The surgeon general of the United States gets a rash around his midriff every time he wears a certain pair of trousers. An aged, bedridden man mysteriously comes down with eczema on the backs of his hands. A sexually active youth develops a condition that he thinks is a venereal disease.

What do these three individuals have in common? All of them -- and as many as 7 million other Americans -- have a sensitivity to one of the most commonly encountered of substances: rubber.

Actually, it isn't raw rubber itself, or latex, that causes skin rashes, swelling and itching; it's the chemicals used to process this harmless material into the many products loosely categorized as "rubber."

Probably three Americans out of every 100 have some sort of sensitivity to rubber-containing products. Rubber thus is one of the chief causes of contact dermatitis, or inflammation of the skin due to contact with an offensive substance, affecting 3 percent of adults and 6 percent of children.

The three cases mentioned above are typical rubber contact dermatoses. There is something in the elastic waistband of one particular pair of trousers that irritates Dr. C. Everett Koop's skin. The bedridden elderly man got a rash on his hands because he liked to tuck them under a rubber hot water bottle to keep them warm. And the young man's disease wasn't venereal at all; it was an allergic reaction to a condom.

Although rubber articles are sometimes labeled "genuine latex," they are not pure latex as it comes from the tree. For rubber to have the elasticity that makes it so useful, and to be durable under ordinary conditions, it must be processed with a wide variety of chemicals.

Synthetic rubbers, more widely used today than the latex-based kind, also contain antioxidants, preservatives and vulcanizers to give them the "natural" characteristics people expect.

Some properties of the very chemicals that help make rubber useful also cause unpleasant reactions in susceptible people. The metal nickel is a good example. Nickel compounds are used as antioxidants, to protect rubber from the deleterious effects of exposure to air; unfortunately, sensitivity to nickel is one of the most common types of allergy.

Some household detergents contain the same allergenic chemicals as some rubber articles, notably gloves. So the sensitive person who pulls on a pair of gloves to avoid the effect of dishwater can find that he or she has traded one problem for another. Non-allergenic vinyl gloves are available, and wearing lightweight cotton gloves inside the rubber ones can be an alternative solution to the problem.

To prevent reaction to nickel, some eyeglass manufacturers coat the nickel-containing metallic side pieces of their spectacles with a synthetic rubber, thus trading one potential allergy for another.

One of the more bizarre reactions to rubber products was documented in Contact Dermatitis, a leading medical text: a raccoon-like bleaching of the skin around the eyes, caused by chemicals in swim goggles. The condition is common enough, apparently, that one company produces a line of non-allergenic goggles. They are not available in stores but can be obtained on special order, through optometrists and ophthalmologists, from the Hilsinger Corp., Plainville Mass. 02672. Phone (617) 699-4406.

Dermatitis of one kind or another also has been traced to wet suits, swimming caps, scuba masks, nose clips and earplugs. Medical journals have also reported reactions to hearing-aid pads, breast-shaped prostheses for women who have had mastectomies, shampoo headrests in beauty parlors, cosmetic sponges, eyelash curlers and balls used in the game of squash.

In the 1960s there was a brief flurry of a contact dermatitis called Flubberitis (probably the only ailment named for a toy) that was traced to a bouncy, pliable plaything known as Flubber.

Sometimes the rubber is concealed. Shoes, for example, may be held together with high-strength adhesives that chemically resemble synthetic rubber.

When dermatitis strikes the feet, says skin specialist Ted Rosen of Houston, it usually shows up on the top of the foot where the skin is relatively thin. The stratum corneum -- literally "horny layer" of skin on the palms of the hands and soles of the feet -- tends to protect the sensitive underlying layers where allergic reactions manifest themselves.

A common condition, sometimes called "jock itch," that bothers men wearing snug-fitting shorts is often mistaken for rubber allergy but really is a reaction to bleach. Chemicals in the bleach react with compounds in the rubberized portion of the shorts to create an irritating deposit on the abric. The remedy, doctors say: buy a new pair of shorts and wash them without bleach.

For treating dermatitis, Rosen warns against over-reliance on drugstore remedies, many of which "can occasionally cause allergic problems."

He warns particularly about one of the most common preparations for skin disorders, calamine lotion:

Plain calamine lotion is totally innocuous, says Rosen, but not calamine that contains diphenhydramine, an antihistamine. Diphenhydramine can be given orally or in shots without sensitizing the patient allergically. But when lotion containing it is rubbed onto the skin, the user may become sensitized, after which "he can't take it orally without running the risk of developing a severe systemic {whole-body} reaction."

Once an allergist or dermatologist has pinpointed the substance to which a patient is sensitive, it is possible to find substitute articles that do not contain the allergen. Doctors often know or can easily find out where non-allergenic products can be purchased.

William Hines is a Washington free-lance writer.