Guides

The questions that come before choosing a model. Written from published standards and manufacturer documentation, with the assumptions behind each figure stated — and with the parts nobody can answer honestly left marked as such rather than filled in.

• Do you actually need an air purifier at home? An honest answer: what a home air purifier does, what it cannot do, and the three questions worth asking before you spend anything.
• How to size an air purifier for your room Work the sizing out from CADR instead of the square footage on the box, with the arithmetic written out and the assumptions stated.
• "True HEPA" vs "HEPA-type": what the labels are worth There is no public certification a purifier maker can obtain. Here is what each label actually rests on and how to check a specific unit.
• How to check your air purifier is actually working A 30-minute PM2.5 test that shows whether your air purifier delivers its rated CADR in your own room, with the numbers to expect and the arithmetic.