What is the Air Quality Index (AQI)?
The Air Quality Index (AQI) is a scale used to measure and communicate how clean or polluted the air is and what are the health risks to the public. It works like a thermometer that runs from 0 to 500 degrees. However, it displays pollution levels and corresponding health risks instead of temperature.
Importance
In the US, poor air quality contributes to over 100,000 premature deaths annually. It incurs costs exceeding $150 billion related to healthcare and lost productivity. AQI and proper air quality monitoring practices helps mitigate this impact by providing essential information to the public, regulators, and industries.
Here are more reasons why AQI is important:
- Simplifies Data: Monitoring air quality means gathering data from monitoring stations, sensors, and satellites. The AQI simplifies these complex datasets into a single numerical value that is easy to understand and interpret.
- Public Health Protection: The main purpose of monitoring air pollution levels is to protect human health at home and work. It alerts individuals, especially those with preexisting health conditions, to adjust their activities to avoid exposure.
- Risk Indicator: Workplaces use the AQI as a risk indicator to assess potential hazards to their employees’ health and safety. High AQI values can prompt companies to implement protective measures, such as providing Personal Protective Equipment (PPE) or limiting outdoor work activities.
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How Does the Air Quality Index (AQI) Work?
Under the guidance of the US Environmental Protection Agency (EPA), AQI is calculated for five major air pollutants regulated by the US Clean Air Act (CAA). These are:
- Ground-level ozone (O₃)
- Particulate matter (PM₂.5 and PM₁₀)
- Carbon monoxide (CO)
- Sulfur dioxide (SO₂)
- Nitrogen dioxide (NO₂)
Each pollutant has an associated AQI value, and the overall AQI is determined by the highest measured value among these pollutants at a given time and location. The AQI converts air quality data into a simple color-coded scale that ranges from good (green) to hazardous (maroon). This makes it easier for the public to understand and take precautions.
In the US, in addition to the CAA, the AQI also supports compliance with health regulations and standards set by the National Ambient Air Quality Standards (NAAQS). For workplace safety in particular, the Occupational Safety and Health Administration (OSHA) also considers AQI levels in creating and implementing its regulations.
In many countries, government agencies are responsible for setting air quality standards and ensuring compliance. For example, in the European Union, the Ambient Air Quality Directive sets air quality standards and requires member states to assess and manage air quality. Meanwhile, in India, the Air (Prevention and Control of Pollution) Act empowers the Central Pollution Control Board (CPCB) to set air quality standards and monitor air pollution levels.
Scoring and Categories
| Color | Level of Concern | AQI Range | Description |
| Green | Good | 0–50 | Air quality is satisfactory, posing little or no risk. |
| Yellow | Moderate | 51–100 | Air quality is acceptable; some pollutants may concern sensitive groups. |
| Orange | Unhealthy for Sensitive Groups | 101–150 | Members of sensitive groups may experience health effects; the general public is less likely to be affected. |
| Red | Unhealthy | 151–200 | Everyone may experience health effects; sensitive groups may experience more serious effects. |
| Purple | Very Unhealthy | 201–300 | Health alert: Everyone has an increased risk of health effects. |
| Maroon | Hazardous | 301 and above | Health warnings of emergency conditions: everyone is more likely to be affected. |
An AQI value of 100 or below is generally considered safe for most people. In contrast, values above this threshold indicate increasing health risk levels.
Industries such as manufacturing and construction have the highest potential to contribute to poor air quality. Activities like welding, painting, and coating release hazardous chemicals into the air that could impact workers’ health. Monitoring AQI levels helps these industries comply with regulations and protect their employees.
US Regulating Bodies
The AQI monitoring is conducted by various organizations at different levels, primarily in the US. The key entities involved include the following:
- AirNow: The EPA and its partner agencies operate AirNow, which provides real-time air quality data and AQI values for over 400 cities in 46 states across the US.
- State and Local Agencies: State and local air quality agencies are essential for checking air quality in their areas. They run more monitoring stations and share data with the EPA’s national network.
- National Oceanic and Atmospheric Administration (NOAA): NOAA works with the EPA to track air quality, focusing on how weather affects pollution.
- Other Federal Agencies: Several other federal entities contribute to AQI monitoring efforts, including:
- National Aeronautics and Space Administration (NASA): Uses satellite data to assess air quality on a broader scale.
- Centers for Disease Control and Prevention (CDC): Provides health-related guidance based on AQI data.
- US Forest Service: Monitors the quality of air in forests and wildfire impacts.
Air Quality Monitoring Methods
Agencies and organizations track the AQI through various methods. This process starts with deploying sensors and equipment specifically designed to measure particular air pollutants, including:
Satellite Monitoring
Satellites equipped with remote sensing technologies can detect particulate matter and other air pollutants from space. This method provides a broader perspective on air quality over large areas.
Regulatory Monitoring Stations
Regulatory air quality monitoring stations, often referred to as Federal Reference Method (FRM) or Federal Equivalent Method (FEM) monitors, are established by government agencies. These stations are strategically located and use specific instruments to measure criteria pollutants.
Open-Path Monitoring Systems
Unlike traditional point sensors, which measure air quality at a single location, open-path systems utilize advanced technology to cover larger areas. These systems work by emitting a beam of light, typically lasers or infrared beams, across an open path between a transmitter and a receiver.
Manual Air Quality Monitoring Systems (AAQMS)
Manual systems involve collecting air samples over a defined period. For instance, air samples are collected on a filter and weighed before and after sampling to determine the mass of particulate matter collected. This can result in longer turnaround times for data availability compared to real-time automated systems.
Sensor-Based Monitoring
Sensor-based air quality monitors represent a modern and accessible approach to tracking air quality. They are distinguished by their compact and user-friendly design. These monitors are small, portable devices easily deployed in diverse locations, making them ideal for individuals, schools, and community groups eager to engage in environmental monitoring.
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Who Must Monitor AQI?
Monitoring AQI requires the involvement of multiple entities working together, including:
- Government Agencies: Establish standards and ensure compliance.
- Environmental Organizations: Advocate for policies and conduct independent monitoring.
- Educational Institutions: Conduct research and educate the public.
- Businesses and Employers: Monitor emissions and ensure workplace safety.
- Individuals: Use personal sensors and advocate for cleaner air.
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