What causes dandruff? Dianna Kenneally, principal scientist for P&G Beauty, explains.

Dandruff-irritated scalp is not just dry skin. A naturally occurring microscopic fungus called malassezia [pronounced mal-uh-SEZ-ee-uh] is the real cause.

This fungus, she says, feeds on the scalp's natural oils and creates by products that irritate the scalp. The body fights back by increasing the amount and rate of dead skin cell flaking. Every human being has malassezia fungus, Kenneally says, but not every person's immune system will respond to the irritation with dandruff. And that's why so many myths have arisen around dandruff. The condition is not contagious, nor is it brought about by poor circulation, an improper diet or bad hygiene, she says.

A client who complains of dandruff may have other issues, Kenneally says, especially if she cites an itching, burning or a dry feeling in her scalp. Research shows that 25 percent of the U.S. population describes their scalp as sensitive, she says. This is often associated with the medical condition called atopy, where people have inherited a predisposition to react to certain allergens and develop such conditions as eczema, hay fever and asthma.

Such clients could benefit from daily use of an anti-dandruff shampoo specifically formulated for sensitive scalp, as long as the scalp shows minimal irritation. A severely irritated scalp might be seborrheic dermatitis, a condition that affects up to 10 percent of the U.S. population, Kenneally notes.Symptoms of seborrheic dermatitis include large, oily, yellow flakes and itching accompanied by redness and inflammation. If this appears to be the case with any client, immediately refer him or her to a physician. Shampoos designed for dandruff today contain pyrithione zinc, which removes the dandruff, but also offer conditioning ingredients to leave the hair both beautiful and dandruff free.”