This Hypoallergenic Filter is designed for the ResMed S9™ Series positive airway pressure machine to keep out finer particles in the air that contribute to allergy symptoms. It is disposable and should be changed regularly when soiled. The Ultrafine CPAP Filter is made from electrostatically charged medias that attract harmful particles out of the air and capture them. The ultrafine term refers to the mechanical make-up of the media or fiber weave.