Understanding u.s. air quality index

Tips on understanding the U.S. AQI


  • Good AQI does not mean zero risk of health effects will absolutely occur.
  • The AQI should not be used for low-cost sensor readings indoors or readings less than 1 hour.
  • The AQI of multiple pollutants should not be added together.
  • There are many reasons why an air quality index reported by the host country government differs from the U.S. AQI.


  • The AQI is based on scientific evidence, which has not identified a level of air pollution that has zero risk of health effects, The science describes how risk differs between higher and lower air pollution levels in a group of people and does not predict how any individual will be affected.
  • The U.S. AQI is based on the science for outdoor air pollution and longer periods of time.
  • The science is not clear whether the health risks are larger when multiple pollutants are high at the same time.
  • Different formulas or pollutants may be used to calculate the index. Also, pollutant levels can vary across locations in a city.

Key Facts about the U.S. AQI

Why It is useful

  • Describes how clean or polluted the air is in a consistent way across locations
  • Higher AQI means higher air pollution levels and higher concern for effects on health¬†
  • Describes possible health concerns for sensitive groups and everyone else¬†
  • Gives advice on decreasing time and activity level outdoors to lower exposure

How U.S. embassies & consulates report the U.S. AQI

  • Post report the U.S. Environmental Protection Agency’s (EPA) NowCast AQI to describe recent air quality.
  • The NowCast AQI is based on fine particles (PM2.5) or ozone levels averaged up to several hours. 
  • Pollutant levels are converted to an index value and put into a color-coded category: good to Hazardous. 
  • Good and Moderate means air pollution is below the standards that EPA has set to protect human health.
  • Good AQI poses low health risk.
  • Hazardous AQI poses high risk for everyone.
  • All posts report a NowCast AQI for PM2.5
  • Posts that measure ozone report a separate Nowcast AQI for ozone.
  • EPA also has a daily AQI based on daily levels of 5 pollutants: PM2.5, ozone, nitrogen dioxide, sulfur dioxide, carbon monoxide. The highest index value is reported.