San Diego, California vs San Francisco, California Air Quality
Side-by-side air quality comparison using 10 years of EPA monitoring data. San Francisco, California has the edge with an Air Quality Grade of B (70/100).
| Metric | San Diego, California | San Francisco, California |
|---|---|---|
| Air Quality Grade | D (36/100) | B (70/100) |
| Current Median AQI | 67 (Moderate) | 32 (Good) |
| 5-Year Average AQI | 67 | 38 |
| 10-Year Trend | → Stable (+2) | ↓ Improving (-7) |
| Unhealthy Days/Year | 32 | 2 |
| Primary Pollutant | Fine Particulate Matter (PM2.5) | Fine Particulate Matter (PM2.5) |
Side-by-Side Analysis
San Francisco, California outperforms San Diego, California on overall air quality with a Grade B (70/100) versus D (36/100). San Francisco, California's 5-year median AQI of 38 sits in the "Good" range, while San Diego, California averages 67 ("Moderate") — a 30-point gap that shows up consistently in year-over-year readings, not just in a single year.
The two cities are moving in opposite directions: San Diego, California is stable (+0.1 AQI/yr) while San Francisco, California is improving (-0.6 AQI/yr). Over time, today's ranking may flip if these trends hold.
What's in the Air
Both cities share the same dominant pollutant: Fine Particulate Matter (PM2.5). These cities' dominant issue is fine particulate matter — typically driven by combustion (vehicles, wildfire smoke, industry, residential wood burning). PM2.5 is the air pollutant most strongly linked to long-term cardiovascular and respiratory disease because the particles penetrate the bloodstream.
Health Implications
Over a 5-year window, San Diego, California averages roughly 32 unhealthy air days per year (AQI above 100, where sensitive groups should limit outdoor exertion) versus 2 for San Francisco, California. That 30-day gap matters most for residents with asthma, COPD, heart disease, or pregnancy — and for outdoor workers, who accumulate the most cumulative exposure. The city with fewer unhealthy days offers a meaningfully different baseline risk picture for sensitive populations. For long-term residents, the cleaner-air city is associated with measurably better outcomes on respiratory disease, cardiovascular events, and life expectancy — Harvard cohort research consistently finds 0.5 to 1.0 years of additional life expectancy for each 10-µg/m³ reduction in long-term PM2.5 exposure.
Frequently Asked Questions
San Francisco, California has better air quality with a Grade B (70/100) compared to San Diego, California's Grade D (36/100). San Francisco, California has a current median AQI of 32 and is improving over the past decade.
San Diego, California averages 32 unhealthy air days per year (5-year average), while San Francisco, California averages 2. Unhealthy days are those when AQI exceeds 100 and sensitive groups should limit outdoor activity.
San Diego, California's primary pollutant is Fine Particulate Matter (PM2.5), while San Francisco, California's is Fine Particulate Matter (PM2.5). Both cities share the same dominant pollutant.
Source: EPA Outdoor Air Quality Data, 2026.
Comparing entity A and entity B on U.S. air quality and AQI history requires lining up the underlying EPA Air Quality System data data side by side. The table above runs the comparison on the canonical fields; the narrative below identifies the factor or factors that drive the most meaningful difference between the two.
Practical use of the comparison: read the data above, then drill into the individual entity A and entity B detail pages for the underlying breakdown. A pairwise comparison answers the relative question; the per-entity pages answer the absolute question.