PDFWAC 296-820-860

Appendix B: Calculating the Air Quality Index for PM2.5 (nonmandatory).

The Air Quality Index (AQI) for PM2.5 is calculated as follows:
 
IPM2.5
=
IHi - ILo
(Cp - BPLo) + ILo
 
 
BPHi - BPLo
 
Where:
 
IPM2.5
is the Air Quality Index value for PM2.5
Cp
is the concentration of PM2.5 in μg/m3 truncated to 1 decimal place
BPHi
is the concentration breakpoint that is greater than or equal to Cp
BPLo
is the concentration breakpoint that is less than or equal to Cp
IHi
is the AQI value corresponding to BPHi
ILo
is the AQI value corresponding to BPLo
PM2.5 THRESHOLDS1
AQI1
AQI CATEGORY1
WA DOH HEALTH MESSAGING2
0.0-12.0
0-50
Good
It is a great day to be active outside and a good time to make a plan if worse air quality is in the forecast.
12.1-35.4
51-100
Moderate
Some people are especially sensitive to lower levels of particle pollution and should reduce exposure. For example, limit time outside and avoid strenuous outdoor activity. All sensitive groups should watch for symptoms.
35.5-55.4
101-150
Unhealthy for sensitive groups
Sensitive groups should take steps to reduce exposure. Limit time outside, avoid strenuous outdoor activity, and follow tips for cleaner indoor air. Everyone should watch for symptoms as a sign to reduce exposure.
55.5-150.4
151-200
Unhealthy
Everyone should reduce exposure. Limit time outside, avoid strenuous outdoor activity, and follow tips for cleaner indoor air.
150.5-250.4
201-300
Very unhealthy
Everyone should reduce exposure. Stay inside and filter indoor air to keep it cleaner. Go elsewhere for cleaner air, if needed.
250.5-350.4
301-400
Hazardous
Everyone should reduce exposure. Stay inside and filter indoor air to keep it cleaner. Go elsewhere for cleaner air, if needed.
350.5-500.4
401-500
Hazardous
Everyone should reduce exposure. Stay inside and filter indoor air to keep it cleaner. Go elsewhere for cleaner air, if needed.
˃ 500.4
Beyond the AQI
Hazardous (beyond the AQI)
 
1
U.S. EPA. September 2018. Technical Assistance Document for the Reporting of Daily Air Quality – The Air Quality Index (AQI). EPA 454/B-18-007. Research Triangle Park, North Carolina.
2
Washington Department of Health. April 2022, accessed April 2023. Washington Air Quality Guide for Particle Pollution: https://doh.wa.gov/sites/default/files/legacy/Documents/4300/waqa%20infographic%5fEnglish.pdf?uid=64384c71c8715
[Statutory Authority: RCW 49.17.010, 49.17.040, 49.17.050, 49.17.060, and chapter 49.17 RCW. WSR 24-01-070, § 296-820-860, filed 12/14/23, effective 1/15/24.]