The Air Quality Index (AQI) is a standardized, health-based tool used globally to report daily air quality in a simple, understandable format. Its primary purpose is to convert complex measurements of air pollutant concentrations into a single numerical value and color scale that communicates the level of pollution and its associated health risks to the public. By translating raw scientific data into an accessible number, the AQI helps individuals make informed choices about their outdoor activities and potential exposure to airborne contaminants.
Required Pollutant Data and Units
Calculating the standard AQI requires current concentration data for a set of airborne contaminants known as the “criteria pollutants.” These pollutants include ground-level ozone (O3), particulate matter (specifically PM2.5 and PM10), carbon monoxide (CO), sulfur dioxide (SO2), and nitrogen dioxide (NO2). The AQI calculation is performed separately for each of these pollutants.
The raw data collected by air monitoring stations must be in specific units for conversion into the index value. Gaseous pollutants are typically measured in parts per million (ppm) or parts per billion (ppb), while particulate matter is measured in micrograms per cubic meter (µg/m³). The averaging time for the concentration measurement, such as a 1-hour, 8-hour, or 24-hour average, also varies by pollutant to reflect its specific health impact profile.
Understanding the Indexing Scale and Breakpoints
The AQI is a segmented scale ranging from 0 to 500, not a direct, linear measurement of pollutant concentration. This scale is divided into specific health-based categories, such as Good, Moderate, and Unhealthy for Sensitive Groups. To facilitate the conversion from pollutant concentration to the index number, a set of standardized “breakpoints” are used.
Breakpoints are the specific concentration values (C_low and C_high) that correspond to the upper and lower limits of each AQI category (I_low and I_high). These pollutant-specific breakpoints are established by regulatory bodies, like the U.S. Environmental Protection Agency (EPA). The concentration range for PM2.5 corresponding to an AQI of 0 to 50 (Good) is different from the concentration range for SO2 corresponding to the same index range. These standards are based on national ambient air quality standards designed to protect public health.
Step-by-Step Calculation Method
The core mathematical process for converting a measured pollutant concentration into an index value is linear interpolation. This method calculates a Pollutant Index Value (Ip) by finding where the measured concentration (Cp) falls proportionally between two established breakpoints. The formula is: Ip = [(I_high – I_low) / (C_high – C_low)] (Cp – C_low) + I_low. This equation essentially finds the position of the concentration within its specific breakpoint range and maps that position onto the corresponding AQI range.
To illustrate, consider a measured PM2.5 concentration of 40.0 µg/m³. This value falls within the concentration breakpoints for the “Unhealthy for Sensitive Groups” category, which has an I_low of 101 and an I_high of 150. Using the EPA’s PM2.5 breakpoints, the C_low is 35.5 µg/m³ and C_high is 55.4 µg/m³. Plugging these numbers into the formula yields an Ip that is slightly higher than 101. This calculation is performed for every measured pollutant to generate its own sub-index value.
Determining the Overall Air Quality Index
After calculating a separate index value for each criteria pollutant, the final reported Air Quality Index is determined by the maximum index value. The overall AQI is the highest individual Pollutant Index Value (Ip) among all the pollutants measured at a particular location. This highest value is designated as the “critical pollutant” for that period.
For example, if the PM2.5 index is 125, the ozone index is 90, and the CO index is 50, the overall reported AQI is 125. This maximum index rule ensures that the public is always informed of the pollutant that poses the greatest and most immediate health threat at that time.
Interpreting the Health Message
The final AQI number is paired with a specific color and descriptive category to communicate the corresponding health message. An AQI value of 0 to 50 is coded Green and described as “Good,” indicating minimal health risk. As the number rises, the colors and warnings intensify, moving through Yellow for “Moderate” (51-100) and Orange for “Unhealthy for Sensitive Groups” (101-150).
When the AQI reaches 101 or higher, specific health advisories are issued, first for people with pre-existing conditions like asthma or heart disease, and then for the general population.
- Good (0-50, Green): Indicates minimal health risk.
- Moderate (51-100, Yellow): Warnings intensify.
- Unhealthy for Sensitive Groups (101-150, Orange): Advisories issued for people with pre-existing conditions.
- Unhealthy (151-200, Red): Everyone may begin to experience health effects.
- Very Unhealthy (201-300, Purple): Indicates serious health effects are likely.
- Hazardous (301-500, Maroon): Indicates emergency conditions requiring avoidance of all outdoor activity.