Air Quality in California

California earns an average Air Quality Grade of C, with a 5-year median AQI of 49 across 53 monitored areas — 8 points above the national average of 41. The grade combines four signals — 5-year median AQI, 10-year trend direction, count of unhealthy days per year, and dominant pollutant — into a single A-F score. California's 53 monitored areas collectively logged 6,581 days at "Unhealthy for Sensitive Groups" or worse over the last five years.

California is on a clear improving trajectory: 38 of 53 monitored areas are showing measurably cleaner air over the past decade, versus only 9 that are getting worse. That mirrors the broader national pattern of falling particulate and ozone pollution as cleaner vehicles, cleaner power generation, and tighter industrial standards take effect.

The dominant pollutant across 27 of 53 California areas is Ground-Level Ozone. Ground-level ozone forms when sunlight reacts with vehicle and industrial emissions. It is worst on hot, sunny, stagnant summer days. Ozone irritates the lungs and triggers asthma — even healthy adults can feel it after exercising on high-ozone days. Other monitored areas in the state report Fine Particulate Matter (PM2.5) (26) as their dominant pollutant.

Within California, the gap between best and worst is meaningful: Humboldt, California tops the state with a Grade A and 5-year median AQI of 28, while Inyo, California sits at the bottom with a Grade F and 5-year median AQI of 57. Local terrain, prevailing winds, and proximity to industrial or wildfire emission sources drive most of that within-state variation.

Napa, California is the fastest-improving area in California, with median AQI falling by 2.3 points per year over the EPA reporting period. Steady improvement at that pace usually reflects fleet turnover (older diesels retiring), upwind power-plant retirements, and tighter local emissions controls.