Is the Air Quality Good in Contra Costa, California?
Mostly — air quality in Contra Costa, California is fair, not pristine. The city earns a Grade of C (fair) on a 5-year median AQI of 48 (Good), with 31 unhealthy-air days over five years (about 6 per year). Healthy adults are fine most of the time, but sensitive groups should watch the daily forecast.
Who Can Safely Breathe the Air in Contra Costa, California?
Healthy adults can continue normal outdoor activity in most weather, but should pay attention to AQI alerts during the worst pollution windows. People with asthma, heart disease, or pregnancy should reduce prolonged or intense outdoor exertion on flagged days, and consider running an indoor HEPA air cleaner during peak season. Because PM2.5 penetrates deep into the lungs and bloodstream, an N95 or KN95 mask provides meaningful protection on smoky or high-particulate days — surgical masks do not.
Across the past five years, this area has logged 31 days where AQI rose into the "Unhealthy for Sensitive Groups" range or worse — about 6 days per year. That is roughly typical for a U.S. metro, with most caution days clustered in summer (ozone) or wildfire season.
Contra Costa, California Air Quality Snapshot
| Air Quality Grade | C59/100 |
| 5-Year Median AQI | 48 (Good) |
| Most Recent Median AQI (2023) | 51 (Moderate) |
| Dominant Pollutant | Fine Particulate Matter (PM2.5) |
| 10-Year Trend | Stable (-0.12 AQI/yr) |
| Unhealthy Days (last 5 yr) | 31 |
| National Rank (cleanest = #1) | #880 of 1,020 (86th most polluted percentile) |
| California Rank | #28 of 53 |
What Does the C Grade Mean?
Contra Costa, California earns a C — air quality is fair, but not great. With a 5-year median AQI of 48, the city sees a meaningful number of "Moderate" days each year, when the EPA flags air as a concern for unusually sensitive people.
Contra Costa, California's 5-year median AQI of 48 is 7 points above the national average of 41 — meaningfully more polluted than the typical U.S. metro tracked here. Within California, Contra Costa, California's air quality is roughly typical for the state, where the average city posts a 5-year median AQI of 49.
For context within California: Humboldt, California currently holds the state's cleanest grade (A, AQI 28), while Inyo, California sits at the bottom (F, AQI 57).
What's in Contra Costa, California's Air?
The dominant pollutant in Contra Costa, California is Fine Particulate Matter (PM2.5). Fine particulate matter — particles less than 2.5 micrometers across — comes mostly from combustion: vehicle exhaust, wildfire smoke, residential wood burning, and industrial emissions. Because these particles are small enough to enter the bloodstream, PM2.5 is the pollutant most strongly linked to cardiovascular disease, respiratory illness, and premature death.
Days by Dominant Pollutant (2023)
| Pollutant | Days as Dominant | Share of Year |
|---|---|---|
| Fine Particulate Matter (PM2.5) | 270 | 74% |
| Ground-Level Ozone | 94 | 26% |
| Nitrogen Dioxide | 1 | 0% |
Is the Air Getting Better or Worse?
Air quality in Contra Costa, California has held roughly steady over the past decade, with year-to-year shifts in median AQI of less than half a point. That stability makes the city's long-run grade a reliable signal of what residents can expect day-to-day.
In 2014, Contra Costa, California posted a median AQI of 53. By 2023 that figure was 51 — a drop of 2 AQI points cleaner across 10 years of EPA records.
Year-by-Year AQI in Contra Costa, California
| Year | Median AQI | Good Days | Unhealthy Days | Dominant Pollutant |
|---|---|---|---|---|
| 2014 | 53 | 150 | 5 | PM2.5 |
| 2015 | 48 | 193 | 8 | PM2.5 |
| 2016 | 46 | 210 | 3 | PM2.5 |
| 2017 | 54 | 131 | 12 | PM2.5 |
| 2018 | 55 | 111 | 18 | PM2.5 |
| 2019 | 43 | 238 | 4 | PM2.5 |
| 2020 | 49 | 192 | 18 | PM2.5 |
| 2021 | 47 | 195 | 5 | PM2.5 |
| 2022 | 51 | 175 | 2 | PM2.5 |
| 2023 | 51 | 173 | 2 | PM2.5 |
How This Grade Is Calculated
The AirHistory Air Quality Grade combines four signals: the 5-year median AQI (40% of the score), the 10-year trend direction (30%), the count of unhealthy days per year (20%), and the dominant pollutant type (10%). All four come directly from the EPA Air Quality System (AQS), which aggregates readings from federally certified monitors. Read the full methodology.
More about Contra Costa, California
Contra Costa, California has an Air Quality Grade of C (fair) with a 5-year median AQI of 48. The dominant pollutant is Fine Particulate Matter (PM2.5), and air quality has been stable over the past decade.
The data source behind this answer is the EPA Air Quality System (AQS). Every figure on the page traces back to that source; the methodology page describes the inputs and the refresh cadence in full detail.
A practical caveat: the headline answer above reflects the most recent the EPA Air Quality System (AQS) vintage; underlying data is often revised for months after first publication, and the right reference for any specific decision is whichever vintage is current at the time of the decision. The as-of date is stamped on every page.
Source: EPA Outdoor Air Quality Data, 2026.