Air purifier buyer's guide
Every air purifier does the same thing: a fan pulls room air through a filter. The differences that matter are how much air it moves, how clean the filter gets it, what the filters cost over time, and how loud it is while doing it. Four decisions, in order.
1. Size it to the room with CADR
Clean Air Delivery Rate is the one spec that tells you how much clean air a unit delivers. Measure your room, multiply its square footage by two-thirds, and that is the minimum smoke CADR to look for (8-foot ceilings). A 12-by-15 bedroom (180 sq ft) needs about 120 CADR; an open 30-by-20 living area needs 400. Buying one size up is cheap insurance, because it lets you run the unit on a quieter speed and still get the same clean air. Our room-size classes follow this rule. More in CADR explained.
2. Price the filters, not the box
Replacement filters commonly cost $30 to $130 a year. Over five years that often exceeds the purchase price, and it is where cheap units get expensive. Every model page shows filter cost per year and five-year cost of ownership, and the cheapest-to-run list sorts on it. A washable prefilter catches hair and dust before the main filter and stretches its life.
3. Check noise at the speed you'll actually use
The CADR on the box is measured at top speed, which on most units is 50 to 60 dB: loud enough that people turn it down. What you want is enough CADR at a speed you'll tolerate. Oversizing (step 1) is the usual fix. For bedrooms, a lowest setting under 26 dB is near-silent. See noise ratings explained.
4. Decide whether you need carbon
HEPA filters catch particles: smoke, dust, pollen, dander. They do nothing for gases and odors. Cooking smell, wildfire smoke's irritants, VOCs from new furniture and pet odors need activated carbon, and only a real bed of it (a pound or more) works for long. A thin carbon-coated mesh is a marketing feature. The odors and VOCs list ranks only units with a substantial carbon stage.
Features worth paying for
- Auto mode with a particle sensor: runs the fan only as hard as the air requires. Most useful feature on the list.
- AHAM Verifide certification: the CADR was measured by an independent lab. See AHAM explained.
- Energy Star: a reasonable proxy for an efficient motor; a unit running all day costs real money.
Features to skip
Ionizers and "plasma" modes (some produce ozone; turn them off), UV lights (too brief an exposure to matter at these airflows), "HEPA-type" or "HEPA-like" filters (not HEPA; see HEPA vs HEPA-type), and filter-life claims measured in years without a stated airflow.
Placement
Put the unit a foot or more from walls, not in a corner, on the floor or a low table, with nothing blocking the intake. Keep doors to the room closed when air quality matters. One purifier per room beats one large unit for a whole floor, because CADR does not carry through doorways.