For the daily AQI forecast, the Air District takes the anticipated concentration measurements for each of the major pollutants, converts them into AQI numbers, and posts the highest AQI number for each reporting zone.
The specific pollutant listed in the forecast is associated with the highest AQI value for the day.
In the Bay Area, there are really only two or three pollutants that are present in concentrations high enough to show up in the daily forecast.
Ozone - Ground-level ozone, commonly called "smog," is the major summertime cause of poor air quality in the Bay Area. It's an unstable molecule made of three oxygen atoms. It is created in the lower atmosphere by a combination of pollutants called reactive organic compounds and oxides of nitrogen that react in the presence of sunlight.
At high concentrations, ozone can inflame the lungs and aggravate respiratory illnesses such as asthma. Long-term exposure to high levels of ozone has been linked with the development of asthma in children.
PM2.5 - PM2.5, or fine particle pollution, is a category of microscopically small mixtures of solid particles or liquid droplets that are emitted directly into the air or formed from secondary reactions involving gaseous pollutants that combine in the atmosphere. PM2.5 are particles 2.5 microns in size, or about 1/100 the diameter of a human hair.
PM2.5 is primarily a problem in the wintertime, when residential wood burning is the largest source. PM2.5 can also be elevated at other times in the year when wildfires or other fires affect the region.
PM2.5 is associated with a wide range of respiratory and cardiovascular health problems, and long-term exposure to PM2.5 is the leading air pollution-related cause of premature deaths in the Bay Area.
NO2 - Nitrogen dioxide is an oxide of nitrogen associated with the formation of ozone. It is primarily emitted from cars, trucks, trains, construction equipment, and other motor vehicles. It is only very rarely present in quantities that would create a health concern in the Bay Area.
It is also possible that other pollutants such as carbon monoxide or sulfur dioxide may show up on rare occasions in the Daily AQI Forecast, but these are rarely present in sufficient quantities to cause health concerns. in the Bay Area.