Understanding EPA Air Quality Index: Monitoring Seasonal Weather Effect on Quality of Air for Health Risk

Guide to Air Quality Index

The Air Quality Index (AQI) provides daily and local air quality data, monitored by the EPA, or United States Environmental Protection Agency. The AQI presents data on how clean or polluted the air is on a national level or for a given location. Air quality changes daily, even by the hour, so the EPA monitors the data closely. Monitoring air quality is important, especially for health concerns.

The Air Quality Index is calculated for five important air pollutants. These major air pollutants are regulated by the Clean Air Act, and the standard set by the EPA.

  • Ground level ozone
  • Particle pollution
  • Carbon monoxide
  • Sulfur dioxide
  • Nitrogen dioxide

Air Quality Index Values

Air quality data from monitoring stations nationwide is charted by value on the AQI. The AQI values run from 0-500, the higher the value meaning more air pollution and health risk. Each AQI category has been assigned a color, for easy identification. Air quality conditions are as follows

  • Good, 0-50 AQI value, signified by Green
  • Moderate, 51-100 AQI value, signified by Yellow
  • Unhealthy for Sensitive Groups, 101-150 AQI value, signified by Orange
  • Unhealthy, 151-200 AQI value, signified by Red
  • Very Unhealthy, 201-300 AQI value, signified by Purple
  • Hazardous, 301-500 AQI value, signified by Maroon

The condition “Unhealthy for Sensitive Groups” is important to note, as people with lung disease, heart disease or asthma “should avoid prolonged or heavy exertion outdoors” (EPA). People belonging to this “Sensitive Group” should pay attention to air quality reports made during local weather forecasts.


Accessing Air Quality Data Online

Airnow.gov provides a national overview and outlook for air quality, for conditions on ozone and particle pollution. This site is a group effort between the EPA, NOAA, NPS and tribal, state and local agencies, working together to provide reports on air quality. The Weather Channel also provides AQI data from the EPA. Air quality index values are reported by most meteorologists during local forecasts, combining weather conditions and air quality data. In some cases, the weather has a direct effect on air quality.

Seasonal Effect on Air Quality

According to the EPA’s publication A Guide to Air Quality and Your Health, “AQI values can vary from one season to another”. Air quality is affected by weather conditions and geographical location. During the summer, ozone air pollutants can be higher because of ozone forms with more heat and sunlight. Particle pollution can be higher during the winter when more wood is burned under clear weather conditions. With an absence of wind, particle pollutants can increase. Carbon monoxide pollutants can also be trapped close to the ground during colder weather.

The Wind is especially important in improving air quality. The Wind helps move pollutants away, keeping unsafe levels down. When inversion happens, it can trap pollutants close to the ground. For example, in California, when inversion happens, the inversion layer acts as a trap, preventing air near the ground from rising or dispersing. This keeps air pollutants trapped near the surface.

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