Air Quality Index – A Guide to Your Health

Why is air quality important?

Local air quality affects how you live and breathe. Like the weather, it can change from day to day or even hour to hour. The U.S. Environmental Protection Agency (EPA) and your local air quality agency have been working to make information about outdoor air quality as easy to find and understand as weather forecasts. A key tool in this effort is the Air Quality Index, or AQI. EPA and local officials use the AQI to provide simple information about your local air quality, how unhealthy air may affect you, and how you can protect your health.

What is the AQI?

The AQI is an index for reporting daily air quality. It tells you how clean or unhealthy your air is, and what associated health effects might be a concern. The AQI focuses on health effects you may experience within a few hours or days after breathing unhealthy air. The AQI is calculated for four major air pollutants regulated by the Clean Air Act: ground level ozone, particle pollution, carbon monoxide, and sulfur dioxide. For each of these pollutants, EPA has established national air quality standards to protect public health. EPA is currently reviewing the national air quality standard for nitrogen dioxide. If the standard is revised, the AQI will be revised as well.

How does the AQI work?

Think of the AQI as a yardstick that runs from 0 to 500. The higher the AQI value, the greater the level of air pollution and the greater the health concern. For example, an AQI value of 50 represents good air quality with little or no potential to affect public health, while an AQI value over 300 represents air quality so hazardous that everyone may experience serious effects.


An AQI value of 100 generally corresponds to the national air quality standard for the pollutant, which is the level EPA has set to protect public health. AQI values at or below 100 are generally thought of as satisfactory. When AQI values are above 100, air quality is considered to be unhealthy—at first for certain sensitive groups of people, then for everyone as AQI values increase.


1Due to the normal aging process, older adults may experience increased health risks from exposure to unhealthy air. Studies indicate that some people become more sensitive in their mid-60s. However, the risk of heart attacks, and thus the risk from particle pollution, may begin as early as the mid-40s for men and mid-50s for women.

How is a community’s AQI calculated and reported?

Each day, monitors record concentrations of the major pollutants at more than a thousand locations across the country. These raw measurements are converted into a separate AQI value for each pollutant (ground-level ozone, particle pollution, carbon monoxide, and sulfur dioxide) using standard formulas developed by EPA. The highest of these AQI values is reported as the AQI value for that day. 2
In large cities (more than 350,000 people), state and local agencies are required to report the AQI to the public daily. Many smaller communities also report the AQI as a public health service.


When the AQI is above 100, agencies must also report which groups, such as children or people with asthma or heart disease, may be sensitive to that pollutant. If two or more pollutants have AQI values above 100 on a given day, agencies must report all the groups that are sensitive to those pollutants. For example, if a community’s AQI is 130 for ozone and 101 for particle pollution, the AQI value for that day would be announced as 130 for ozone. The announcements would note that particle pollution levels were also high and would alert groups sensitive to ozone or particle pollution about how to protect their health.


Many cities also provide forecasts for the next day’s AQI. These forecasts help local residents protect their health by alerting them to plan their strenuous outdoor activities for a time when air quality is better.


2For more information on how the AQI is calculated, see “Technical Assistance Document for the Reporting of Daily Air Quality—the Air Quality Index (AQI)” in the “Publications” section of www.airnow.gov.


The AQI is a national index, so the values and colors used to show local air quality and the levels of health concern are the same everywhere in the United States.

Checking local air quality is as easy as checking the weather. You can find the latest AQI values on the Internet, in your local media, and on many state and local telephone hotlines. You can also sign up to receive AQI forecasts by e-mail:

  • AQI on the Internet. EPA and its federal, tribal, state, and local partners have developed an AirNow Web site to provide the public with easy access to national air quality information. On the web site, you will find daily AQI forecasts and real-time AQI conditions for over 300 cities across the United States, with links to more detailed state and local air quality Web sites. AIRNow’s reports are displayed as maps you can use to quickly determine if the air quality is unhealthy near you.
  • AQI via e-mail. Sign up for EnviroFlash (www.enviroflash.info), a free service that will alert you via e-mail when air quality is forecast to be a concern in your area.
  • AQI in the media. Many local media—television, radio, and newspapers—and some national media (such as USA Today, The Weather Channel, and CNN) provide daily air quality reports, often as part of the weather forecast.

Here’s the type of report you might hear:

Tomorrow will be a code red air quality day for Center City. The cold winter air, morning traffic, and wood smoke are expected to cause particle pollution to rise to unhealthy levels. People with heart or lung disease, older adults, and children should avoid prolonged or heavy physical activities.

What are typical AQI values in most communities?

In many U.S. communities, AQI values are usually below 100, with higher values occurring just a few times a year. Larger cities typically have more air pollution than smaller cities, so their AQI values may exceed 100 more often. AQI values higher than 200 are infrequent, and AQI values above 300 are extremely rare—they generally occur only during events such as forest fires. You can compare the air quality of U.S. cities and find out about quality trends in your area by visiting “AirCompare” at www.epa.gov/aircompare/.


AQI values can vary from one season to another. In winter, carbon monoxide may be high in some areas because cold weather makes it difficult for car emission control systems to operate effectively. Ozone is often higher in warmer months, because heat and sunlight increase ozone formation. Particle pollution can be elevated any time of the year.


AQI values also can vary depending on the time of day. Ozone levels often peak in the afternoon to early evening. Carbon monoxide may be a problem during morning or evening rush hours. And particle pollution can be high any time of day, and is often elevated near busy roadways, especially during morning or evening rush hours.

How can I avoid being exposed to unhealthy air?

You can take simple steps to reduce your exposure to unhealthy air. First, you need to find out whether AQI levels are a concern in your area. You can do this, as described previously, by visiting the AIRNow Web site, signing up for EnviroFlash, or checking your local media. If the AQI for ozone, particle pollution, carbon monoxide, or sulfur dioxide is a concern in your area, you can learn what steps to take to protect your health by checking the charts on the following pages. Two important terms you will need to understand are:

  • Prolonged exertion. This means any outdoor activity that you’ll be doing intermittently for several hours and that makes you breathe slightly harder than normal. A good example of this is working in the yard for part of a day. When air quality is unhealthy, you can protect your health by reducing how much time you spend on this type of activity.
  • Heavy exertion. This means intense outdoor activities that cause you to breathe hard. When air quality is unhealthy, you can protect your health by reducing how much time you spend on this type of activity, or by substituting a less intense activity—for example, go for a walk instead of a jog. Be sure to reduce your activity level if you experience any unusual coughing, chest discomfort, wheezing, breathing difficulty, or unusual fatigue.