Air purifiers are devices employed in homes and business all over the world. Due to the amount of dust etc indoors, air purifiers are used to clean and 'purify' the air inside the building, circulating it until it is free from unnecessary particles.

An air purifier usually takes the form of a mechanical wall unit, but can be stood on the floor and vary in size, some squat box shapes and some slender towers. Prices vary in a wide range but there are many websites devoted to their sale, as well as a large variety of manufacturer. They use a variety of methods to remove contaminants from the air. Some use electricity to suck particles out of the air, while some use absorbent materials. All air purifiers must be cleaned regularly to keep them working properly, as most manufacturers boast at least 95% filtration of known impure air particles.

The industry benchmark is called the HEPA standard, a system that filtrates 99.97% of particles with a size of 0.3 micrometers (Department of Energy). The original HEPA filter was used in the Manhattan Project to stop airborne radioactive particles. Certain purifiers use UV light that is fatal to some germs. Some purifier manufacturers have come into controversy over units that generate ions in order to react with the contaminants in the air, as some machines produce the harmful pollutant Ozone.

Airborne particles such as dust, pollen and spores build up inside rooms. Many people use air purifiers to reduce the quantity and density of these specks of matter in order to provide a safer, less cloying atmosphere. This is very much the case for sufferers of asthma and allergy sufferers, to whom these particles can be dangerous allergens or hazards. Other uses are to prevent more direct threats, such as gases and even second hand smoke.