Coverage area is the biggest trap when buying an air purifier: the number cannot be compared across brands. Convert the same machine at two air changes per hour versus five and the square-footage claim more than doubles. Most US brands use 2 ACH, Blueair uses 5 ACH, Dyson skips AHAM's CADR entirely in favour of its in-house POLAR whole-room test (sampled at multiple points in a 27 m² room), and Chinese makers publish CADR in m³/h under GB/T 18801. So Alen's 1,300 sq ft does not automatically beat an IQAir that publishes no square-footage figure at all.

A second widely misunderstood point: the 0.3 µm in the HEPA standard is not a lower limit but the hardest particle size to catch. Particles smaller than that are jostled by Brownian motion into the filter fibres and are actually easier to trap. That is why IQAir can claim ≥99.5% efficiency down to 0.003 µm — no physics is being broken, the test point has simply moved somewhere more extreme.

The national split is a mild surprise too: three of the ten are Korean brands (two Coway plus Winix), matching the three American ones, with Switzerland, Sweden, the UK and China taking the remaining four slots.