Health effects . . . learn more
Behind the Colors & Numbers
How we determine healthy vs. unhealthy air
The colors are based on the Air Quality Index (AQI). The AQI is calculated from the pollution level and the pollutant standards. Pollutants have different standards, but the AQI is a number that allows levels of different pollutants to be put on the same scale.
Think of the AQI as a yardstick that runs from 0 to 500. The higher the AQI value, the greater the level of air pollution and the greater the health concern. For example, an AQI value of 50 represents good air quality with little potential to affect public health.
An AQI value of 100 generally corresponds to the national air quality standard for the pollutant, which is the level US EPA has set to protect public health. AQI values below 100 are generally thought of as satisfactory. When AQI values are above 100, air quality is considered to be unhealthy at first for certain sensitive groups of people, then for everyone as AQI values get higher.
For example: an AQI of 100 for ozone corresponds to an ozone level of 0.075 parts per million (averaged over 8 hours). An AQI of 100 for PM2.5 (particles up to 2.5 micrometers in diameter) corresponds to a level of 35 micrograms per cubic meter (averaged over 24 hours).
Source: USEPA . . . learn more