An air purifier is a device which removescontaminants from the air in a room. These devices are commonly marketed as being beneficial to allergy sufferers andasthmatics, and at reducing or eliminatingsecond-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 (AHU) 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 CO2 from air before processing.Pressure swing adsorbers or other adsorption techniques are typically used for this.