The 1977 amendments to the Clean Air Act called for daily analysis and reporting of air quality based on a uniform air quality index (see Section 319, "Air quality monitoring," https://www.epa.gov/air/caa/title3.html). The first index was called the Pollutant Standards Index (PSI). In 1999, a new health category and two pollutants were added, and the name was changed to the Air Quality Index (AQI).
For an overview of the current AQI, please see https://airnow.gov/aqi/aqi-basics/.
For more background, try the Federal Register notice on AQI Reporting, https://www.gpo.gov/fdsys/pkg/FR-1999-08-04/pdf/99-19433.pdf