Air purifiers: size it by CADR and plan for the filter cost

The one number that matters, how to match it to a room, why filter price is the real price, and which features are noise.

An air purifier is a fan and a filter. The whole purchase comes down to moving enough air through a good enough filter, quietly, at a running cost you'll keep paying.

Size by CADR

Look for the CADR (clean air delivery rate) for smoke or fine particles. A practical rule: CADR in cubic feet per minute should be at least two-thirds of the room's square footage — a 300 sq ft bedroom wants roughly 200 CFM. The number lets the unit do four to five air changes an hour on a mid setting instead of screaming on high. Manufacturer "room size" claims usually assume one change per hour; halve them.

Filter

• True HEPA (H13 class) for particles. "HEPA-type" and "HEPA-like" are marketing.
• Carbon for odours, cooking and VOCs — the weight of carbon matters; a thin mesh does little.
• Replacement price and interval. Multiply filter cost by changes per year; that number is the real price of the purifier over three years. Some brands make the filter cheap and the unit dear, others the reverse.

Noise

You will run it on low or medium almost always, so the dB rating at those settings is what matters; under about 30 dB on low is bedroom-quiet. Bigger units running slowly are quieter than small units running fast for the same clean air.

What to check

• Sleep mode that kills the display and holds the fan at its quietest.
• Auto mode and a particle sensor — useful, but laser sensors read fine particles only; they're blind to gases.
• Pre-filter you can vacuum; it extends the main filter's life.
• Certification (CARB for ozone in California; AHAM Verifide for CADR).

Ignore, mostly

• Ionizers and "plasma" modes — switch them off; some produce ozone.
• UV lamps in consumer units; the air passes too quickly to matter.
• Apps beyond a filter-life reminder.

Placement

A foot from the wall, not in a corner, in the room where you spend the most hours — usually the bedroom. One good unit per room beats one big unit for the whole house.

Today's prices

Every seller we found, ranked by total price: air purifier.

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