Key Facts: Air Purifiers
This page is Pure Air Lab's maintained fact reference — the specification and certification facts our guides are built on, restated in plain prose. We report what a purifier is rated or certified to do, never health-outcome claims.
CADR: three numbers, one that matters for sizing
CADR (Clean Air Delivery Rate) is an AHAM-administered test reported in CFM for three separate particle classes: smoke (0.1–1.0 microns), dust (0.5–3.0 microns), and pollen (5.0–11.0 microns). Size by the Smoke CADR — the smallest, hardest-to-capture class and the most conservative number — not the flattering Pollen figure many listings lead with. The full breakdown is in What CADR Do I Need for My Room? and the normalized reference table is The Air Purifier Numbers.
The AHAM 2/3 rule and wildfire sizing
AHAM's standard sizing guidance: Smoke CADR should be at least two-thirds of a room's square footage (multiply room sq ft by 0.67). During wildfire smoke events, AHAM recommends sizing up to a full 1:1 ratio — Smoke CADR at least equal to the room's square footage. More in Air Purifier Sizing for Wildfire Smoke.
ACH: the real throughput math
Air Changes per Hour converts CADR into how many times per hour a unit filters a room's entire air volume: ACH = (CADR in CFM × 60) ÷ room volume in cubic feet. Common guidance targets 4–5 ACH for allergy-sensitive households and 5–6 ACH during wildfire smoke. A purifier that looks adequately sized by square footage alone can still deliver a disappointing ACH if the room has high ceilings, which the CADR-per-square-foot rule doesn't account for.
True HEPA vs "HEPA-type"
"True HEPA" is a verifiable U.S. DOE/IEST claim: at least 99.97% removal of particles at 0.3 microns (the most-penetrating particle size). "HEPA-type," "HEPA-like," and "HEPA-grade" are unregulated marketing terms with zero required efficiency behind them. The European EN 1822 standard separately grades filters H13 (≥99.95% at MPPS) and H14 (≥99.995%) — both genuine HEPA-grade classes, with H13 the more common consumer tier. Full explainer: Is "HEPA-Type" Real HEPA?
Ionizers and the CARB ozone limit
Ionizers, electrostatic precipitators, and other "electronic" air-cleaning technologies can generate ozone — itself a lung irritant — as a byproduct. Since 2010, California's CARB requires certification for all portable air cleaners sold in-state, and electronic/ionizing models must test at or below 0.050 ppm (50 ppb) ozone emission to pass. A CARB listing confirms testing against that ceiling, not a zero-ozone guarantee; an unlisted device hasn't been verified against it at all. See Do Ionizers Make Ozone?
Filter cost is the recurring number that matters
The purifier's sticker price is often the cheap part; the filter is the recurring cost. A True HEPA cartridge typically runs 6–12 months depending on use and air quality, and combined HEPA+carbon cartridges cost more per swap since you replace fresh carbon media whether or not it's saturated. The real comparison metric is (filter price ÷ months of life) × 12 — annualized cost — not the sticker price of the machine. See Why Filters Cost More Than the Machine and Filter Cost-Per-Year Comparison Method.
Carbon filters: gases and odor, not particles
Activated carbon filters work by adsorption, capturing gases, VOCs, and odor molecules that pass straight through a particulate HEPA filter. They do not meaningfully capture particles like dust, pollen, or smoke soot — that's the HEPA layer's job. A thin carbon panel bonded to a HEPA filter handles light odor; heavy smoke, cooking, or pet odor calls for a deeper dedicated carbon stage. Details in Carbon Filters: What They Actually Remove.
Houseplants do not meaningfully purify room air
NASA's often-cited 1989 Clean Air Study tested plants in sealed lab chambers, not real rooms with normal air exchange — follow-up research estimates you would need dozens to hundreds of houseplants per room to match even a modest mechanical purifier's air-changes-per-hour. Plants are a negligible, not a meaningful, contributor to indoor air purification. See Do Plants Purify Air?
How these facts are maintained
These facts are drawn from AHAM's public CADR and sizing guidance, the U.S. DOE/IEST True HEPA specification, the EN 1822 European filter standard, CARB's public regulation and certified-devices materials, and manufacturer specification sheets, reviewed against our guides and updated on the date shown above when standards or the market change. We do not test purifiers ourselves, we do not publish prices or star ratings, every pick names an honest drawback, and we report specification/certification facts — never health-outcome claims. Our full methodology is at How We Evaluate Air Purifiers.