An air quality index (AQI) is an estimate of how polluted the air is now or how polluted it is expected to become. When air pollution increases, the AQI also increases, and this means greater risks to public health.

Each country has its own air quality index based on its own air quality standards. Examples include Canada's Air Quality Health Index, Malaysia's Air Pollution Index, and Singapore's Pollutant Standards Index. Common pollutants that are monitored include ground-level ozone, particulates, sulfur dioxide, carbon monoxide, and nitrogen dioxide.

Children, older adults, and people with breathing or heart problems are often the first to be affected by poor air quality. When the AQI is high, government agencies usually advise people to limit outdoor activities or avoid going outside entirely.

Overview

Calculating the AQI requires knowing the concentration of an air pollutant over a specific time period, measured by air monitors or models. The amount of pollutant and the time it is present together show how much exposure someone has to the pollutant. Health effects linked to a certain level of exposure are determined through research. Different air pollutants have different levels of harm, and the method used to convert pollutant levels into AQI values changes depending on the pollutant. AQI values are usually grouped into ranges. Each range has a label, a color, and a public health message.

The AQI can rise when more air pollutants are released. For example, during heavy traffic, when a forest fire is upwind, or when pollutants are not spread out by wind. Air that does not move, often caused by high-pressure systems, temperature inversions, or low wind speeds, can trap pollution in an area. This leads to high pollutant levels, chemical reactions between pollutants, and hazy conditions.

On a day when the AQI is expected to be high because of fine particle pollution, an organization or health group might:
• tell people who are more likely to be harmed, such as the elderly, children, and those with breathing or heart conditions, to avoid outdoor activities.
• announce an "action day" to encourage people to take steps that reduce air pollution, like using public transportation.
• suggest wearing masks outdoors and using air purifiers indoors to keep fine particles out of the lungs.

During very poor air quality, such as during an air pollution event, when the AQI shows that short-term exposure could seriously harm health, agencies may use emergency plans. These plans allow them to require major sources of pollution, like coal-burning industries, to reduce emissions until conditions improve.

Most air pollutants do not have an AQI. Many countries monitor ground-level ozone, particulates, sulfur dioxide, carbon monoxide, and nitrogen dioxide, and calculate AQI values for these pollutants.

The way AQI is defined in a country reflects the process used to create air quality standards in that country. A website that lets government agencies worldwide share real-time air quality data using a common AQI definition has recently been created.

Indices by location

Each state and territory in Australia is responsible for checking air quality and sharing data according to the National Environment Protection (Ambient Air Quality) Measure (NEPM) standards.

Each state and territory shares air quality data for specific monitoring locations. Most also share air quality indexes for each monitoring location.

In Australia, air quality indexes use a simple scale where 100 represents the highest allowed pollution level for each pollutant, as set by the NEPM. These levels are:

The air quality index (AQI) for a location is the highest value among all pollutants measured there.

There are six AQI bands, each with health advice:

In Canada, air quality has been reported using provincial air quality indexes (AQIs) for many years. These indexes focus on reducing pollution emissions to the lowest possible level, not just protecting human health. The Air Quality Health Index (AQHI) helps people understand how air quality affects health. It is a tool used to make decisions that reduce short-term exposure to pollution by changing activities during high pollution levels. The AQHI also suggests ways to improve air quality by changing behaviors to reduce environmental harm. It especially helps people who are sensitive to air pollution by giving them advice to protect their health during low, moderate, high, and very high health risk levels.

The AQHI uses numbers from 1 to 10+ to show the health risk level of local air quality. Sometimes, when pollution is very high, the number may go above 10. The AQHI also provides current air quality values and forecasts for today, tonight, and tomorrow, along with health advice.

On December 30, 2013, Hong Kong changed its Air Pollution Index to the Air Quality Health Index (AQHI). This index, managed by the Environmental Protection Department, uses a scale of 1 to 10+ and considers four pollutants: ozone, nitrogen dioxide, sulfur dioxide, and particulate matter (PM10 and PM2.5). For each hour, the AQHI is calculated based on the combined risk of hospital admissions linked to the average pollution levels of these four pollutants over three hours. The AQHI is grouped into five health risk categories with health advice for each.

China's Ministry of Environmental Protection (MEP) measures air pollution levels in the country. Starting January 1, 2013, the MEP monitors daily pollution levels in 163 major cities. The AQI level is based on six pollutants: sulfur dioxide (SO2), nitrogen dioxide (NO2), particulate matter smaller than 10 μm (PM10), particulate matter smaller than 2.5 μm (PM2.5), carbon monoxide (CO), and ozone (O3).

An individual score (IAQI) is calculated using specific pollution levels and the same method as the US AQI scale. The final AQI value can be calculated hourly or over 24 hours and is the highest score among the six pollutants.

The score for each pollutant and the final AQI score are not directly proportional. For example, an AQI of 300 does not mean twice the pollution or harm compared to an AQI of 150. Similarly, a pollutant's concentration when its IAQI is 100 is not twice as high as when its IAQI is 50.

If an AQI is 50 for 182 days and 100 for 183 days, the annual average is 75. However, this does not guarantee the pollution is safe, even if the 100 benchmark is considered acceptable. The benchmark is based on 24-hour targets, and the annual average must meet annual goals. It is possible to have safe air daily but still fail the annual pollution standard.

The Common Air Quality Index (CAQI) is used in Europe since 2006. In 2017, the European Environment Agency introduced the European Air Quality Index (EAQI) and encouraged its use to inform the public.

A 2012 EU-supported project, CiteairII, stated the CAQI was designed to be easy for the public to understand, without focusing on complex health-based indexes. The CAQI's goal was to highlight urban air pollution and its main source (traffic) to help people reduce exposure.

The CAQI uses a scale from 0 to 100, where low values mean good air quality and high values mean poor air quality. It has hourly and daily versions, and separate indexes for areas near roads ("roadside" or "traffic") and away from roads ("background"). As of 2012, the roadside index included two required pollutants (NO2 and PM10) and three required pollutants for the background index (NO2, PM10, and O3). Optional pollutants like PM2.5, CO, and SO2 were also included. The CAQI is the worst sub-index among the required and optional pollutants.

Key pollutant concentrations, sub-indices, and CAQI ranges are described in detail. Updated CAQI values and maps are available on websites like www.airqualitynow.eu. A Year Average Common Air Quality Index (YACAQI) is also used, where pollutant levels are normalized to a value near 1. For example, yearly averages of NO2, PM10, and PM2.5 are divided by 40 μg/m³, 40 μg/m³, and 20 μg/m³, respectively. The overall YACAQI for a city is the average of these normalized values.

The National Air Quality Index (NAQI) was launched in New Delhi on September 17, 2014, as part of the Swachh Bharat Abhiyan. The Central Pollution Control Board and State Pollution Control Boards operate the National Air Monitoring Programme (NAMP), covering 240 cities with over 342 monitoring stations. An expert group, including medical professionals and air quality experts, recommended an AQI scheme in 2014. The new index measures eight pollutants, compared to the earlier three. Continuous monitoring systems provide real-time data in cities like New Delhi, Mumbai, Pune, Kolkata, and Ahmedabad.

The NAQI has six categories: Good, Satisfactory, Moderate, Poor, Severe, and Hazardous. The index considers eight pollutants: PM10, PM2.5, NO2, SO2, CO, O3, and others.