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
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"
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.