1. Aggregated owner reviews, not a personal test unit
For every model on this site, I read through the aggregated reviews left by pet owners on Amazon — often numbering in the thousands or tens of thousands per model. I'm not claiming to have run these units in a lab or measured particle counts myself; I'm reading what owners who actually live with the device report after weeks and months of daily use. Where you see a phrase like "owners report" or "based on X reviews," that's exactly what's happening.
2. CADR math: the 2/3 rule
Manufacturers publish a CADR (Clean Air Delivery Rate) number in CFM, and a "recommended room size." Those recommended sizes are frequently optimistic. The rule of thumb we use instead: for continuous, meaningful air changes, the unit's CADR (in CFM) should be at least two-thirds of your room's area in square feet. A purifier rated for a 300 sq ft room under that math should comfortably handle a 200 sq ft room at a more aggressive filtration rate — which matters a lot more when there's a litter box or a bird cage in it.
3. Real cost of ownership, not just the price tag
A cheaper purifier with an expensive, frequently-replaced filter can cost more over three years than a pricier unit with a cheap annual filter. Every review on this site includes a filter-cost-per-year calculation: published filter price × how often the manufacturer (and owners, in practice) say you need to replace it. We don't publish device prices — those change constantly and Amazon's own listing is the accurate source — but filter economics are a genuinely useful, durable number.
4. Noise: manufacturer data plus the "gets loud" complaints
We report the manufacturer's decibel range across fan speeds, then cross-reference it against how often owners specifically complain about noise at the speed they actually run the unit on (usually not the lowest one).
5. Reading the 1- and 3-star reviews on purpose
Five-star reviews tend to say "works great." One- and three-star reviews say what actually breaks, what support was like, and what the marketing copy conveniently left out. We specifically mine the lower-rated reviews for recurring patterns — the same complaint showing up dozens or hundreds of times is a signal; one angry review about a defective unit usually isn't.
6. No sponsored placements
We don't accept payment from manufacturers to influence rankings. Rankings are based on data, not deals. Where we earn money is through the Amazon Associates program when you buy through a link on this site — see our Affiliate Disclosure for the details.