AQI

The U.S. EPA developed the Air Quality Index, or AQI scale, to make the public health impacts of air pollution concentrations easily understandable.

Air Quality Index

The Air Quality Index, or AQI, much like an air quality "thermometer", translates daily air pollution concentrations into a number on a scale between 0 and 500. The numbers in this scale are divided into six color-coded ranges, with numbers 0-300 as seen below.

(0-50)
Good
No health impacts are expected when air quality is in this range.
(51-100)
Moderate
Unusually sensitive people should consider limiting prolonged outdoor exertion.
(101-150)
Unhealthy for Sensitive Groups
Active children and adults, and people with respiratory disease, such as asthma, should limit outdoor exertion.
(151-200)
Unhealthy
Active children and adults, and people with respiratory disease, such as asthma, should avoid prolonged outdoor exertion; everyone else, especially children, should limit prolonged outdoor exertion.
(201-300)
Very Unhealthy
Active children and adults, and people with respiratory disease, such as asthma, should avoid all outdoor exertion; everyone else, especially children, should limit outdoor exertion.
The AQI numbers refer to specific amounts of pollution in the air. It's based on the federal air quality standards for six major pollutants - ozone, carbon monoxide, nitrogen dioxide, sulfur dioxide, and two sizes of particulate matter.

In most cases, the federal standard for these air pollutants corresponds to the number 100 on the AQI chart. If the concentration of any of these pollutants rises above its respective standard, it can be unhealthy for the public.

When the Air District prepares its daily AQI forecast, we take the anticipated concentration measurements for each of the major pollutants, convert them into AQI numbers, and post the highest AQI number for each reporting zone.

Readings below 100 on the AQI scale should not affect the health of the general public (although readings in the moderate range of 50 to 100 may affect unusually sensitive people). Levels above 300 rarely occur in the United States, and readings above 200 have not occurred in the Bay Area in decades.

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