EPA annual releases, a change of -16 points.
- Contra Costa County's median AQI fell from 51 to 48 between the 2023 and 2024 EPA annual releases, a change of -3 points.
Data updated August 2026
- Good · 193 d · 53.5%
- Moderate · 168 d · 46.5%
How did Contra Costa County's air break down in 2024?
Across 361 monitored days, 193 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 94 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2024). 361 days monitored. Median AQI: 48 (Good).
Insights: Contra Costa County, California. EPA monitors recorded a median Air Quality Index of 48 across 361 measured days in 2024, earning a Good classification and a Moderate health grade. Of those days, 193 (53%) fell in the Good band (AQI 0–50), 168 (47%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 63, while the worst single-day peak hit 94 (Moderate). Residents averaged about 16.0 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (348 days), Ozone (13 days). Across the 2020–2024 trend window, the share of monitored days rated Good shifted by +1.0 points and the median AQI moved -1 points, placing the multi-year trajectory at stable. The worst single reading over the 2020–2024 window was AQI 94, moderate but never Unhealthy. Nearby counties show comparable readings: Alameda (53.0% good, AQI 49); Amador (87.4% good, AQI 38); Butte (65.2% good, AQI 46); Calaveras (82.2% good, AQI 40); Colusa (79.2% good, AQI 40); Del Norte (81.8% good, AQI 32).
Air Quality Summary
Good Days
53.5%
193 of 361 monitored days
-21.5pp vs 75% target
Max AQI Recorded
94
Moderate
Health Grade
Moderate
90th-pct AQI 63
Contra Costa County, Good days share 53.5%
National avg
Share of monitored days at AQI 0-50 (Good band).
Contra Costa 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 (2024)
Distribution of 361 monitored days across EPA AQI categories.
Good
193
53%
Moderate
168
47%
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 Contra Costa County.
- PM2.5 · 348 days · 96.4%
- Ozone · 13 days · 3.6%
Health Impact Assessment
Moderate
Contra Costa County's air quality is rated Moderate based on 53.5% good air days. No unhealthy air days were recorded in 2024.
Air quality was acceptable on 53.5% of monitored days in 2024, with a median AQI of 48. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM2.5, driving AQI on 348 of 361 monitored days.
5-Year Air Quality Trend
Air quality is stable in Contra Costa County. Highest Good-day share: 2024 (53.5%, 193 of 361 monitored days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 192 | 156 | 7 | 10 | 49 | 221 |
| 2021 | 195 | 165 | 5 | 0 | 47 | 150 |
| 2022 | 175 | 188 | 2 | 0 | 51 | 129 |
| 2023 | 173 | 190 | 2 | 0 | 51 | 110 |
| 2024 | 193 | 168 | 0 | 0 | 48 | 94 |
How does Contra Costa County compare?
How Contra Costa County compares
Good-day share worse than US avg
Contra Costa County 53.5%
Alameda County, California 53.0%
US average 76.0%
Median AQI worse than US avg
Contra Costa County 48
Alameda County, California 49
US average 39
Unhealthy days / yr better than US avg
Contra Costa County 0
Alameda County, California 0
US average 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Contra Costa County (48), not geographic neighbors.
Alameda County, California
Median AQI 49 · 53.0% Good-day share
Santa Clara County, California
Median AQI 47 · 55.4% Good-day share
Rio Blanco County, Colorado
Median AQI 46 · 72.7% Good-day share
Johnston County, Oklahoma
Median AQI 44 · 74.7% Good-day share
Yancey County, North Carolina
Median AQI 44 · 81.9% Good-day share
Pontotoc County, Oklahoma
Median AQI 43 · 71.6% 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).
California counties on the map
Geographic containment only, other monitored counties in California, independent of AQI or pollutant match.
What Contra Costa 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 48 means for exposure. Health guidance
- See how Contra Costa County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Contra Costa County with its California neighbours before drawing a conclusion about the region. All of California
Based on 361 EPA-monitored days in 2024; 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 Contra Costa County, California? ▼
Contra Costa County has a median Air Quality Index (AQI) of 48, rated "Good." 53.5% of monitored days (193 out of 361) had good air quality in 2024.
What is the main pollutant in Contra Costa County? ▼
The primary pollutant in Contra Costa County is Fine Particles (PM2.5), which was the main AQI driver on 348 days (96% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Contra Costa County have? ▼
In 2024, Contra Costa County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 94.
Is the air quality in Contra Costa County getting better or worse? ▼
Air quality in Contra Costa County is stable across floor-qualifying years (2020-2024); years with fewer than 100 monitored days are excluded from that direction. Highest qualifying Good-day share: 2024 at 53.5% (193 of 361 days). Lowest qualifying Good-day share: 2023 at 47.4% (173 of 365 days).
What does AQI 48 mean for health? ▼
Contra Costa County's median AQI of 48 falls in the "Good" category. See the AQI category guide for what each range means for health.
How does Contra Costa County compare to other counties in California? ▼
Contra Costa County has 53.5% good air days. Nearby counties in California: Alameda (53.0% good), Amador (87.4% good), Butte (65.2% good), Calaveras (82.2% good), Colusa (79.2% good), Del Norte (81.8% good).
What is the 90th percentile AQI in Contra Costa County? ▼
The 90th percentile AQI for Contra Costa County is 63, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 94 (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. Spotted a figure that looks wrong? Report a correction.
Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files for Contra Costa County, California · 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.