Room size and air changes
Purifiers are rated by room size, from around 25 square metres for a bedroom model to 250 for the largest office units. To check one, divide its airflow in cubic metres an hour by the room's volume (floor area times ceiling height) to see how many times an hour it changes the air. As a rough guide, aim for four or five changes an hour for allergies, which often means choosing a model rated for a larger room than yours.
Particles, smells or dry air
A HEPA filter traps fine particles such as dust, pollen, pet dander and smoke, while a carbon filter absorbs the smells and cooking odours that pass straight through a HEPA filter. Models with a particle sensor can speed the fan up when the air gets worse and slow it down once it clears, so they run quietly in a clean room. Humidifiers, and combined purifier and humidifier models, add moisture to air dried out by central heating.
Filter changes and running costs
At full speed most draw under 150 watts from an ordinary socket, so running one all day uses only a few units of electricity. Filters are the main running cost: as a rough guide a HEPA or carbon filter lasts six to twelve months, less in a smoky or dusty room, and a clogged one moves far less air. Humidifiers need fresh water and a regular tank clean to stop bacteria growing.
