The main cause of dandruff is probably not what you think. Here’s what it is.

Ugh, dandruff. Looking in the mirror and seeing those small, white or gray flakes of skin sticking out like a sore thumb against your dark hair can be so frustrating. This might feel especially true if you go through cycles of dandruff-free hair and not-so-dandruff-free hair. You might ask yourself "Where is this dandruff coming from?"

Dandruff is often mistaken for a dry scalp, says Dr. Chris Adigun, a board-certified dermatologist in Chapel Hill, North Carolina. But dandruff and dry scalp are two different conditions. Dandruff flakes are a manifestation of the skin condition seborrheic dermatitis.

Here’s what you need to know about this cause of dandruff so you can properly treat it.

What is dandruff?

Seborrheic dermatitis is a common skin condition that can show up as dandruff on the scalp and-or as scaly patches on the skin. These patches vary in color based on skin tone and can be dry, greasy or itchy. Seborrheic dermatitis usually appears in bodily areas that produce a lot of oil, like your scalp, sides of your nose, eyebrows, in ears, on your chest and even on your eyelids.

What causes dandruff?

Dandruff — or seborrheic dermatitis more broadly — is usually caused by an inflammatory reaction to Malassezia furfur, a yeast that lives and grows on everyone’s skin. Adigun says that those who experience this response tend to either grow more yeast on the skin, have skin that “just doesn’t like the yeast” or some combination of the two.

Why am I getting so much dandruff all of a sudden?

Seborrheic dermatitis flares up because of various triggers. A common trigger for seborrheic dermatitis on the scalp is going long periods between shampoos. This allows for the yeast to build up and reach a point that the skin can no longer tolerate. “Your body gets to the point where it just can’t tolerate [the yeast]. It becomes inflamed and angry,” she adds. “It’s kind of like a friend that you get a little too much of, and you get annoyed at them and snap at them.” This leads to dandruff.

Why do I have dandruff if I wash my hair daily?

How often someone needs to wash their hair to prevent dandruff varies by person. “Some people can go two weeks before they get seborrheic dermatitis. Other people, they’re going to get raging seborrheic dermatitis on the scalp … if they just skip one or two days,” says Adigun. “So, you have to figure out the schedule that’s best for you.”

For those who are very prone to dandruff, Adigun recommends using a shampoo with antifungal ingredients. Examples include Head & Shoulders and Selsun blue. She also suggests shampooing as often as possible. These steps will help reduce the build-up of yeast on the scalp and, in turn, lead to a more dandruff-free head of hair.

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