What Causes Dandruff?
Over the last century, medical opinion has shifted regarding the reasons for seborrheic dermatitis, explains Dr. Roderick Hay, a consultant dermatologist and professor of cutaneous infection at King’s College in London, England. Early 20th century medicine pinpointed a fungus on the skin as the culprit, but in the 1950s and ’60s, according to Hay, “It was regarded as a proliferate disorder, a condition in which the epidermis replicates much faster, so something like psoriasis.” In the last 30 years, the fungus theory has resurfaced because doctors realized anti-fungal medications reduced dandruff.
The particular yeast—a kind of fungus—associated with dandruff is called Malassezia. “Our skin is covered with literally millions of bacteria,” says Hay, and for the most part they are harmless. In some people, though, the Malassezia yeast can produce enzymes that irritate the skin by either stripping it of its natural fats or by triggering the immune system, which then develops a proactive response. “The two together are probably part and parcel of what we call dandruff,” says Hay. People who are stressed and tired are often more prone to the condition, and while cold weather can aggravate dry, scaly skin, dandruff is not usually seasonal, says Hay.