The PAPR uses a battery-powered fan to pull ambient air through filters or cartridges, and circulates it over the user’s face within a full-face mask, protective helmet, or hood. PAPRs may be suitable for particulate dusts and mists, or chemical gases and vapors, depending on the particulate filters or chemical cartridges used. A breathing hose connects the cartridge/filter to the mask, hood, or helmet. 
While the respirators discussed previously are negative pressure respirators (the user inhales, drawing in air), the PAPR is considered a positive pressure respirator because air is delivered to the wearer. The battery pack to power the fan is strapped to the user’s waist or back. Batteries for PAPR range from rechargeable batteries to adapters for using 12V or 24V DC.
With the PAPR, filtered air surrounds the head area within the hood, full-face mask, or helmet. If there is a leak in the respirator, filtered air is forced out through the leak; contaminated air is not drawn in. People with health conditions that prohibit them from wearing a restrictive air mask such as a half- or full-facepiece respirator may be able to use this type. The PAPR is not to be worn in areas considered immediately dangerous to life or health (IDLH).
From: CDC/OSHA.gov Illus. by Atilliis & Associates