The modularity of<unk> s allows them to be customized for different working environments. Regardless of type, a<unk>  consists of: * some kind of headgear (mask or hood), * a<unk>  (motor-driven) fan which forces incoming<unk>  into the device, * a filter (or multiple filters) for delivery to the user for breathing, and * a battery or other power source. The mask may be hard and tight-fitting, or flexible and loose-fitting. The former affords a higher level of protection, but is less comfortable. Tight-fitting<unk> s require a fit test, while loose-fitting ones do not. This makes loose-fitting<unk> s useful when a fit test for a tight-fitting<unk>  cannot be successfully passed, for example when facial hair is present. Masks may be reusable or disposable. Some masks allow the full face to be seen by others, aiding in interpersonal communication. The fan, filter, and power-pack may be carried around freely by the user, often secured by a belt around the waist. Alternatively, with certain units, the<unk>  is fed to the user via lengthy tubing while the fans and filters are remotely mounted.<unk> s have low breathing resistance, unlike filtering facepiece<unk> s such as N95 masks. A<unk>  may have adjustable<unk>  flow rates for added comfort. While they are often referred to as positive pressure masks, they are not true positive-pressure devices as overbreathing can overcome the pressure supplied by the fan.
Powered air-purifying respirator
Flow-restricted, oxygen-powered ventilation device
Liquid ventilator
Exhalation delivery system
Elastomeric respirator
Open-source ventilator
Analytical nebulizer
Air filtration in operating rooms