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Air purifier marketing is a swamp of pseudo-science: "HEPA-type" filters, ionizers, UV wands, and impressive-looking particle counts with no measurement conditions attached. I test purifiers with a laser particle counter, and the good news is that the physics of air cleaning is simple. Buy based on three numbers, ignore everything else, and you'll get a machine that actually works.
The Only Spec That Matters Most: CADR
Clean Air Delivery Rate (CADR) measures how many cubic feet of air the purifier cleans per minute, tested with standardized smoke, dust, and pollen. It's the one number you can't fake. Everything else — filter layers, marketing terms, "covers 1,500 sq ft" claims — derives from or hides behind it.
Here's the room-size math: divide smoke CADR by two to get the maximum room area in square feet for four air changes per hour (what allergy and asthma sufferers need). A CADR of 200 handles a ~100 sq ft (10×10) bedroom properly. A CADR of 300 handles ~150 sq ft. Ignore any "covers" claim that isn't consistent with this math.
"HEPA" vs. "HEPA-Type": Read This Before Buying
True HEPA is a defined standard: the filter must capture 99.97% of particles at 0.3 microns. "HEPA-type," "HEPA-like," or "HEPA-style" mean nothing — those filters often capture 60–90% at best. For allergy sufferers, this single word difference is the difference between a purifier that works and one that makes noise. Always look for "True HEPA" or the standard designation (EN 1822 / ISO 29463).
Features Worth Paying For (and Worth Skipping)
Worth it
- Auto mode with a real particle sensor — purifier ramps up when air quality drops
- Quiet sleep mode under 30 dB — a bedroom purifier you turn off at night is useless
- Filter life indicator — better than guessing
- Washable pre-filter — extends the expensive HEPA's life
Skip it
- Ionizers / plasma / "PCO" — marginal benefit, some generate trace ozone; switch these off where possible
- UV-C lights — air moves too fast past the bulb to meaningfully sterilize
- App control — nice, but never pay extra for it
- Washable "HEPA" filters — a contradiction; washing destroys HEPA media
The Hidden Cost: Filters
The purchase price is half the story. A purifier running year-round needs 1–3 filter replacements annually, at $20–90 each. Compare the 3-year cost of ownership (unit + filters + electricity), not the sticker price. Our testing favorite's filters cost $35/year; a cheap competitor's cost $120/year — over three years, the "expensive" purifier was $100 cheaper.
The Short Version
Buy a True HEPA purifier whose smoke CADR is at least twice your room's area in square feet, run it continuously on a low quiet setting, and budget for filters. Placement matters too: keep it at least a foot from walls and never in a corner. For our tested picks, see the full air purifier guide.