AQI Calculations Overview- Ozone, PM2.5 and PM10

Overview

The AQI was developed by the EPA and is used to convey how clean or polluted the air around you is. There are 6 separate AQI levels: Good, Moderate, Unhealthy for Sensitive Groups (USG), Unhealthy, Very Unhealthy, and Hazardous. The thresholds for each category were developed based on established national air quality standards. For more information on the basics of the AQI, click here.

This topic will focus on ozone, PM2.5, and PM10. However, there are also standards for CO, NO2, and SO2. Information on all pollutants and a detailed guide by the EPA is also available: Technical Assistance Document for the Reporting of Daily Air Quality.

AQI Calculator

The EPA has developed a quick and easy AQI to Concentration and Concentration to AQI calculator. Click below to access this page.

AQI Equation

The AQI equation, breakpoints, and information on truncating vs rounding of concentrations/AQI are located here.

Ozone

• Daily Ozone AQI
  The daily ozone AQI is calculated by taking the maximum 8 hour concentration from 07:00-23:00 LST, and then converting to AQI. There are 17 8-hour averages considered in each day. Each 8-hour average requires 6 of 8 hours (75%) for a valid calculation. In addition, 75% or 13/17 of the 8-hour averages are needed for a valid daily ozone AQI calculation.

  For examples of a Daily Ozone AQI calculation, click here.

• Ozone NowCast (Hourly AQI)
  If viewing/calculating the hourly ozone AQI, these calculations are done using the ozone NowCast concentration converted to AQI. As of August 1, 2019, the ozone Nowcast uses the Partial Least Squares (PLS) method. NowCast concentrations can be calculated using an AirNow-Tech data query. More information on this method can be found here.

PM2.5 and PM10

• Daily PM2.5 and PM10 AQI
  The daily PM2.5/PM10 AQI is calculated by taking the 24-hour concentration average from midnight to midnight (Local Standard Time) and converting to AQI. 75%, or 18/24 hours of data are needed for a valid daily AQI calculation.

  For examples of a Daily PM2.5 and PM10 calculation, click here.

• PM2.5 and PM10 NowCast (Hourly AQI)
  If viewing/calculating the hourly PM2.5/PM10 AQI, these calculations are done using the PM2.5/PM10 NowCast concentration converted to AQI. This is a weighted average of the most recent 12 hours. NowCast concentrations can be calculated using an AirNow-Tech data query or by using the steps here.