AQI stands for Air Quality Index, and it is the number Bay Area residents increasingly rely on to decide whether to open windows, exercise outdoors, or pick up masks before a wildfire smoke event. But the number itself is often misunderstood. An AQI of 100 is not twice as bad as 50. An AQI of 200 does not mean the air is four times worse than 50. And an AQI of 50 does not mean the air is safe for everyone. How AQI is calculated, what the color categories actually mean, and how Bay Area air quality relates to both weather and health are the foundation for making good decisions during the wildfire smoke events and winter inversion days that are increasingly part of life in the region.
How AQI Is Calculated
The AQI is a standardized scale from 0 to 500, developed by the EPA to communicate air pollution levels in a consistent way across different pollutants. The scale covers six major pollutants: ground-level ozone, fine particulate matter (PM2.5), coarse particulate matter (PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. For any given measurement period, each pollutant is converted to its own AQI value, and the overall AQI is the highest of these individual values.
In the Bay Area, the pollutants that most frequently drive elevated AQI readings are PM2.5 and ground-level ozone. During wildfire smoke events, PM2.5 is almost always the controlling pollutant, sometimes to extremes. During summer, ozone can drive AQI in the inland valleys on hot afternoons when sunlight converts vehicle emissions to smog. Winter inversion days typically feature elevated PM2.5 from wood burning and vehicle emissions trapped under the inversion layer.
AQI Categories and What They Mean
The AQI scale uses six color-coded categories that communicate both the pollution level and the health implications. Green (0-50, Good) means air quality poses little or no risk. Yellow (51-100, Moderate) is acceptable for most people, though some unusually sensitive individuals may experience symptoms from ozone. Orange (101-150, Unhealthy for Sensitive Groups) means people with heart disease, lung disease, older adults, and children may experience health effects. Red (151-200, Unhealthy) means everyone may begin to experience health effects. Purple (201-300, Very Unhealthy) means health warnings for everyone, with serious effects for sensitive groups. Maroon (301-500, Hazardous) means health emergency conditions, with everyone likely to be seriously affected.
For most Bay Area days outside of wildfire events, AQI readings fall in the Good to Moderate range (0-100). Wildfire smoke events can push AQI to Very Unhealthy or Hazardous levels within hours; the Camp Fire in November 2018 pushed Bay Area AQI above 200 for multiple consecutive days, with some inland stations recording above 300. Winter inversion days with heavy wood burning typically reach the Unhealthy for Sensitive Groups range (101-150).
PM2.5: The Most Important Bay Area Pollutant
Fine particulate matter with a diameter of 2.5 micrometers or less, PM2.5, is the most health-relevant pollutant for Bay Area residents in most real-world situations. Particles at this size penetrate deep into the lungs and can enter the bloodstream. The health effects of PM2.5 exposure include aggravated asthma, decreased lung function, increased respiratory and cardiovascular hospitalization, and in severe long-term exposure, premature death. Children, older adults, and people with existing cardiopulmonary conditions are most vulnerable.
The Bay Area's two main PM2.5 sources are wood burning (in winter, during inversion events) and wildfire smoke (in summer and fall). Vehicle emissions also contribute to PM2.5 but are a smaller fraction than in many other urban areas. The Bay Area Air Quality Management District's real-time AQI monitoring network covers dozens of locations throughout the region, and AQI readings can vary significantly between monitoring stations; a coastal station may read 30 while an inland station on the same day reads 120.