A barrier device is one type of personal protective equipment designed to protect rescuers from exposure to infection when in close contact with victims. HIV and the hepatitis family of viruses are carried in blood and some other body fluids.
Since there is no way to know if the victim of a cardiac arrest is carrying these or any other communicable diseases, it's wise to use a barrier device to keep body fluids out of the rescuer's mouth during rescue breathing.
There are 2 types of barrier devices used by lay rescuers:
A flat plastic barrier that lays across the victim's mouth and nose. It conforms to the face and allows the rescuer to blow through a hole in the middle. The hole has either a one-way valve or a filter to protect the rescuer, depending on the brand. These flat barriers are popular with lay rescuers (and off-duty professionals) because they are so portable. Most of the time, they can fold up tight and fit on a keychain.
A mask shaped like a pear that fits over the mouth and nose. With proper technique, it seals around the mouth and nose. The rescuer blows through a one-way valve at the top to provide rescue breaths.
http://firstaid.about.com/od/cprbasics/f/08_Barriers.htm
