in a dishwasher or a bath tub, rather than being used and thrown away. The down-side? They're the most expensive type of filter, costing from a couple hundred to over a thousand dollars.

Hybrid units

Because each type of filter captures a different thing, many manufacturers offer hybrid units which hold both a gas and a mechanical filter. They are available in all types, from simple portable models - such as the one from Enviracare which filters particles such as smoke, pollen, and dust as well as gases - to the large, complex Sun Pure Air Purifier which holds six filters arranged in increasingly strict order to filter out just about everything harmful you can imagine!

There is one important note to add about filters: recent research has shown that living in an environment which is too hygienic can have negative effects on your health, and especially on the developing immune systems of babies and children. Filtering everything that is potentially nasty out of the air – including minor infectious disease bacteria – may actually cause allergies and asthma by weakening the immune system.