Definition: Annual average concentration of fine particulate matter in the air (e.g., in 2023, the average concentration of fine particulate matter in the air in Los Angeles County was 11.1 micrograms per cubic meter).
Data Source: California Air Resources Board, custom tabulation (Oct. 2020) & iADAM: Air Quality Data Statistics (Mar. 2025); U.S. Environmental Protection Agency, Particulate Matter (PM2.5) Trends (Mar. 2025).
Footnote: 'Fine particulate matter' (PM2.5) refers to inhalable particles with a diameter of less than 2.5 microns. Most PM2.5 is formed from chemicals emitted from vehicles, power plants, and industrial facilities. The national primary ambient air quality standard for PM2.5 is 9 micrograms per cubic meter; annual concentrations above the primary standard are considered unhealthy, especially for sensitive groups such as children, people with asthma, and older adults. Data are limited because (i) monitoring stations are usually in urban areas, and (ii) PM2.5 samples are taken every three days or during times of the year when air pollution is very high. In addition, not all counties are monitored, and data are only provided for counties with monitors that meet completeness criteria. County-level data for 2019 and earlier years—annotated here with an asterisk (*)—may not represent finalized values; visit iADAM for the most up-to-date data. State-level data are averaged from county-level data and should be treated with caution. N/A means that data are not available.
