Air Quality Index

Overview

The Air Quality Index (AQI) is a numerical scale used for reporting daily air quality. It focuses on health effects one might experience within a few hours or days after breathing polluted air. The AQI is calculated for five major air pollutants regulated by the Clean Air Act: ground-level ozone, particle pollution (also known as particulate matter), carbon monoxide, sulfur dioxide, and nitrogen dioxide. For each of these pollutants, the United States Environmental Protection Agency (EPA) has established national air quality standards to protect public health.

Purpose

The primary purpose of the AQI is to provide information about air quality in a simple, easy-to-understand format. It is designed to help individuals understand what local air quality means to their health. The AQI is divided into six categories, each corresponding to a different level of health concern. Each category is assigned a specific color to make it easier for people to understand quickly whether air pollution is reaching unhealthy levels in their communities.

Calculation

The AQI is calculated using a complex mathematical formula that converts raw data on pollutant concentrations into a number on the AQI scale. The formula takes into account the relative importance of each pollutant in causing adverse health effects. The AQI value for each pollutant is calculated separately, and the highest of these values is reported as the AQI. For example, if the AQI values for ozone and particulate matter are 60 and 65 respectively, the reported AQI would be 65.

AQI Categories

The AQI is divided into six categories, each of which corresponds to a different level of health concern. The categories are:

Good (0-50): Air quality is considered satisfactory, and air pollution poses little or no risk.
Moderate (51-100): Air quality is acceptable; however, for some pollutants there may be a moderate health concern for a very small number of people who are unusually sensitive to air pollution.
Unhealthy for Sensitive Groups (101-150): Members of sensitive groups may experience health effects. The general public is not likely to be affected.
Unhealthy (151-200): Everyone may begin to experience health effects; members of sensitive groups may experience more serious health effects.
Very Unhealthy (201-300): Health alert: everyone may experience more serious health effects.
Hazardous (301-500): Health warnings of emergency conditions. The entire population is more likely to be affected.

Health Implications

The health implications of the AQI vary depending on the pollutant and the AQI category. For example, an AQI in the "Unhealthy" range indicates that everyone may begin to experience health effects, while an AQI in the "Hazardous" range indicates that the entire population is more likely to be affected. Health effects can range from minor irritation of the eyes, nose, and throat to serious heart and lung damage. People with heart or lung disease, older adults, and children are at greater risk from air pollution.

Global Use

While the AQI originated in the United States, it is now used by countries around the world to communicate air quality to their citizens. However, the specific pollutants measured and the methods of calculation can vary from country to country. For example, the AQI in China includes additional pollutants such as PM2.5, PM10, and ozone 8-hour, and uses different breakpoints.