What Is the Air Quality Index?

Definition & Examples of the AQI

Woman wearing an N95 mask on a dusty, smoggy city street

 Phynart Studio / Getty Images

The air quality index (AQI) is what the Environmental Protection Agency (EPA) uses to measure the relative safety of the air we breathe in a given location on a given day. It tracks levels of various pollutants to come up with a score, which changes throughout the day.

Air quality alerts on TV or radio are familiar occurrences in many cities. This information is more significant for some people than others. Learn what these ratings mean and when they should matter to you.

What Is the Air Quality Index?

The air quality index is an index used to report the current condition of the air in terms of its threat to our health. It provides people with a snapshot of how clean or how polluted that air is, as well as associated health risks.

Checking the AQI is a frequent practice in some cities and during certain seasons or weather conditions. AQI helps us better plan our outdoor activities.

The U.S. Environmental Protection Agency (EPA) tracks five major air pollutants regulated by the Clean Air Act. These pollutants include ground-level ozone, particle or particulate matter pollution, carbon monoxide, sulfur dioxide, and nitrogen dioxide. To help protect public health, the EPA has established national air quality standards for each of the five toxin categories. Among these pollutants, ground-level ozone and airborne particles represent the greatest risks to human health in the United States.

  • Acronym: AQI

How the Air Quality Index Works

AQI values run from 0 to 500. The higher the number, the greater the level of air pollution, which brings increased risk to human health. Within this range, the AQI is divided into six different categories, spanning from “Good” to “Hazardous.”

Air Quality Index Levels
Range Quality Level Description
0-50 Good Air quality is considered satisfactory, posing little or no risk.
51-100 Moderate Air quality is acceptable; however, for some pollutants, there may be a moderate health concern for a very small number of people. For example, people who are unusually sensitive to ozone may experience respiratory symptoms.
101-150 Unhealthy for Sensitive Groups Although the general public is not likely to be affected at this AQI range, people with lung disease, older adults, and children are at a greater risk from exposure to ozone, whereas persons with heart and lung disease, older adults, and children are at greater risk from the presence of particles in the air.
151-200 Unhealthy Everyone may begin to experience some adverse health effects, and members of the sensitive groups may experience more serious effects.
201-300 Very Unhealthy This would trigger a health alert signifying that everyone may experience more serious health effects.
301+ Hazardous This would set off a health warning of emergency conditions. The entire population is more likely to be affected.
Source: AirNow.gov

AQI also operates on a six-color system that corresponds to the six danger categories. Colors range from green (good) and yellow (moderate) through orange (unhealthy for sensitive groups), red (unhealthy), purple (very unhealthy), and maroon (dangerous).

There are monitors at more than 1,000 locations across the U.S. recording air pollutant levels every day. These values are converted to a score for each of the five pollutants, and the highest of these scores is reported as the AQI level for that area on that day.

Why Air Quality Is Important

Air pollution presents a serious health risk. It is associated with stroke, heart disease, lung cancer, and both chronic and acute respiratory diseases, including asthma. Around the world, 91% of people lived in places that did not meet the World Health Organization's air quality guideline levels in 2016, which resulted in an estimated 4.2 million premature deaths.

Local air quality can seriously affect your immediate and long-term health. And, like the weather, the EPA notes, it can fluctuate frequently. The EPA uses the AQI to make air quality easy for the public to access and understand.

You will want to take notice if you are active outdoors, or have children who play outside. Likewise, if you have heart or lung disease, or you are a senior, a failure to pay attention to air quality could pose substantial immediate health risks.

Typical AQI Levels

According to the EPA, AQI values are usually below 100, with higher values registered just a handful of times annually. Major cities usually suffer more air pollution than smaller ones, and as a result, may exceed 100 more frequently. Values over 200 are infrequent, and those over 300 are extremely rare. Such readings may occur during a forest fire, for example.

Air quality can also differ seasonally, as well as locally. For example, ozone values may be higher during warm months as sunlight and heat boost ozone formation. Carbon monoxide, on the other hand, may be higher during cold weather, which can restrict the efficiency of car emission control systems. Particle pollution can be more intense near high-traffic thoroughfares, especially during peak daily traffic.

It is unfortunate that the AQI is needed. It is an important tool, however, and health risks associated with poor air quality can be mitigated through its use.

Key Takeaways

  • The air quality index (AQI) is a metric following five major pollutants to report the relative danger the air poses to our health.
  • These data are collected and reported each day in cities all over the U.S.
  • AQI helps all people, and especially those with sensitive health conditions, to assess the risks of being outside ant any given time.