Helena Joyce
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 concentration of six atmospheric pollutants: sulfur dioxide (SO2), nitrogen dioxide (NO2), suspended particulates smaller than 10 μm in aerodynamic diameter (PM10), suspended particulates smaller than 2.5 μm in aerodynamic diameter (PM2.5), carbon monoxide (CO), and ozone (O3). These pollutants are measured by instruments on the ground and satellites orbiting Earth, such as the GOES-R Series and JPSS satellites. The AQI is then divided into six color-coded categories, each indicating a different level of health concern. For example, a low AQI of 50 or below is generally considered safe, while an AQI above 100 is deemed unhealthy.
| Characteristics | Values |
|---|---|
| Purpose | To communicate to the public about the current and forecasted air pollution and associated public health risks. |
| Governing Body | Developed by government agencies. In the US, it is the EPA that establishes an AQI for five major air pollutants regulated by the Clean Air Act. |
| Categories | Six color-coded categories, each corresponding to a range of index values. |
| Health Risk | The higher the AQI value, the greater the health concern. Children, the elderly and individuals with respiratory or cardiovascular problems are typically the first groups affected by poor air quality. |
| AQI Values | AQI values at or below 100 are generally thought of as satisfactory. Below 50 is considered safe. |
| Action | When the AQI is high, governmental bodies generally encourage people to reduce physical activity outdoors or avoid going out altogether. |
| Measurement | Instruments on the ground and satellites orbiting Earth collect information about air quality. |
| Pollutants | Ground-level ozone and airborne particles are the two air pollutants that pose the greatest risk to human health in the US. However, other pollutants such as PM2.5, PM10, NO2, CO, SO2, etc., are also measured. |
| Global Initiatives | UNEP supports the deployment of sensors and aims to provide technical support to countries to address the air pollution crisis. |
| National Differences | Different countries have their own air quality indices, corresponding to different national standards. For example, China's AQI level is based on six atmospheric pollutants, while Hong Kong's Air Quality Health Index considers four. |
| Calculation | An individual score (Individual Air Quality Index, IAQI) is calculated using breakpoint concentrations and a piecewise linear function to calculate intermediate values. |
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What You'll Learn
- Air quality data is collected by ground instruments and satellites orbiting Earth
- AQI values are categorised using a colour-coded system, with green indicating safe and maroon indicating an emergency
- AQI values are calculated using a piecewise linear function
- The AQI is based on the level of six atmospheric pollutants
- The World Health Organisation estimates that 99% of the global population breathes unclean air
Air quality data is collected by ground instruments and satellites orbiting Earth
The World Health Organization (WHO) estimates that 99% of the global population breathes unclean air, and air pollution causes 7 million premature deaths a year. To address this crisis, governments must strengthen air quality regulation and monitoring capacity. While air quality monitoring is not a legal requirement in 37% of countries, organisations like the United Nations Environment Programme (UNEP) are taking steps to address this. UNEP has supported the deployment of 48 low-cost sensors across Kenya, Costa Rica, Ethiopia, and Uganda since 2020, and aims to provide technical support to over 50 countries.
Satellite data is increasingly being used to support air quality monitoring efforts. Satellites can provide data on criteria air pollutants like PM2.5 and NO2, and greenhouse gases like CH4 and CO2. For example, NASA's Aura Ozone Monitoring Instrument (OMI) has collected data on nitrogen dioxide (NO2) from 2005 onwards, and this data indicates that levels of this trace gas have changed significantly over the last decade in many places around the world. Satellite data can also be used to estimate emissions, track pollutant plumes, support air quality forecasting, and monitor long-term trends.
Ground-based instruments are also used to collect air quality data. For example, the California Air Resources Board (CARB) has funded research projects using remote sensing techniques, with instruments deployed on the ground, in aircraft, and on satellites.
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AQI values are categorised using a colour-coded system, with green indicating safe and maroon indicating an emergency
The Air Quality Index (AQI) is a public tool developed by government agencies to communicate the level of air pollution and associated health risks. The AQI is calculated using data from air quality monitors, which track pollutants such as PM2.5, ozone, nitrogen dioxide, sulfur dioxide, and carbon monoxide. PM2.5, or particulate matter with a diameter of 2.5 micrometres or less, is of particular concern as it can be inhaled and absorbed into the bloodstream, leading to serious health issues.
To make it easier for people to understand the health implications of air quality, the AQI values are categorised using a colour-coded system. This system varies slightly between countries, but generally, green indicates safe air quality, with low levels of pollution and associated health risks. On the other hand, maroon or purple indicates an emergency situation, with hazardous levels of pollution that are unsafe for everyone.
In the United States, the AQI is divided into six colour-coded categories, each representing a different level of health concern. An AQI value of 50 or below, indicated by the colour green, represents good air quality. Values above 100 are considered unhealthy, first for sensitive groups and then for everyone as values increase. An AQI value of over 300, indicated by maroon or purple, represents hazardous air quality, posing a serious health risk to the general population.
Other countries have similar colour-coded systems. For example, in Hong Kong, the Air Quality Health Index (AQHI) uses a scale of 1 to 10+ to indicate the level of health risk, with colours ranging from green for low risk to maroon for very high risk. Similarly, China's Ministry of Environmental Protection (MEP) monitors the level of air pollution in 163 major cities, with health advice provided for different risk categories.
It is important to note that different countries have their own AQI standards, reflecting their national air quality standards and regulations. However, the overall purpose of the AQI is to provide a clear indication of air quality and potential health risks, allowing individuals to take appropriate actions to protect their health.
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AQI values are calculated using a piecewise linear function
The Air Quality Index (AQI) is an indicator 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 five "criteria" pollutants regulated under the Clean Air Act: ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide. Each country has its own AQI, reflecting the discourse surrounding the development of national air quality standards in that nation.
The specific formulas and domains used in the AQI calculation can vary depending on the specific pollutants being measured and the standards set by the relevant government agencies. However, the general idea is that the AQI value increases as the concentration of pollutants in the air increases.
For example, in the United States, the Environmental Protection Agency (EPA) has established National Ambient Air Quality Standards (NAAQS) for each of the five criteria pollutants. An AQI value of 100 generally corresponds to the level of the NAAQS for a particular pollutant. As the concentration of that pollutant increases, the AQI value also increases, indicating a higher level of air pollution and associated health concerns.
It is important to note that the relationship between AQI values and pollutant concentrations is not always linear. While the overall AQI score is calculated using a piecewise linear function, the score for each individual pollutant is non-linear. This means that an AQI of 300 does not necessarily mean twice the pollution or harm compared to an AQI of 150. The AQI values are meant to provide a general indication of air quality and associated health risks, but the specific health impacts can vary depending on various factors.
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The AQI is based on the level of six atmospheric pollutants
The Air Quality Index (AQI) is an indicator used by government agencies to communicate to the public about the current and forecasted level of air pollution. It is designed to reflect the level of air pollution and the associated health risks, with higher AQI values indicating greater pollution and health concerns.
The AQI is based on the levels of six primary atmospheric pollutants:
- Sulfur Dioxide (SO2)
- Nitrogen Dioxide (NO2)
- Suspended Particulates smaller than 10 μm in aerodynamic diameter (PM10)
- Suspended Particulates smaller than 2.5 μm in aerodynamic diameter (PM2.5)
- Carbon Monoxide (CO)
- Ozone (O3)
These pollutants are monitored at various locations, and the AQI value represents the highest level among these pollutants at a given location. PM2.5, in particular, poses a significant health threat as it can be inhaled and absorbed into the bloodstream, leading to serious illnesses such as stroke, heart disease, lung disease, and cancer.
The AQI is divided into six categories, each represented by a specific colour. These categories indicate increasing levels of health concern. For instance, an AQI value of 50 or below is generally considered good air quality, while values above 100 are deemed unhealthy, initially for sensitive groups and then for everyone as values increase further. Values above 300 indicate hazardous air quality.
Different countries have their own variations of the AQI, with unique national air quality standards. These indices provide valuable information to help individuals make informed decisions about their health and well-being, especially for those who are more vulnerable to the effects of air pollution, such as children, the elderly, and individuals with respiratory or cardiovascular issues.
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The World Health Organisation estimates that 99% of the global population breathes unclean air
The Air Quality Index (AQI) is a tool used by government agencies to communicate to the public about the current and forecasted level of air pollution. The AQI is designed with six color-coded categories, each representing a range of index values that indicate the level of health risk associated with air pollution. An AQI of 50 or below is generally considered safe, while values above 100 are deemed unhealthy.
The World Health Organization (WHO) has reported that 99% of the global population breathes air that exceeds its recommended air quality limits, threatening their health. This means that almost everyone in the world breathes air that does not meet the WHO's standards for air quality. This is due to the presence of pollutants generated by fossil fuel use, which can cause respiratory and blood-flow problems, leading to millions of preventable deaths each year.
To address this issue, the WHO has called for urgent action to reduce fossil fuel use and strengthen air quality regulations. According to the organization, air pollution causes about 7 million premature deaths annually, with low- and middle-income countries experiencing the highest exposures to unhealthy air.
In 2021, the United Nations Environment Programme (UNEP) developed the first real-time air pollution exposure calculator, utilizing data from over 6,400 locations in 117 countries and territories. This initiative aims to provide early warning information and promote international cooperation to tackle the air pollution crisis.
Despite these efforts, a 2022 report by the WHO revealed that over 6,000 cities in 117 countries are now monitoring air quality, but the residents are still exposed to unhealthy levels of fine particulate matter and nitrogen dioxide. The report highlighted the need to curb fossil fuel use and implement additional measures to improve air quality, especially in regions with persistently poor air quality, such as the WHO's Eastern Mediterranean and Southeast Asia regions, followed by Africa.
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Frequently asked questions
AQI stands for Air Quality Index, an indicator developed by government agencies to communicate to the public how polluted the air currently is or how polluted it is forecast to become.
Ground instruments and satellites orbiting Earth collect information about the composition of our air. Satellites like GOES-R and JPSS monitor particle pollution and provide measurements of carbon monoxide, respectively. This data is then processed by air quality databanks, which may weigh data differently based on reliability and the type of pollution measured.
The AQI is divided into six color-coded categories, each corresponding to a different level of health concern. Code Green and Yellow mean the air is generally safe for everyone. Code Orange is unhealthy for sensitive groups, including children, the elderly, 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.
