Air Purifier Buying Guide: What to Know Before You Buy

CADR stands for Clean Air Delivery Rate, measured in cubic feet per minute, and it is certified by an independent testing body under a standardized protocol. A purifier gets three CADR numbers: smoke (the smallest particles, 0.09 to 1 micron), dust (0.5 to 3 microns), and pollen (5 to 11 microns). Smoke CADR is the most demanding and the one worth comparing. Square footage coverage claims, by contrast, are calculated by the manufacturer using whatever air-change assumption they choose. A unit quoting two air changes per hour can legally claim roughly two and a half times the coverage of the same machine quoted at the 4.8 air changes per hour that allergy guidance uses. CADR is a measurement; coverage is an interpretation. Compare the measurement.

True HEPA filters typically last 6 to 12 months of continuous operation, and carbon layers saturate faster, usually 3 to 6 months. Manufacturer intervals assume average conditions; a home with pets, smokers, or an active construction site nearby will need replacements sooner. Real annual costs vary widely: Coway AP-1512HH runs about 60 dollars a year, Levoit Core 300 about 50, Winix 5500-2 about 45 to 60 with its washable carbon pre-filter, and Blueair 211+ about 60 to 80. Some budget units with proprietary cartridges run 150 to 200 dollars a year, which exceeds the machine's purchase price. Before buying any purifier, look up the exact replacement filter part number, note its price and interval, and multiply by five years. Add that to the sticker price to get the number that actually distinguishes these products.

Pure mechanical filtration purifiers, meaning a fan pulling air through HEPA and carbon media, produce no ozone. Ozone comes from electronic air-cleaning features: ionizers, plasma generators, electrostatic precipitators, and some UV lamps produce it as a byproduct. Ozone is a respiratory irritant that can worsen asthma and reduce lung function, and there is no established concentration at which it usefully cleans indoor air without also being an irritant. California's Air Resources Board maintains a certification list for indoor air cleaners sold in the state precisely because of this. The practical advice: prefer mechanical filtration. If a purifier you want includes an ionizer or plasma feature, verify it can be switched off independently of the fan, then switch it off and leave it off. The HEPA filter is doing the work regardless.

For a single enclosed room, one correctly sized unit is better: a large fan moving a given volume of air is quieter and more efficient than a small fan straining to move the same volume. For an open-plan space or multiple rooms, two smaller units placed apart usually outperform one large unit in a corner, because air distribution matters as much as raw CADR. Purifiers clean the air that reaches them, and a single unit in a 900 sq ft open-plan space creates a well-filtered zone near itself and a poorly filtered far end. Two units at opposite ends produce a more uniform result even at a lower combined CADR. The rule: match one unit per enclosed room, and split coverage across large or irregular spaces.