Air-Quality Index (AQI) Scale

The hourly air quality readings shown on this site are based on a national system called the Air Quality Index (AQI):

  • The AQI compares pollutant levels to their health standards
  • It takes into account multiple pollutants
  • It assigns an air quality rating like "good" or "unhealthy."
The five pollutants used in the AQI are those for which there are national health standards: carbon monoxide, nitrogen dioxide, ground-level ozone, particulates and sulfur dioxide. U.S. EPA has also set a health standard for lead, but we don't report lead levels in near-real time or include them in the AQI because it takes several weeks or more to collect and analyze lead samples.

Comparing pollutant levels to their health standards

We calculate the AQI using data from our network of continuous air monitoring sites, where levels of some or all of the five pollutants are measured around the clock. The measurements are made in various units, such as "parts per million" or "micrograms (of pollutant) per cubic meter (of air)," and are averaged over different durations, depending on the individual pollutant's health standard.

A relatively small amount of a pollutant such as ozone can be unhealthy, while other pollutants such as carbon monoxide need to reach much higher concentrations to become unhealthy. The health standard for ozone is 0.08 parts per million, while the carbon monoxide health standard is 9 parts per million -- more than 100 times higher than for ozone.

The AQI puts each reading in the context of its health standard by converting it from its original units to the AQI scale, where the health standard for each pollutant equals 100. AQI values over 100 have exceeded the health standard (the exception is particulates - see explanation below). Lower AQI values mean less pollution.

Particulates: There is one exception to the rule that AQI levels over 100 mean a health standard has been exceeded. In the case of particulates smaller than 2.5 microns ("PM2.5"), U.S. EPA set an AQI of 101 to correspond to a reading of 41 micrograms per cubic meter (below the health standard of 65 micrograms per cubic meter), to be more protective of people with particular susceptibility. For PM2.5, AQI levels over 150 correspond to levels over the standard.

Taking into account multiple pollutants

The AQI also recognizes that we are breathing in more than one different kind of pollutant. Having all of the pollutants on a common scale lets you tell at a glance which one has the highest levels with respect to the health standards.

Assigning air quality ratings

We assign air quality ratings based on the AQI values: if a reading is above 100 AQI, air quality is rated "Unhealthy" (a health standard has been exceeded, except for particulates as noted above). When AQI values decrease, pollutant levels are going down and air quality ratings improve. As levels drop below the health standard, ratings progress from "Moderate" to "Good."

Green is used to show areas rated Good, yellow for Moderate, orange for the lower part of the Unhealthy range, and red for the upper part of the Unhealthy range, where health effects are likely to be more widespread among people who are exposed (not necessarily more widespread geographically).

The AQI scale is an example of an "environmental indicator" that provides an understandable index of environmental quality, and is more powerful and more realistic than looking at levels of just one or two pollutants.

Copyright © 2015 Micmac Environmental Health Department