What Is the Air Quality Index, and How Is It Calculated?
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What Is the Air Quality Index, and How Is It Calculated?

Air pollution is a significant problem across the United States. To help combat air pollution, the Environmental Protection Agency (EPA) developed the U.S. Air Quality Index (AQI).  The purpose of this index is to provide easy-to-understand, accessible information on daily air quality for U.S. residents to utilize. In this article, we will review the origins of the AQI, how AQI calculates air quality values, the AQI category system, the five major pollutants covered by AQI, U.S. cities with the worst and best AQIs, and how to use the AQI tool to protect yourself from air pollution.

History of AQI

The AQI was first introduced in 1968 by the National Air Pollution Control Administration, with the dual goals of drawing public attention to the air pollution crisis and call on public officials to help mitigate air pollution. In 1976, the EPA began issuing the national index for air quality, and the AQI we use today was released in 1999. Since 1999, the AQI has consistently been updated to reflect current health-based air quality standards. Additionally, the EPA offers historical AQI data dating back to 1980 for anyone interested in a snapshot of local air quality over time.

How AQI (and the Value Index) is Calculated

The AQI is calculated on a scoring system, ranging from a 0 value to a 500 value, with 0 being the best value possible and 500 the worst. These values are determined by the number of pollutants in the air. Lower AQI values have lower air pollution and lower associated health concerns, and higher AQI values have greater health pollution and greater associated health risks. An AQI value of around 50 or lower represents good air quality in the area, and an AQI value of 300 or more represents hazardous air quality. The value of 100 is considered to be the national air quality standard for the protection of public health, where values of 100 or less are typically considered satisfactory. People sensitive to poor air quality will be the first to feel the impacts of AQI values above 100, and if the values continue to rise the general population will experience negative health impacts as well.

AQI Categories

Each AQI has six categories associated with values 0 through 500. Green is considered good with values of 0 to 50, yellow is considered moderate with values of 51 to 100, orange is considered unhealthy for sensitive groups and has values of 101 to 150, red is unhealthy with values of 151 to 200, purple is very unhealthy at values of 201 to 300, and maroon is hazardous at values of 301 or higher.

Below is a standard AQI chart for Ozone and Particle Pollution:

These are the air quality index basics for ozone and particle pollution.
Image: AirNow.gov

The Five Pollutants with AQIs

There is an AQI available for each of the five main pollutants regulated by the Clean Air Act: ground-level ozone, particle pollution (also known as particulate matter), carbon monoxide, sulfur dioxide, and nitrogen dioxide. The EPA has implemented national, standard regulations on all of these air pollutants to protect the public from associated negative health impacts.

Cities with the Best and Worst AQI

While the U.S. does have significant air pollution, U.S. cities are not the most polluted in the world. Countries with the worst pollution, based on data from the U.S. AQI system, include Bangladesh, India, and Pakistan. Based on real-time AQI data averaged from all five pollutants, current U.S. cities with the worst air quality are West University Place, Texas with a 162 value, Rathdrum, Idaho with 161, and New Paltz, New York with 160. Current cities with some of the best air quality (with a value of 0) include Amesbury, Massachusetts, Anderson, Indiana, and Big Bend National Park, Texas.

Utilizing AQI in Daily Life

There are a few ways that you can easily check your local AQI. The first is using your preferred weather app, which should provide you with the Air Quality Index levels for your area for the day. If you want more detailed information, use the tool on the AQI website by simply entering your ZIP Code, City, or State into the toolbar. Once entered, a tool indicating the highest pollutant and current color, level of concern, and value will appear in your area. For example, a score of 47 for particle pollution was generated for the Dallas-Fort Worth area. This is the highest-scoring pollutant (and therefore the pollutant with the greatest threat to resident health) currently in the area. An ozone value was calculated as well (33), but no significant carbon monoxide, sulfur dioxide, or nitrogen dioxide values were calculated.

To protect yourself on days where values are 100 or more, try to limit time spent outside. If not possible to limit your time outdoors on heavily polluted days, invest in an N95 mask to wear while outside. The mask will help protect you from inhaling pollutants, particularly on dusty or hazy air pollution days. Avoid spending time in areas of higher pollution concentration levels, like highways and congested areas of large cities. Avoid exercising outdoors on heavily polluted days, and keep your windows and doors closed to limit outdoor air exposure.

While staying indoors during high air pollution days is important, it’s also imperative to maintain healthy indoor air quality (IAQ). This can be done by regularly cleaning your home, regularly replacing your air filters, and utilizing an air purifier in high-traffic rooms.

The AQI is a powerful tool that allows updated, daily insight into air quality for any city in the U.S. and is an excellent resource to understand local area air pollution health risks. Utilize this tool to better understand local air quality and to implement proactive measures to reduce exposure to unhealthy pollution levels.

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