This information is taken from Amercian Lung Association website, www.LungsUSA.org.
They are from the Air Quality Index, shown at right. We compare the monitored data against the Air Quality Index, a color-coded scale that EPA developed to help the public understand daily air pollution forecasts and protect themselves. Each color provides a specific warning about the risk associated with air pollution in that range.
In our analysis of ozone and short-term levels of particle pollution, we assign increasing weights to the days when air pollution levels reach the higher ranges to calculate our grades. We add those together and calculate the weighted average, then assign grades based on that weighted average. For year-round levels of particle pollution, we use annual average levels calculated by EPA.