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
According to the 2024 EPA AQS county release on PlainAirData, Contra Costa County, California ranks #14 of 15 by median AQI (48) and #14 by good-day share (53.5%). San Mateo County, California is #1 of 15 by median AQI (27, 88.1% Good days) at good-day-share #5 ≠ Chattooga County, Georgia #1 by good-day share (96.3%, median AQI 39) at median-AQI #5.
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 →Share of monitored days at AQI 0-50 (Good band).
Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).
Where Contra Costa County sits nationally
COUNT-derived placement among 951 US counties with at least 100 monitored days in 2024. Contra Costa County's Good-day share is 53.5% - cleaner than 7% of that comparison set.
Contra Costa County Good-day share vs. every US county
Share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release
53.5% 7th percentile higher than 7% of 951 US counties
Each bar is a band; taller bars hold more US counties. The dashed line + filled bar mark this entry. Hover or tap any bar for its full count, share, and where it sits relative to this entry.
Source U.S. Environmental Protection Agency, Air Quality System (AQS) · 2024
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%
2024 daily-monitor rhythm in Contra Costa County
Month-by-month median AQI from EPA day-level county monitors (361 reported days in 2024). Jun posted the highest typical readings (median AQI 61, peak 75); Mar was cleanest at median 29. Summer months (Jun–Aug) averaged median AQI 56 vs 47 in Dec–Feb.
Five-year seasonal record
EPA day-level county files · 2020–20241,822 daily readings show a recurring local pattern: Sep has the highest matched-month median AQI (59), while Mar is lowest (39). Each cell is that month’s median daily AQI, not a current-air forecast.
Lower AQI is better. The colors summarize EPA AQI bands; gaps mean the county had no day-level record for that month.
Health Impact Assessment
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
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
Download this county's annual AQI row as CSV , with the same EPA fields and denominator shown on this page.
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.