“it’s an unhealthy air quality day” or “local air quality is unhealthy for sensitive groups”

"With summer upon us, we find ourselves in the midst of ozone season and we may be hearing phrases such as “it’s an unhealthy air quality day” or “local air quality is unhealthy for sensitive groups” on the radio and television. But what do these warnings mean and why should we care?"

How does the AQI work?
The AQI operates on a scale of 0 to 500, with higher AQI numbers corresponding to poorer air quality and greater the health concerns.  Typically, an AQI level of 100 corresponds to the National Ambient Air Quality Standard for that pollutant.  To simplify understanding of the AQI and the potential health effects, it has been broken up into six color-coded categories as follows:
Each day pollutant concentrations are measured, converted into AQI values, and reported for the day in that area.  The AQI can be found in numerous places, including: