DISCUSSION: The purpose of the Environmental Protection Agency's (EPA) Air Quality Index (AQI) is to quickly alert people to general air quality conditions and provide guidance that they may need to limit their exposure to outside air. Specifically, the index takes into account observations of various air pollutants, including near-surface ozone, particulate matter (small and larger particles), carbon monoxide, sulfur dioxide, and nitrogen oxide. A calculation converts the observed concentration of each of the above six pollutants to an AQI value. The pollutant with the highest AQI is associated with the scale above where a smaller AQI value (~25) indicates good air quality that poses little risk. A higher value (up to 500) indicates hazardous conditions even for healthy people.
As described by Physics Today's Mika McKinnon here, the recent fires in California highlighted several issues with the AQI. First, the AQI doesn't specify which pollutant triggered the value given. The pollutant with the highest concentration is important for determining which groups are most at risk. For example, carbon monoxide is especially dangerous for those with a heart attack risk, while ozone is especially dangerous for the very young and old. Thus, the AQI as given doesn't specify which people are most at risk on a given day. In addition, how frequently the AQI is updated depends on the number of observations in an area. In remote locations with few observations, it can take 24 hours or longer for the AQI to update. In the case of California fires, there was visibly hazy, dangerous air, while the AQI indicated good air quality conditions before it eventually updated to a higher value. Obviously, an AQI that doesn't necessarily match current conditions could pose a problem.
The point of this article is not to discourage people from paying attention to the AQI. It does provide some indication of air quality conditions and actions that may need to be taken by certain groups of people. It is easy to read and understand. However, it is important to understand the limitations of this tool in order to use it most effectively and to spur further improvements of the tool.
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©2017 Meteorologist Dr. Ken Leppert II