An air purifier or air cleaner is a device which removes contaminants from the air in a room. These devices are commonly marketed as being beneficial to allergy sufferers and asthmatics, and at reducing or eliminating second-hand tobacco smoke. The commercially graded air purifiers are manufactured as either small stand-alone units or larger units that can be affixed to an air handler unit or to an HVAC unit found in the medical, industrial, and commercial industries. Air purifiers may also be used in industry to remove impurities such as carbon dioxide from air before processing. Pressure swing absorbers or other absorption techniques are typically used for this. There are two types of air purifying technologies, Active and Passive. Active air purifier use ionisation for cleaning the air. Passive air purification units on the other hand use air filters to remove pollutants.