An air quality action day, sometimes called an air quality alert day, is put into effect when the Air Quality Index (AQI) reaches unhealthy ranges.
There are numerous air pollution control agencies across America, each having slightly different terminology for such days and using varying AQI levels when considering them.
According to AirNow.gov, action days are declared in some places when the AQI is predicted to be unhealthy for sensitive groups, or Code Orange (AQI value of 101 to 150). In this example, the groups that are sensitive to the dominant pollutant of the day should reduce exposure to it by limiting any time spent outdoors.
If ozone is the instigator for an air quality action day, the sensitive groups consist of children and adults who are active outdoors, as well as people with lung diseases such as asthma. For particle pollution, the sensitive groups would include children, older adults and anyone with heart or lung disease.
Occasionally, an action day might be declared when the AQI is forecast to be moderate, or Code Yellow (AQI value of 51 to 100), or if there’s a chance that levels could approach Code Orange levels.
Most commonly, according to AirNow.gov, air quality action days are put into effect when the AQI is expected to be unhealthy, or Code Red (AQI value of 151 to 200). In this case, everyone should reduce exposure to air pollution, especially the members of the sensitive groups for that day’s particular pollutant.
AirNow.gov provides several suggestions at this link on what you can do to help keep the air cleaner every day.