Greta Jimenez
The Air Quality Index (AQI) is a unit of measurement that communicates to the public how polluted the air currently is or how polluted it is forecast to become. The AQI is based on the concentrations of five pollutants: ozone, nitrogen dioxide, sulfur dioxide, PM2.5, and PM10. The higher the AQI value, the greater the level of air pollution and associated health risks. For example, an AQI value of 50 or below represents good air quality, while an AQI value over 300 indicates hazardous air quality. The AQI is calculated using hourly measurements of these five pollutants, and the pollutant with the highest AQI value determines the overall AQI for that hour.
| Characteristics | Values |
|---|---|
| Purpose | To communicate to the public about the current and future air pollution levels and associated health risks |
| Pollutants measured | Ozone, Nitrogen Dioxide, Sulphur Dioxide, Carbon Monoxide, PM2.5, PM10 |
| Unit of measurement | Ratio of the pollutant to a million parts of atmospheric air (ppm) |
| AQI value range | 0-500 |
| AQI categories | Six categories with specific colours |
| Calculation | Sum of the percentage excess risk of daily hospital admissions attributable to the 3-hour moving average concentrations of pollutants |
| Data sources | Ground instruments, satellites |
| Reporting frequency | Hourly |
| Alert mechanism | Air quality alert issued when AQI exceeds 101 |
| Health recommendations | Reduce outdoor physical activity, use masks and air purifiers, avoid going out |
| Geographical coverage | Over 10,000 stations in more than 80 countries |
| Limitations | Data may be amended without notice due to quality assurance |
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What You'll Learn
The Air Quality Index (AQI)
The AQI is based on the measurement of six key pollutants: particulate matter (PM2.5 and PM10), ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), and carbon monoxide (CO). These pollutants are monitored by air quality stations worldwide, and the data is used to calculate the AQI for each location. The AQI for a specific location is determined by identifying the highest value among the individual pollutant indices being tracked at that site.
The AQI is typically divided into six categories, each represented by a distinct colour code and accompanied by health advice. These categories indicate increasing levels of health concern, with specific recommendations aimed at protecting the health of vulnerable individuals, such as children, the elderly, and those with respiratory or cardiovascular issues.
The calculation of the AQI involves obtaining air pollutant concentration data over a specified averaging period, using air monitors or models. The concentration and exposure time together represent the dose of the air pollutant. Epidemiological research then establishes the health effects associated with a given dose. It is important to note that the function used to convert pollutant concentration to AQI varies depending on the specific pollutant.
The definition and application of the AQI vary across different nations, reflecting the discourse and development of national air quality standards in each country. For example, Hong Kong employs the Air Quality Health Index (AQHI), which is measured on a scale of 1 to 10+ and considers four pollutants: ozone, nitrogen dioxide, sulfur dioxide, and particulate matter.
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Calculating the AQI
The Air Quality Index (AQI) is a tool used by government agencies to communicate to the public how polluted the air currently is or how polluted it is forecast to become. The AQI is based on the concentrations of five pollutants: ozone, nitrogen dioxide, sulfur dioxide, PM2.5 and PM10. The breakpoints between index values are defined for each pollutant separately, and the overall index is the maximum value of these indices.
The AQI is calculated from the concentrations of these pollutants, which are measured using laser particle sensors. These concentrations are then converted into a standard statistical distribution with a preset mean and standard deviation. The resultant individual pollutant indices are assumed to be equally weighted, and the overall index is defined as the maximum value of the individual pollutant indices. 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 or below represents good air quality, while an AQI value over 300 represents hazardous air quality.
The AQI is divided into six categories indicating increasing levels of health risk: Good, Moderate, Unhealthy for Sensitive Groups (USG), Unhealthy, Very Unhealthy, and Hazardous. When the AQI is high, governmental bodies generally encourage people to reduce physical activity outdoors or even avoid going outside altogether. In the case of wildfires, the use of masks and indoor air purifiers may also be recommended.
The AQI is calculated differently in various parts of the world, as different countries have their own air quality indices corresponding to different national air quality standards. For example, Hong Kong's Air Quality Health Index (AQHI) provides a number from 1 to 10+ to indicate the level of health risk associated with local air quality. The AQHI is calculated from the sum of the percentage excess risk of daily hospital admissions attributable to the 3-hour moving average concentrations of four pollutants: ozone, nitrogen dioxide, sulfur dioxide, and particulate matter (including PM10 and PM2.5).
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AQI categories
The Air Quality Index (AQI) is a tool used by government agencies to communicate to the public how polluted the air currently is or how polluted it is forecast to become. The AQI is calculated from the concentrations of the following pollutants: ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, nitrogen dioxide, PM2.5, and PM10. The higher the AQI value, the greater the level of air pollution and the greater the health concern.
The AQI is divided into six categories, each with a specific colour code, indicating increasing levels of health concern:
- Good: AQI value of 0-50, indicating satisfactory air quality, with little to no risk from air pollution.
- Moderate: AQI value of 51-100, indicating acceptable air quality, although certain pollutants may pose a moderate health concern for sensitive individuals, such as those with respiratory issues.
- Unhealthy for Sensitive Groups: AQI value of 101-150, where the general public is not likely to be affected, but sensitive groups such as people with lung disease, older adults, and children may be at a higher risk from exposure to certain pollutants.
- Poor: AQI value of 151-200, indicating that air pollution levels are unhealthy for everyone, and that everyone may begin to experience health effects, not just sensitive groups.
- Severe: AQI value of 201-300, indicating very unhealthy air quality, with potential significant health risks for the general public.
- Hazardous: AQI value of above 300, representing hazardous air quality, with potential serious health risks for everyone.
It is important to note that different countries may have their own specific AQI categories and corresponding health concerns, as national air quality standards vary. Additionally, the AQI values and categories are subject to change over time as health effects research evolves.
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AQI and health
The Air Quality Index (AQI) is a tool used by the United States Environmental Protection Agency (EPA) to communicate to the public about outdoor air quality and health. The AQI is based on the concentrations of five pollutants: ozone, nitrogen dioxide, sulfur dioxide, PM2.5, and PM10. The index is calculated using the breakpoints between the concentrations of each pollutant, with the overall index defined as the maximum value. The higher the AQI value, the greater the level of air pollution and the associated health risks.
The AQI includes six color-coded categories, each indicating a range of index values and an increasing level of health concern. An AQI value of 50 or below represents good air quality, while a value of 51-100 is considered moderate. At this level, sensitive individuals, such as those with respiratory diseases like asthma, may experience health effects, and it is recommended that they limit outdoor exertion.
AQI values between 101 and 150 are considered unhealthy for sensitive groups, and everyone else may start to be affected. Values between 151 and 200 are unhealthy for everyone, and it is recommended that everyone limit outdoor physical activity. Values between 201 and 300 are very unhealthy, and values above 300 are considered hazardous. At these levels, everyone is advised to avoid outdoor physical activity, and in emergency conditions, everyone is advised to stay indoors and avoid exertion.
The AQI is an important tool for the public to quickly assess the air quality in their communities and take appropriate actions to protect their health. It is also used by government agencies to provide health advice and recommendations to the public during periods of poor air quality, such as encouraging the use of masks and air purifiers during wildfires.
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Monitoring air quality
The AQI is designed to help people understand the current air quality and make informed decisions about their activities. It is based on the concentrations of five major pollutants: ozone, nitrogen dioxide, sulfur dioxide, PM2.5, and PM10. These pollutants are known to have significant impacts on human health and the environment. The AQI is typically divided into six color-coded categories, with each category representing a range of index values. For instance, an AQI value below 50 indicates good air quality, while a value over 300 represents hazardous air quality with potential health risks, particularly for children, the elderly, and individuals with respiratory or cardiovascular issues.
To obtain AQI data, monitoring stations are strategically placed in population centers, such as busy roads and city centers, to support human health objectives. These stations may also be established away from urban areas to determine background pollution levels. The data collected includes pollutant concentrations, meteorological information, and geographic locations. This data is then made accessible to the public through websites and interactive maps, allowing people to check the air quality in their specific locations.
In addition to the AQI, other indices such as the Air Quality Health Index (AQHI) are used in certain regions. The AQHI, used in Hong Kong, provides a number from 1 to 10+ to indicate the level of health risk associated with local air quality. It considers the same four pollutants as the AQI but provides a more localized assessment of air quality and associated health advice.
With the increasing availability of real-time air monitoring data, websites like WAQI.info offer global air quality information for over 80 countries. This data is collected using laser particle sensors that measure PM2.5 and PM10 particle pollution, which are considered some of the most harmful air pollutants. By providing access to historical data and interactive maps, WAQI.info enables users to make informed decisions about their health and well-being based on air quality conditions in their regions.
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Frequently asked questions
The AQI is the ratio of the pollutant to a million parts of atmospheric air. The concentrations of the pollutants are measured in ppm or parts per million.
The AQI measures five major air pollutants: particulate matter (PM2.5 and PM10), ground-level ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), and carbon monoxide (CO).
The AQI is calculated from the concentrations of the above pollutants. The pollutant with the highest AQI value determines the overall AQI for that hour. The AQI is based on six color-coded categories that indicate increasing levels of health concern.
Code Green and Yellow mean the air is generally safe for everyone. Code Orange is unhealthy for sensitive groups, including children, senior citizens, and people with heart and lung diseases. Code Red and Purple mean the air is unhealthy for everyone, and Code Maroon is a health warning of emergency conditions.
The data is collected through ground-based instruments and satellites orbiting the Earth, such as the GOES-R and JPSS satellites.
