AQI Breakpoints
The specific pollutant concentration values that correspond to each AQI level, used to convert raw measurements to the 0-500 scale.
Detailed Explanation
AQI breakpoints are the specific pollutant concentration thresholds that map raw measurement data to the AQI scale. Each criteria pollutant has its own set of breakpoints because the health effects of each pollutant occur at different concentrations. For example, a PM2.5 concentration of 9.0 micrograms per cubic meter corresponds to an AQI of 50 (the upper limit of "Good"), while an ozone concentration of 0.054 parts per million over 8 hours corresponds to AQI 50. To calculate the AQI for a given pollutant, the measured concentration is compared against the breakpoint table, and a linear interpolation formula converts the concentration to an AQI value. The AQI is calculated separately for each pollutant, and the highest value becomes the reported AQI for that location and time period. Breakpoints are periodically revised when the EPA updates the NAAQS — for instance, the 2024 revision of the PM2.5 annual standard from 12 to 9 micrograms per cubic meter was accompanied by updated AQI breakpoints. Understanding breakpoints is important because they explain why the same AQI number can represent very different pollutant concentrations depending on which pollutant is driving it. AirHistory reports both the overall AQI and the dominant pollutant for each city, giving users the context needed to understand which breakpoints are relevant to their local air quality conditions.
Related Terms
Air Quality Index (AQI)
A standardized EPA scale from 0 to 500 that communicates daily air quality and associated health risks.
AQI Categories
The six color-coded health concern levels — Good, Moderate, Unhealthy for Sensitive Groups, Unhealthy, Very Unhealthy, and Hazardous — used by the EPA to describe air quality.
Criteria Pollutants
The six common air pollutants regulated by the EPA under the Clean Air Act: ozone, PM2.5, PM10, CO, NO2, and SO2.
National Ambient Air Quality Standards (NAAQS)
EPA-established limits on concentrations of six criteria pollutants that must be met nationwide to protect public health and the environment.
Frequently Asked Questions
The specific pollutant concentration values that correspond to each AQI level, used to convert raw measurements to the 0-500 scale.
AQI breakpoints are the specific pollutant concentration thresholds that map raw measurement data to the AQI scale. Each criteria pollutant has its own set of breakpoints because the health effects of each pollutant occur at different concentrations. For example, a PM2.5 concentration of 9.0 micrograms per cubic meter corresponds to an AQI of 50 (the upper limit of "Good"), while an ozone concentration of 0.054 parts per million over 8 hours corresponds to AQI 50.