Why does dandruff only appear on your head and not, say, your knees or back?
鈥� Dandruff is simply flakes of the outer skin layer that is continually shed as older cells die and are replenished. Look in any enclosed space visited by people and you will notice how much is suspended in the air.
It is estimated that each of us sheds around 450 million of these cells daily 鈥� or up to 0.2 grams. House dust is up to 90 per cent human cells. Not surprising given that we change our skins monthly.
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鈥淗ouse dust is up to 90 per cent human cells 鈥� not surprising given that we change our skins monthly鈥�
Skin sloughing varies with several factors including age and body part, as well as individual differences in skin dryness and replenishment rate. Knees become noticeably rough from protracted kneeling as dead skin flakes are constantly rubbed off. On our backs they are dislodged by clothing. We smooth the soles of our feet and our heels with pumice stones and a close shave removes plenty of dead skin with the stubble.
Hair has at least three properties that conspire to highlight skin flakes. First, it provides a dense jungle that traps them. Second, it is prone to electrostatic charges that make them stick. And third, it is usually darker in colour, which draws attention to them. The scalp and shoulders are also rather level, so loose flakes don鈥檛 fall to the ground readily. Bald skin has less ability to hold and show up flakes.
Underlying skin complaints can increase the degree of flaking. Anti-dandruff shampoos work by combating electrostatics and moistening the skin, and many are hypoallergenic.
House dust is a genuine health hazard 鈥� the droppings of dust mites that eat it can trigger asthma, while the shed skin鈥檚 oily squalene can react with ozone to produce noxious aldehydes. And dandruff may be unduly off-putting because of an instant association with personal hygiene. Sloughed cells still harbour skin bacteria, and not all are benign.
Len Winokur, Leeds, UK
鈥� Dandruff only appears on our heads because this is where our sebaceous glands are concentrated; on the face and scalp. This means that dandruff can even show up in the hair of your eyebrows or beard.
These glands secrete sebum, which lubricates and waterproofs the skin, but also feeds the fungus Malassezia globosa. During a dandruff episode, the population of this fungus can double. It metabolises triglycerides in sebum. Oleic acid is produced as a by-product and penetrates the topmost layer of the skin. This triggers an inflammatory response in susceptible people, increasing the number of cells that are shed.
Normally, about half a million cells per square centimetre of scalp are shed every time that we wash our hair. Individual cells are microscopic, but the oily sebum acts as a glue to stick them together into big enough clumps to be seen as dandruff. And in people with dandruff, skin cells can mature and be shed within a week, when a month is more typical for others.
Mike Follows, Willenhall, West Midlands, UK
This article appeared in print under the headline 鈥淕etting scalped鈥�