Grade F Air Quality Cities
4 cities with poor air quality
Grade F means poor air quality with persistent unhealthy conditions and worsening trends. These cities are scored using 10 years of EPA Air Quality System data, weighted across 5-year median AQI (40%), trend direction (30%), unhealthy days per year (20%), and worst-pollutant severity (10%).
What "Grade F" Actually Means
A Grade F means a 5-year median AQI well above the U.S. average (typically 95+), often combined with worsening trends and a very high count of unhealthy days. These are the most polluted cities tracked by EPA monitoring — places where the typical day, not just the worst day, falls into territory that matters for human health.
Everyone, in proportion to baseline risk. Sensitive groups face the highest incremental risk. Healthy adults will regularly feel effects (eye irritation, breathing difficulty during exercise) and should plan exposure carefully. People in active cancer treatment, recent surgery, or with compromised immunity should consult their physicians about a personalized exposure plan.
Grade F and Daily Life
On the worst days the EPA recommends everyone limit outdoor activity. Sensitive groups should treat AQI forecasts as essential input year-round. Outdoor workers should rotate tasks, take indoor breaks, and use N95/KN95 respirators when conditions warrant. Indoor air quality matters most when outdoor air is consistently bad: a HEPA air cleaner sized to your living space (bedroom + family room minimum) can cut indoor PM2.5 by 80%+ during pollution events. MERV-13+ HVAC filtration is standard equipment.
Long-Term Health Picture
Long-term residence in Grade F cities is associated with the largest measured air-quality health impacts in the U.S.: substantial increases in asthma incidence, lung cancer risk, cardiovascular mortality, and all-cause mortality. The Harvard Six Cities Study and follow-up research find life-expectancy reductions of 1-3 years for residents of the most polluted U.S. metros compared to the cleanest.
Where Grade F Cities Cluster
Grade F cities concentrate in the California Central Valley (the worst air in the country, year over year), parts of the Mountain West affected by chronic wildfire smoke, and a small number of concentrated industrial/freight zones. The 5-year median grade puts these places in the worst tier nationally.
Among the 4 Grade F cities tracked here, the largest concentrations are in CA (2), MX (1), AZ (1). The dominant pollutant in these cities is Ozone (3 cities), followed by PM2.5 (1).
All Grade F Cities
| City | State | 5yr Avg AQI | Trend | Worst Pollutant |
|---|---|---|---|---|
| San Bernardino, California | CA | 82 | Improving | Ozone |
| Inyo, California | CA | 57 | Worsening | Ozone |
| BAJA CALIFORNIA NORTE, Country Of Mexico | MX | 81 | Worsening | PM2.5 |
| Maricopa, Arizona | AZ | 90 | Worsening | Ozone |
Frequently Asked Questions
A Grade F means a 5-year median AQI well above the U.S. average (typically 95+), often combined with worsening trends and a very high count of unhealthy days. These are the most polluted cities tracked by EPA monitoring — places where the typical day, not just the worst day, falls into territory that matters for human health.
4 of 1,020 monitored US cities currently have a Grade F Air Quality rating, representing 0.4% of all tracked areas.
Everyone, in proportion to baseline risk. Sensitive groups face the highest incremental risk. Healthy adults will regularly feel effects (eye irritation, breathing difficulty during exercise) and should plan exposure carefully. People in active cancer treatment, recent surgery, or with compromised immunity should consult their physicians about a personalized exposure plan.
On the worst days the EPA recommends everyone limit outdoor activity. Sensitive groups should treat AQI forecasts as essential input year-round. Outdoor workers should rotate tasks, take indoor breaks, and use N95/KN95 respirators when conditions warrant. Indoor air quality matters most when outdoor air is consistently bad: a HEPA air cleaner sized to your living space (bedroom + family room minimum) can cut indoor PM2.5 by 80%+ during pollution events. MERV-13+ HVAC filtration is standard equipment.
Grade F cities concentrate in the California Central Valley (the worst air in the country, year over year), parts of the Mountain West affected by chronic wildfire smoke, and a small number of concentrated industrial/freight zones. The 5-year median grade puts these places in the worst tier nationally.
/methodology
The this entity category groups every U.S. air quality and pollution monitoring entity sharing this attribute. The list above is the data; the paragraphs below explain what the grouping means against the broader the EPA Air Quality System (AQS) distribution and how to read the relative rankings within the category.
For readers using this category as a starting point, the per-entity detail pages linked from the table above carry the underlying the EPA Air Quality System (AQS) data in full. The category-level view is the filter; the per-entity pages are the actual answer.
Source: EPA Outdoor Air Quality Data, 2026.