HEPA purifiers: the CADR math

A True HEPA filter: the H13 or H14 grade, captures 99.97% of particles at 0.3 µm, with even higher efficiency above and below that size. The "0.3 µm" specification reflects the most-penetrating particle size for HEPA media; smaller and larger particles are captured more efficiently. For indoor PM_2.5, ultrafines, pollen, dander, and mold spores, HEPA filtration is the single most effective consumer intervention available.

The sizing metric is CADR: Clean Air Delivery Rate, in cubic feet per minute, measured separately for tobacco smoke, dust, and pollen. A purifier's air changes per hour are ACH = CADR (cfm) × 60 ÷ room volume (ft³); in metric, ACH = CADR (m³/h) ÷ room volume (m³). AHAM's "2/3 rule" (a smoke CADR of at least two-thirds of the room's area in ft²) works out to about 5 ACH with an 8-ft ceiling and is a conservative starting point. For wildfire smoke, AHAM recommends a smoke CADR equal to the room's area in ft², about 7.5 ACH in the same room.

For specific conditions, target more: 8–10 ACH during wildfire smoke or pollen-allergy seasons, 6 ACH for sensitive groups continuously. Multiple smaller units distributed through living spaces typically outperform one large unit, because air mixing across closed doors is poor. EPA's consumer guide covers selection criteria.

What HEPA doesn't do: it doesn't remove gases (CO₂, VOCs, NO_x), that requires activated carbon or ventilation. It doesn't kill bioaerosols faster than they'd die anyway in still air: the filter just captures them. Ozone generators and ionizers are not HEPA; the EPA cautions explicitly against ozone-emitting units. Watch the CFM rating; some units have impressive cubic meters per hour numbers but tiny actual airflow.

This is general guidance, not a substitute for professional assessment of your specific home. Major interventions (HVAC redesign, sealing a leaky envelope, mold remediation, electrical work for fans or venting) should be done with a certified professional. For chronic problems that don't respond to the steps here, see when to call a pro.