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.
The EPA has developed a quick and easy AQI to Concentration and Concentration to AQI calculator. Click below to access these pages.
The AQI equation, breakpoints, and information on truncating vs rounding of concentrations/AQI are located here.
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.
Hourly Ozone AQI
If viewing/calculating hourly ozone AQI for hours >1 old, these calculations are done using the midpoint 8-hour average ozone concentration which is then converted to AQI. For a given hour, this includes the previous 4 hours, the given hour, and the following 3 hours. 6 of 8 hourly concentrations (75%) are needed for a valid midpoint 8-hour average.
For examples of a midpoint ozone AQI calculation, click here.
Current data does not have a valid midpoint 8-hour average, as only 5/8 hours are available. Thus, the NowCast is used:
If viewing/calculating hourly ozone AQI for the most recent hour or the Current Conditions in 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. More information on this method can be found here.
Method used prior to August 1, 2019: This is a weighted average of the most recent 8 hours. The completeness criteria for the ozone NowCast are that 2 of the past 3 hours of data must be valid. Ozone NowCast concentrations can be calculated using an AN-T data query or by following the steps 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.
Hourly PM2.5 and PM10 AQI
If viewing/calculating hourly PM2.5 or PM10 AQI for hours >5 old, these calculations are done by using the midpoint 24-hour average concentration converted to AQI. For a given hour, this includes the previous 12 hours, the given hour, and the following 11 hours. 18 of 24 hourly concnetrations (75%) are needed for a midpoint 24-hour average.
For examples of a midpoint PM2.5 and PM10 AQI calculation, click here.
The most recent 5 hours of data cannot have a valid midpoint 24-hour average, as <18/24 hourly concentrations are available. Therefore, the PM2.5 and PM10 Nowcast is used:
PM2.5 and PM10 NowCast
If viewing/calculating hourly PM2.5/PM10 AQI for the most recent 5 hours or anywhere that provides the Current Conditions in 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.