RESEARCH STARTER
Air Quality Index (AQI)
The Air Quality Index (AQI) is a vital tool used to communicate the health risks associated with ambient air quality in specific areas at particular times. Developed by the U.S. Environmental Protection Agency (EPA), the AQI provides a standardized method for assessing air pollution levels, which has been adopted and adapted by various countries worldwide. Regular measurements of key air pollutants, such as particulate matter, ozone, nitrogen dioxide, sulfur dioxide, and carbon monoxide, allow authorities to assign a numerical value to air quality, reflecting its potential health impact.
The AQI uses a color-coded system to categorize air quality levels, with higher numbers indicating greater health risks. While the specific pollutants monitored can vary by region, the core methodology remains consistent globally. Monitoring typically focuses on urban areas where pollution levels are more pronounced. However, some countries, like Hong Kong, have faced criticism for having looser standards, leading to discrepancies between reported safe levels and those deemed acceptable by international health organizations like the World Health Organization. The AQI serves as an essential resource for the public, helping individuals make informed decisions regarding outdoor activities based on air quality conditions.
Authored By: Connell, Daniel J. 1 of 4
Published In: 2020 2 of 4
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Full Article
DEFINITION: The US Environmental Protection Agency’s tool for indicating the health risks posed by ambient air quality in given areas at particular times
With air pollution a growing problem the world over, the Air Quality Index and equivalent tools in other nations have become an indispensable part of regional and federal governments’ efforts to convey information on ambient air quality to the general public.
Nations around the world employ a number of variations on the Air Quality Index (AQI) developed by the US Environmental Protection Agency (EPA), but the basic methodology used to assess air quality is the same. The index assesses the most common pollutants, including suspended particulate matter, airborne lead, ground-level ozone, nitrogen dioxide, sulfur dioxide, and carbon monoxide, which impact basic air quality, although individual criteria vary from place to place. These hazardous compounds are capable of causing severe respiratory irritation, heart and circulatory problems, and other negative health effects.
The concentrations of these various air pollutants are measured regularly in testing areas, and, based on this information, the area is assigned an air quality number. Lower numbers mean cleaner air, while very high numbers signal hazardous conditions. The US EPA scale ranges from 0-500, with six categories and colors to communicate health risks: Good (0-50; Green), Moderate (51-100; Yellow), Unhealthy for Sensitive Groups (101-150; Orange), Unhealthy (151-200; Red), Very Unhealthy (201-300; Purple), and Hazardous (301+; Maroon). Values below 100 are generally acceptable, but readings above 100 and the air becomes unhealthy—first for sensitive groups and then, at higher levels, for everyone.
South Korea’s Comprehensive Air-quality Index (CAI) is similar to the US EPA’s Air Quality Index (AQI) in that both measure air pollution and communicate health risks to the public, but there are key differences. The CAI focuses on the highest concentration among pollutants, rather than averaging or calculating a value for each, as the US AQI does. Its numerical ranges and category thresholds also differ slightly, meaning that the same air quality could be rated differently on the CAI than on the US index.
South Korea, China, and some European countries utilize their indices to protect public health during high-pollution or “smog” events. For example, when air quality reaches unhealthy levels, governments may issue advisories urging people to stay indoors, limit outdoor activities, or wear protective masks. Schools and workplaces may adjust schedules or cancel outdoor events, and real-time updates are often provided through apps, websites, or public announcements to help citizens take timely precautions. These measures ensure that vulnerable populations, including children, older adults, and individuals with respiratory conditions, are better protected during periods of poor air quality.
Monitoring sites are typically limited to cities and towns, where pollution levels and population densities are both high. Different levels of pollution may fall into different tiers based on the stringency of a particular agency’s approach to measuring air quality. For example, Hong Kong’s Air Pollution Index has come under heavy criticism for its relatively lax standards, as the air quality it rates as safe sometimes contains pollutants at levels several times higher than those considered acceptable by the World Health Organization.
Bibliography
"Air Pollution: The Invisible Health Threat." World Health Organization, 12 July 2023, www.who.int/news-room/feature-stories/detail/air-pollution--the-invisible-health-threat. Accessed 4 Sept. 2025.
"Air Quality Index." American Lung Association, 2 Nov. 2023, www.lung.org/clean-air/outdoors/air-quality-index. Accessed 12 July 2024.
"Air Quality Index (AQI) Basics." AirNow, www.airnow.gov/aqi/aqi-basics/. Accessed 12 July 2024.
“AQI Basics.” AirNow.gov, www.airnow.gov/aqi/aqi-basics/. Accessed 4 Sept. 2025.
Hawkins, Amanda. “AQI Scales Around The World.” PurpleAir, 30 Nov. 2022, www2.purpleair.com/blogs/blog-home/aqi-scales-around-the-world. Accessed 5 Sept. 2025.
Hu, Shelia. "What Is the Air Quality Index?" Natural Resources Defense Council, 27 Sept. 2023, www.nrdc.org/stories/what-air-quality-index. Accessed 12 July 2024.
“Introduction to the CAI.” AirKorea, 2022, www.airkorea.or.kr/eng/khaiInfo?pMENU_NO=166. Accessed 5 Sept. 2025.
Full Article
DEFINITION: The US Environmental Protection Agency’s tool for indicating the health risks posed by ambient air quality in given areas at particular times
With air pollution a growing problem the world over, the Air Quality Index and equivalent tools in other nations have become an indispensable part of regional and federal governments’ efforts to convey information on ambient air quality to the general public.
Nations around the world employ a number of variations on the Air Quality Index (AQI) developed by the US Environmental Protection Agency (EPA), but the basic methodology used to assess air quality is the same. The index assesses the most common pollutants, including suspended particulate matter, airborne lead, ground-level ozone, nitrogen dioxide, sulfur dioxide, and carbon monoxide, which impact basic air quality, although individual criteria vary from place to place. These hazardous compounds are capable of causing severe respiratory irritation, heart and circulatory problems, and other negative health effects.
The concentrations of these various air pollutants are measured regularly in testing areas, and, based on this information, the area is assigned an air quality number. Lower numbers mean cleaner air, while very high numbers signal hazardous conditions. The US EPA scale ranges from 0-500, with six categories and colors to communicate health risks: Good (0-50; Green), Moderate (51-100; Yellow), Unhealthy for Sensitive Groups (101-150; Orange), Unhealthy (151-200; Red), Very Unhealthy (201-300; Purple), and Hazardous (301+; Maroon). Values below 100 are generally acceptable, but readings above 100 and the air becomes unhealthy—first for sensitive groups and then, at higher levels, for everyone.
South Korea’s Comprehensive Air-quality Index (CAI) is similar to the US EPA’s Air Quality Index (AQI) in that both measure air pollution and communicate health risks to the public, but there are key differences. The CAI focuses on the highest concentration among pollutants, rather than averaging or calculating a value for each, as the US AQI does. Its numerical ranges and category thresholds also differ slightly, meaning that the same air quality could be rated differently on the CAI than on the US index.
South Korea, China, and some European countries utilize their indices to protect public health during high-pollution or “smog” events. For example, when air quality reaches unhealthy levels, governments may issue advisories urging people to stay indoors, limit outdoor activities, or wear protective masks. Schools and workplaces may adjust schedules or cancel outdoor events, and real-time updates are often provided through apps, websites, or public announcements to help citizens take timely precautions. These measures ensure that vulnerable populations, including children, older adults, and individuals with respiratory conditions, are better protected during periods of poor air quality.
Monitoring sites are typically limited to cities and towns, where pollution levels and population densities are both high. Different levels of pollution may fall into different tiers based on the stringency of a particular agency’s approach to measuring air quality. For example, Hong Kong’s Air Pollution Index has come under heavy criticism for its relatively lax standards, as the air quality it rates as safe sometimes contains pollutants at levels several times higher than those considered acceptable by the World Health Organization.
Bibliography
"Air Pollution: The Invisible Health Threat." World Health Organization, 12 July 2023, www.who.int/news-room/feature-stories/detail/air-pollution--the-invisible-health-threat. Accessed 4 Sept. 2025.
"Air Quality Index." American Lung Association, 2 Nov. 2023, www.lung.org/clean-air/outdoors/air-quality-index. Accessed 12 July 2024.
"Air Quality Index (AQI) Basics." AirNow, www.airnow.gov/aqi/aqi-basics/. Accessed 12 July 2024.
“AQI Basics.” AirNow.gov, www.airnow.gov/aqi/aqi-basics/. Accessed 4 Sept. 2025.
Hawkins, Amanda. “AQI Scales Around The World.” PurpleAir, 30 Nov. 2022, www2.purpleair.com/blogs/blog-home/aqi-scales-around-the-world. Accessed 5 Sept. 2025.
Hu, Shelia. "What Is the Air Quality Index?" Natural Resources Defense Council, 27 Sept. 2023, www.nrdc.org/stories/what-air-quality-index. Accessed 12 July 2024.
“Introduction to the CAI.” AirKorea, 2022, www.airkorea.or.kr/eng/khaiInfo?pMENU_NO=166. Accessed 5 Sept. 2025.
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