A bruise is a purple splotch on the skin caused by broken and bleeding blood vessels underneath the surface. Bruises usually result from unexpected clashes with sharp corners on from other forceful encounters. Some people bruise at a level tap, though. Or they frequently wake up with unexplained bruises. Their bruises seem to appear for no reason. Spontaneous bruises are no serious threat to health, yet they're embarrassing – and puzzling.

Could a bruise be a blood vessel's way of reacting to an allergen? Very possibly, says William J. Rea, M.D., a cardiovascular surgeon in Dallas, Texas. For one thing, blood vessels are 15 times more sensitive to certain environmental chemicals such as formaldehyde and pesticides than are other tissue of the body.

And in the course of treating several people for phlebitis (inflammation of the blood vessels, usually in the leg), Dr. Rea noticed two things. All of the people with phlebitis were spontaneous bruisers, and when they were admitted to a chemical free ”environmental control unit” of his hospital, their phlebitis cleared up and they no longer bruised as easily (Annals of Allergy, November, 1981).

So Dr. Rea proposes that in some people, bruising is part of a reaction to chemicals coursing through their blood vessels. ”I’ve seen at least a hundred bruises that were caused by environmental allergy,” Dr. Rea told us. For guidelines on how to purify the air of chemicals, see Clearing the Air. See also Phlebitis.