03/03/2004 - Extra: Hair
Background for students and parents
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Counting hairs: You have more than 100,000 hairs on your head, but you lose about 50 to 100 each day. That’s a good thing, because we’d all look like huge hairballs if it didn’t. Normally, when a hair falls out, a new hair starts forming in the same place.
Hairs with a purpose: Some of the hair on your body is easy to see, like your eyebrows and the hair on your head. But other hair, like the hair on your cheek, is almost invisible. Depending on where it is, hair has different jobs. The hair on your head keeps your head warm and provides a little cushioning for your skull. Eyelashes protect your eyes by decreasing the amount of light and dust that go into them, and eyebrows protect your eyes from sweat dripping down your forehead.
How hair ‘grows’: Whether hair is growing out of your head, arm or ankle, it all rises out of the skin in the same way. It starts at the hair root, a place beneath the skin where cells band together to form keratin (a protein that also makes up your nails and outermost layer of skin). The root is inside a follicle (say: fah-lih-kul), which is like a small tube in the skin. As the hair begins to grow, it pushes up from the root and out of the follicle, through the skin where it can be seen. Tiny blood vessels at the base of every follicle feed the hair root to keep it growing, but once the hair reaches the skin’s surface, the cells within the strand of hair die. The hair your see on your body contains dead cells – that’s why it doesn’t hurt when someone cuts your hair with scissors.
Shiny locks: Nearly every hair follicle is attached to a sebaceous (say: sih-bay-shus) gland, which is sometimes called an oil gland. These glands produce oil, which makes the hair shiny and a bit waterproof.
How long it can go: If the hair on your head is not cut, it can grow up to 5 feet long. But hair grows slowly – only about one-half inch a month – so it would take 10 years to grow your hair to 5 feet. That’s difficult to do, because hairs on the head generally only grow for two to six years before the follicle goes into a rest period and the hair falls out. The hair is later replaced by a new hair growing from the same follicle. This cycle of hair growing, resting, falling out and being replaced helps to maintain just the right number of hairs on your head. At any time, about 85 percent of hairs on your head are in the growth phase, while 15 percent are in the rest period.
Why arm hair stays short: How long a follicle will produce new hair cells depends on where it is located on your body. The hair follicles on your head are programmed to let hair grow for years at a time, but the cells that make the hairs on your arms are programmed to stop growing every couple of months. That’s why the hair on your arms stays short.
Colors and shapes: Hair color comes from melanin (say: meh-luh-nun), the substance that gives hair and skin its pigment. The lighter someone’s hair, the less melanin there is. Older people lose the melanin pigment in their hair as they age, making their hair look gray or white. When it comes to hair shape, the follicle is what makes the difference. Some follicles are structured to produce curly hair, while others send out a straight hair. Follicles also determine if your hair will be thick and coarse or thin and fine.
People losing hair: When someone has hair loss, the hairs don’t grow back after falling out. This can happen all over the head or in just one spot. The fancy name for hair loss is alopecia (say: ah-loh-pee-shuh). The most common type of baldness is in older men. Younger people sometimes lose hair because of stress, malnutrition or medical problems such as cancer – conditions that cause hair not to replace itself after it falls out. Ringworm, a fungal infection on the scalp, causes hairs to break off.
Animals that shed: Some animals have hair follicles that synchronize their rest periods so that all the hair falls out at one time. Many animals can also switch the coloring agent in the follicle on and off, so in the summer the hair is pigmented brown but in the winter is not pigmented, leaving it white. This protects them against summer sunburns.
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