The Air Quality Index (AQI) is measured by converting the concentrations of various air pollutants into a single, uniform index value based on their associated health effects.
Here's a breakdown of the process:
Pollutants Measured
First, monitors measure the concentration of key air pollutants. These pollutants typically include:
- Ground-level ozone (O₃)
- Particulate matter (PM2.5 and PM10) - referring to fine and coarse inhalable particles
- Carbon monoxide (CO)
- Sulfur dioxide (SO₂)
- Nitrogen dioxide (NO₂)
Concentration Measurement
Sophisticated instruments are used to continuously measure the levels of these pollutants in the air. The measurements are typically reported in units like parts per million (ppm) or micrograms per cubic meter (µg/m³).
Conversion to AQI Values
The measured pollutant concentrations are then converted into individual AQI values using pre-defined breakpoints established by environmental agencies like the EPA (Environmental Protection Agency) in the United States. These breakpoints correspond to different levels of health concern. Each pollutant has its own scale and breakpoints.
For example, here's a simplified example of how PM2.5 (fine particulate matter) might be converted to an AQI value:
| PM2.5 Concentration (µg/m³) | AQI Value | Health Concern |
|---|---|---|
| 0 - 12.0 | 0 - 50 | Good |
| 12.1 - 35.4 | 51 - 100 | Moderate |
| 35.5 - 55.4 | 101 - 150 | Unhealthy for Sensitive Groups |
| 55.5 - 150.4 | 151 - 200 | Unhealthy |
| 150.5 - 250.4 | 201 - 300 | Very Unhealthy |
| 250.5 - 500.4 | 301 - 500 | Hazardous |
Determining the Overall AQI
The overall AQI value reported is the highest AQI value calculated for any of the individual pollutants. This is because the pollutant with the worst air quality poses the greatest health risk. This "highest value" is then matched to a corresponding health concern level and color code, which is what's reported to the public.
Health Benchmarks & NAAQS
The health benchmarks used for calculating the AQI are pollutant-specific and established by the EPA through the National Ambient Air Quality Standards (NAAQS). These standards define the acceptable levels of pollutants to protect public health and welfare. The AQI uses these standards to translate pollutant concentrations into an easily understandable measure of air quality.
Reporting the AQI
The AQI is then disseminated to the public through various channels, including websites, mobile apps, and news outlets. This allows individuals to make informed decisions about their outdoor activities and take precautions to protect their health when air quality is poor.
