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The Air Quality Index (AQI) is a standardized indicator of the air quality in a given location. It measures mainly ground-level ozone and particulates but may also include sulfur dioxide, and nitrogen dioxide. Various agencies around the world measure such indices, though definitions may change between places.

•   0-50  Good is usually green
•  51-100 Moderate is usually yellow
• 101-150 Unhealthy for sensitive groups is usually orange
• 151-200 Unhealthy is usually red
• 201-300 Very unhealthy is usually purple
• 301-500 Hazardous is usually maroon

The EPA's AQI 100 corresponds to 0.08 ppm ozone, and to other levels for other pollutants. Source: EPA