Definition: Number of days per year with ground-level ozone concentrations above 0.070 parts per million (e.g., in 2023, unhealthy ozone concentrations were recorded on 70 days in Los Angeles County).
Data Source: California Air Resources Board, custom tabulation (Oct. 2020) & iADAM: Air Quality Data Statistics (Mar. 2025).
Footnote: Ground-level ozone is formed from pollutants emitted from vehicles, power plants, industrial facilities, and other sources. The national ambient air quality standard for ozone is 0.070 parts per million (ppm); concentrations above 0.070 ppm are considered unhealthy, especially for sensitive groups such as children, people with asthma, and older adults. Ozone concentrations are measured and averaged over 8-hour testing periods; then, the number of days per year exceeding the standard is calculated. For counties with more than one monitoring site, data are reported for sites recording the most days above 0.070 ppm. Data are limited because (i) monitoring stations are usually in urban areas, and (ii) ozone 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.
