What is the outdoor Air Quality Index (AQI) and what does each of the colors mean?

The Air Quality Index (AQI) is the system used to warn the public when the air pollution is dangerous. The AQI is divided into six color code categories and advice is given to go along with it :

PM2.5Air Quality IndexPM2.5 Health EffectsPrecautionary Actions
0 to 12.0>Good
0 to 50
Little to no risk.None.
12.1 to 35.4Moderate
51 to 100
Unusually sensitive individuals may experience respiratory symptoms.Unusually sensitive people should consider reducing prolonged or heavy exertion.
35.5 to 55.4Unhealthy for Sensitive Groups
101 to 150
Increasing likelihood of respiratory symptoms in sensitive individuals, aggravation of heart or lung disease and premature mortality in persons with cardiopulmonary disease and the elderly.People with respiratory or heart disease, the elderly and children should limit prolonged exertion.
55.5 to 150.4Unhealthy
151 to 200
Increased aggravation of heart or lung disease and premature mortality in persons with cardiopulmonary disease and the elderly; increased respiratory effects in general population.People with respiratory or heart disease, the elderly and children should avoid prolonged exertion; everyone else should limit prolonged exertion.
150.5 to 250.4Very Unhealthy
201 to 300
Significant aggravation of heart or lung disease and premature mortality in persons with cardiopulmonary disease and the elderly; significant increase in respiratory effects in general population.People with respiratory or heart disease, the elderly and children should avoid any outdoor activity; everyone else should avoid prolonged exertion.
250.5 to 500.4Hazardous
301 to 500
Serious aggravation of heart or lung disease and premature mortality in persons with cardiopulmonary disease and the elderly; serious risk of respiratory effects in general population.Everyone should avoid any outdoor exertion; people with respiratory or heart disease, the elderly and children should remain indoors.

• Green : Good. No advisories.
• Yellow : Moderate. Unusually sensitive individuals should consider limiting outdoor exertion.
• Orange : Unhealthy for sensitive groups. Children, active adults and people with respiratory disease, such as asthma, should limit outdoor exertion.
• Red : Unhealthy. Children, active adults and people with respiratory disease, such as asthma, should avoid outdoor exertion; all others should limit prolonged outdoor exertion.
• Purple : Very Unhealthy. Children, active adults and people with respiratory disease, such as asthma, should avoid any and all outdoor exertion; all others should limit outdoor exertion.
• Maroon : Hazardous. Everyone should avoid all physical activity outdoors.