The San Francisco Bay Area is one of the most polluted metropolitan regions in the country for both ozone smog and year-round fine particle pollution, according to the American Lung Association’s 2025 State of the Air report, which graded counties on federal air quality data collected through 2023. The findings, released in spring 2025 and still shaping regulatory action into 2026, place several Bay Area counties alongside parts of Los Angeles and the Central Valley at the bottom of national rankings.
For the roughly 7.7 million people living across the nine-county region, the report underscores a troubling reality: after decades of gradual improvement, local air quality has stalled or worsened in recent years, even as the federal government has tightened the health benchmarks used to judge it.
Where the Bay Area stands in national rankings
The American Lung Association report ranks metropolitan areas by the number of unhealthy air days recorded at EPA monitors. The San Jose-San Francisco-Oakland combined statistical area received a failing grade for both ozone and short-term particle pollution, landing among the 25 worst metro areas in the nation for each category.
Within the region, individual counties fared poorly on specific measures. Alameda and Santa Clara counties each logged multiple days per year when ozone concentrations exceeded the federal 8-hour standard of 70 parts per billion. Contra Costa County, home to a cluster of oil refineries along the Carquinez Strait, recorded elevated fine particle readings as well. According to EPA design value data, several Bay Area monitors reported annual PM2.5 averages hovering near or above 10 micrograms per cubic meter in recent monitoring periods, a level that now triggers regulatory consequences under a stricter federal standard finalized in March 2024.
That revised standard, which lowered the acceptable annual PM2.5 threshold from 12.0 to 9.0 micrograms per cubic meter, effectively moved the goalposts. Counties that had technically been in compliance under the old limit now face formal nonattainment designations, which require state and local agencies to draft enforceable cleanup plans and can restrict new industrial permits.
Why the numbers have stopped improving
For most of the past half-century, Bay Area air quality followed the same trajectory as the rest of the country: steadily cleaner, driven by catalytic converters, cleaner fuels, and tighter smokestack regulations. The EPA’s national air quality trends report shows that aggregate U.S. emissions of the six major “criteria” pollutants fell by roughly 78% between 1970 and the early 2020s.
But that long arc of progress has bent backward in parts of California. Two forces are primarily responsible.
The first is wildfire smoke. The Bay Area experienced some of its worst air quality days on record during the 2020 fire season, when smoke from the August Complex and other blazes turned San Francisco skies orange and pushed daily PM2.5 readings above 200 micrograms per cubic meter in some locations. Severe fire seasons in 2021 and 2023 added to the toll. Because the EPA judges compliance using a three-year rolling average of monitor readings, consecutive smoky summers can push a county’s “design value” above the federal limit even if local tailpipe and industrial emissions are holding steady or declining.
The second factor is persistent ground-level ozone. Ozone forms when nitrogen oxides and volatile organic compounds react in sunlight, and the Bay Area’s mix of vehicle traffic, refinery operations, and warm inland valleys creates favorable conditions. Data from the California Air Resources Board’s ambient monitoring network show that ozone exceedance days in eastern Contra Costa and southern Santa Clara counties have not declined meaningfully in recent years, even as the vehicle fleet has grown cleaner on average.
Health stakes behind the numbers
Federal air quality standards are set based on extensive reviews of epidemiological research linking pollution exposure to illness and premature death. The EPA’s most recent Integrated Science Assessment for PM2.5, which informed the 2024 standard revision, found “causal” relationships between long-term fine particle exposure and cardiovascular disease, respiratory disease, and mortality. Short-term ozone exposure is linked to asthma attacks, reduced lung function, and increased emergency room visits, particularly among children, older adults, and people who work outdoors.
The American Lung Association estimates that more than 130 million Americans live in counties that received a failing grade for at least one pollutant in its 2025 report. In the Bay Area, that burden is not evenly distributed. Communities near highways, ports, and industrial corridors tend to experience higher concentrations than neighborhoods farther from emission sources. CalEnviroScreen, a state mapping tool that overlays pollution data with socioeconomic indicators, consistently flags census tracts in West Oakland, Richmond, and East San Jose as among the most environmentally burdened in California.
However, the regulatory monitoring network was not designed to capture block-by-block variation. County-level design values can mask significant differences in exposure within a single jurisdiction, a limitation that public health researchers and environmental justice advocates have long highlighted.
What regulators and residents can do now
Nonattainment designations under the revised PM2.5 standard are expected to be finalized by the EPA in 2026, and Bay Area counties are widely anticipated to be on the list. Once designated, the Bay Area Air Quality Management District and CARB will be required to develop State Implementation Plans detailing how the region will reach compliance. Those plans typically include tighter emission limits on refineries and industrial facilities, incentives for zero-emission vehicles and equipment, and enhanced monitoring near pollution hotspots.
At the individual level, residents can track real-time conditions through the EPA’s AirNow platform, which translates raw monitor readings into a color-coded Air Quality Index. The site also provides next-day forecasts that can help people plan outdoor exercise or limit exposure on high-pollution days. During wildfire season or on hot, stagnant afternoons when ozone spikes, public health officials recommend reducing prolonged outdoor exertion, running HEPA-rated air purifiers indoors, and keeping windows closed.
Gaps that still need answers
The available data make the Bay Area’s poor standing clear but leave important questions unresolved. Federal and state monitoring networks confirm that pollution levels challenge current health standards. They do not, on their own, quantify how much of the problem stems from local emissions versus transported wildfire smoke, or predict how quickly the region’s shift toward electric vehicles and cleaner industrial processes will bend the trend lines back downward.
Filling those gaps will require more granular monitoring, updated emissions inventories, and independent health studies that connect pollution exposure to outcomes in specific Bay Area communities. Until then, the regulatory record offers a baseline that is both useful and sobering: millions of residents are breathing air that the federal government has determined is not safe over the long term, and the region is running out of room to meet the standards meant to protect them.
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*This article was researched with the help of AI, with human editors creating the final content.
