When we see that air quality is “good” or “unhealthy”, those public health categories are derived from the Air Quality Index (AQI). In the U.S., the AQI is calculated using four major air pollutants regulated by the Clean Air Act:
- Ground-level ozone
- Carbon monoxide
- Sulfur dioxide
- Particle pollution, also known as particulate matter
Source: https://www.visualcapitalist.com/how-air-quality-index-works/