Air Quality Index (AQI)
Because different nations employ different air quality scoring systems, you should refer to materials for individual countries when assessing the meaning of an AQI score. Many nations used colored coded air quality score indicators to promote ease of public interpretation of these scores. In those schemes, green is generally selected to indicate good air quality, though in some nations green is used for “excellent” and yellow for “good.” Yellow generally indicates that sensitive groups may experience health problems due to poor air quality levels. Orange generally indicates more serious health concerns for at risk populations and the initiation of general health effects for the public. Red typically indicates when air pollution has reached a level where it has general population effects. Purple and brown colors are generally reserved to indicate the two most serious air quality conditions and serious health effects for the population in general.
In the US, air quality data is reported on the AIRNOW network, and the cities with the greatest outdoor air quality risks are listed here as well (see AIRNOW).
For air quality for other cities see the Pollution Standard Index entry in this dictionary. Scroll to the bottom of that entry for links to air quality scores for cities around the world (P S I).