You know the ads. The hunk puts the moves on the pretty girl. She seems to respond and then suddenly catches sight of the telltale snowy speckles on his broad shoulders. Goodgrief, dandruff. End of attraction. End of romance.

Well, as is so often the way with TV commercials, this may be something of an exaggeration, but a recent survey suggests that people believe there must be something to it. According to a survey of people with dandruff conducted by the ICR Survey Research Group of Pennsylvania, half of about 700 people questioned worried about how others reacted to their dandruff. Men, as it turned out, worried more than women -- 58 percent of the men and 48 percent of the women were concerned about the reaction of others -- and were almost as concerned about that reaction as about the condition itself. Only 11 percent of those surveyed knew that most dandruff conditions -- when small bits of dead skin fall from the scalp -- are caused by a yeast infection -- and only few cases are caused by dry skin, oily hair, stress or poor grooming.

Dandruff is a pretty common affliction. According to experts, somewhere in the neighborhood of 50 million Americans have it, to a greater or lesser extent. And although it is a relatively benign condition, about 21 percent of those surveyed said they'd rather have a headache and 17 percent said heartburn would be preferable. Ten percent said they'd be willing to exchange dandruff for an allergy attack, and 9 percent said they'd gladly exchange it for athlete's foot.

In recent years, dermatologists have determined that most dandruff is caused by an overgrowth of a type of fungus found naturally on most scalps. This yeast-like organism is called Pityrosporum ovale, and it seems to thrive mostly in cold, dry weather. It is one of the many types of organisms that make their home on the surface of all humans, but for some unknown reason it goes out of control in some people and creates the overabundance of flaking on the scalp. Dandruff problems can be exacerbated by stress but are not caused by it, although researchers do not understand why.

According to George Washington University dermatologist Carmen Myrie Williams, dandruff problems are often found in families and researchers believe there may be some inherited trait that promotes the growth of the fungus. Williams, who sees a lot of seborrheic dermatitis (dandruff to you) in her practice, notes that it is especially prevalent in fall and winter when the humidity and the temperature drop.

The fungus "is particularly sensitive to sudden changes in temperature and humidity, so that travel from, say, a tropical rain forest to Scandinavia would be likely to induce a flare-up in susceptible people," she said.

For the most part, however, it is a cosmetic problem, "especially for people with dark hair and for all of us who tend to wear darker-colored clothes in the fall and winter," she said. But for some people it can be more serious. People with impaired immune systems -- those having certain cancer treatments, people recovering from organ transplants and those who have contracted the AIDS virus -- may suffer from repeated, severe cases. It can infect eyebrows and lashes and cause an uncomfortable inflammation of the eyelid called blepharitis.

Dandruff may also be caused by an allergic reaction -- to a shampoo, for example. In children, however, the shedding of scalp skin is usually caused by the fungal infection known as ringworm. As a general rule, children are not susceptible to the yeast infection that afflicts their elders, but again researchers do not know quite why. Dandruff usually first occurs in the form we know it sometime during the teenage years.

Treatment for adults is usually a matter of daily shampooing with an assortment of over-the-counter shampoos containing sulfur, selenium, salicylic acid or tar. For severe cases, especially when the skin of the face -- on the forehead, around the nose, in the eyebrows -- is affected, several prescription shampoos containing the anti-fungal chemical ketoconazole are available. These should be used only under a physician's care, Williams said. Sometimes steroid lotions may be applied to the scalp after shampooing.

Meanwhile, the survey, which was conducted for Janssen Pharmaceutica, found that:

More men worry that dandruff is a sign of poor grooming than women (49 percent vs. 41 percent).

Almost one third -- 31 percent -- of those surveyed said they chose their clothing based on the dandruff condition.

Women reported stress more often then men as a cause of dandruff (36 vs. 27 percent).

Men are more likely than women to report having dandruff (44 vs. 33 percent).

Women are more likely than men to take the problem to the doctor (40 vs. 28 percent).

The bottom line, said Williams and other experts, is that nobody has to live with dandruff. Even the worst, most unsightly, itchy, unpleasant cases can be controlled. The doctor may not help the hunk get his girl, but if he doesn't, the dandruff won't be to blame.