PurpleAir PM2.5 correction
PurpleAir EPA Correction Calculator
Apply the EPA U.S.-wide correction to a PurpleAir CF=1 PM2.5 reading and relative humidity. For very high smoke, the calculator transitions continuously to the published extreme-smoke extension.
Correct a PurpleAir CF=1 PM2.5 reading
Apply the EPA U.S.-wide humidity correction, with a continuous transition to the research extension for extreme smoke concentrations.
Corrected PM2.5 concentration
- Correction
- -46.2 µg/m³
- Percent change
- -46.2%
- Formula branch
- U.S.-wide correction
- Raw CF=1 reading
- 100 µg/m³
A corrected single-sensor reading is not an official AQI. Build an hourly series, then use the PM NowCast calculator for a current particle AQI estimate.
Which PurpleAir correction formula is applied
- For CF=1 values below 570 µg/m³, corrected PM2.5 = 0.524 × CF=1 − 0.0862 × relative humidity + 5.75.
- From 570 to below 611 µg/m³, a concentration-dependent blend creates a continuous transition between the U.S.-wide and extreme-smoke equations.
- At 611 µg/m³ and above, the extreme-smoke extension uses 0.000421 × CF=1² + 0.392 × CF=1 + 3.44.
- Negative corrected outputs are displayed as zero. The page reports concentration only and never presents a single reading as an official AQI.
Air quality tools for the decision in front of you
Start with the number or question you already have. Every tool leads to a concrete calculation, comparison, or action plan instead of another generic article.
PM NowCast calculator
Turn the latest 12 hourly PM2.5 or PM10 readings into an EPA-weighted NowCast and AQI.
U.S. multi-pollutant AQI calculator
Calculate AQI sub-indices for ozone, particles, carbon monoxide, sulfur dioxide, and nitrogen dioxide.
Indoor PM2.5 calculator
Estimate steady-state indoor PM2.5 from outdoor air, room volume, infiltration, sources, and purifier CADR.
Trust Signals
Confirm CF=1, humidity, sensor placement, and data quality
Review policy
August 8, 2026. We update supporting pages when calculator logic, live data behavior, or source framing changes.
Primary sources
AQI uses EPA / AirNow breakpoint logic. Cigarette-equivalent framing follows Berkeley Earth’s PM2.5 interpretation.
Live data layer
City pages use EPA AirNow where supported, with cached fallbacks and planning baselines if the API fails or coverage is unavailable.
PurpleAir correction questions
Should I use CF=1 or ATM data?
Use the PurpleAir CF=1 PM2.5 concentration. The published U.S.-wide correction equation is defined for CF=1 data.
Why does relative humidity matter?
Optical particle sensors can respond to water associated with particles. Humidity is an input to the U.S.-wide correction equation.
When is the extreme-smoke formula used?
The extension begins blending at 570 µg/m³ and is used fully from 611 µg/m³ upward, where the ordinary linear correction underestimates extreme concentrations.
Is corrected PM2.5 the same as official AQI?
No. It remains a corrected concentration estimate from a sensor. A particle AQI needs the appropriate reporting average or NowCast workflow.