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Guides

What the numbers on the box mean, which of them matter where you live, and what the machine will cost you after you have bought it. Every figure here comes from physics, a published standard, or the same 25-machine catalogue the finder ranks.

Air purifiers

Smog, dust, allergens and the smell of a new flat.

How to choose an air purifier: the decision in orderSmog, dust, allergens or formaldehyde? Size, filter cost and noise decide the rest. The whole air purifier decision, in the right order.12 min
Understanding CADR and room size: the sizing maths, shownCADR is measured on the loudest setting, per pollutant. Here is the air-changes arithmetic that turns it into a room size you can trust.11 min
Where to put an air purifier — and how to live with itA purifier in a corner behind the sofa does a fraction of its job. Clearance, closed doors, bedroom versus living room, and sleep-mode noise.11 min
HEPA grades explained: what H13, H14 and 0.3 microns really meanHEPA is rated at its worst particle size, not its best. What H13 and H14 mean, why a sealed path matters, and what the grade labels hide.12 min
Formaldehyde and VOC removal: what a purifier can and cannot doFormaldehyde is a gas, so HEPA does nothing for it. What activated carbon does, why it saturates, and why this matters far more in China.11 min
Air purifier running costs: the number nobody checksFilters, electricity and three-year totals. Why the cheapest air purifier on the shelf is often not the cheapest one to own.12 min

Water purifiers

What is actually in your supply, and what it takes to remove it.