Portable air purifiers' CADR ratings don't account for wildfire smoke particle sizes
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Consumers buying portable HEPA air purifiers for smoke season rely on CADR (Clean Air Delivery Rate) ratings printed on the box, but CADR is tested using standardized tobacco smoke, dust, and pollen particles — not the sub-1-micron ultrafine particles that dominate wildfire smoke. A purifier rated at 250 CADR for smoke may deliver effectively 150 CADR for wildfire-specific PM0.1-PM1.0 particles because the HEPA filter's single-pass efficiency drops for particles in the 0.1-0.3 micron range (the most penetrating particle size). A family sizes their purifier for a 400 sq ft bedroom based on the CADR number, but the room never actually reaches clean air because the real-world wildfire smoke delivery rate is 30-40% lower. This persists because AHAM (the trade association that administers CADR testing) has not updated its test protocol since 2002, and purifier manufacturers have no incentive to adopt a more realistic test that would produce lower headline numbers.
Evidence
The AHAM AC-1 CADR test standard uses three particle types (tobacco smoke 0.09-1.0um, dust 0.5-3.0um, pollen 5.0-11.0um) but wildfire smoke is dominated by ultrafine particles below 0.5um where HEPA filter efficiency dips. A 2020 study in Building and Environment by Liang et al. found that real-world PM2.5 removal rates during California wildfire events were 25-45% lower than manufacturer CADR ratings suggested. AHAM's last major revision to the CADR standard was in 2002.