An air purifier is a device which removes contaminants from the air. This benefits those people with asthma or allergies. For those who hate smoke, this device can also be helpful.
The High Efficiency Particulate Air (HEPA) filters are one of the common types of purifiers. These are said to be experts in trapping pollens, mold spores and dusts. However, constant filter changing is needed.
The Charged Media Filters make use of static electricity to enable the collection of fine particles. Like the (HEPA) filters, this too needs constant filter change.
The Electrostatic Precipitators is another variant. It uses electric charge for the particles to be trapped on a collector plate. Its benefit is that the plates can be taken out, eliminating the frequent filter changes.
When purchasing these filters, things that you need to keep in mind are the noise it makes, the filters and their respective prices, and your room size. As for austerity measures in terms of energy consumption, check out the Energy Star Logo.