Wikipedia has more on Claustrophobia
Claustrophobia (from Latin claustrum, "a shut in place") is the fear of having no escape, being in small spaces or rooms, or being confined. The fear is usually most intense in young children, and it is common in adults too. The fear is often triggered due to being locked in a dark room, closet, or even in a box, wearing clothes that are too tight on them, being trapped in an elevator due to blackout, being alone in a vehicle, or getting stuck in the holes or between bars of a fence. Sufferers would avoid being in a small room (even when lit especially without windows), small spaces, vehicle, elevator, crowds, cellar, and caves. Since suffocation is commonly caused by limited amount of air available in small spaces, claustrophobics may often fear being suffocated as well.
Studies suggest that about 5-7% of all people suffer severe claustrophobia, and it is believe to be more commonly suffered by fat people. Not just fat people suffer from claustrophobia, skinny people too.
Claustrophobia is treated by gently facing their fears.