At once deeply personal and publicly scrutinised, hair sits at the boundary between biology and identity. Few attributes are as tied up in culture, beauty and expression as the strands that frame our faces — and dot our skin.
You have about 1,00,000 hair follicles on your scalp and many more across your body, said Maksim Plikus, a cell biologist at the University of California, Irvine, US. Each is like a 3D printer, he said, home to a growing strand of hair and pigment that gives it colour.
These strands are made mostly of keratin, a tough protein also found in your nails. But they’re much more than dead matter; hair is a wonder of biology, serving as a sensor for your brain and an archive of your body’s secrets.
We asked experts what everyone should know about hair.
Protects you
Inside your hair follicles is a microbial zoo, teeming with bacteria, viruses and fungi, said Dr Ralf Paus, a dermatologist at the University of Miami in the US. While you may have heard of the gut microbiome, you also have a hair microbiome, which crowds out harmful germs, reducing the risk of infections. The microbes inside your hair follicles might even help your hair grow and enhance your hair colour, Dr Paus said.
When you get scrapes, cuts or other minor injuries, your body mobilises stem cells inside your hair follicles to migrate to the wound and to transform into new skin cells, Plikus said. “When the wound heals, the stem cells fold back to their day job of growing hair,” he added.
Acts as antenna
Your hair follicles are like motion detectors for your brain, wrapped in nerve endings that fire with the slightest touch or breeze, said Angela Christiano, a molecular geneticist who studies hair disorders at Columbia University, US. For instance, if an insect drifts near your eyes, your lashes sense it and reflexively set off a blink.
Hairy skin has nerves that feed into the brain’s emotion centres, which is why a gentle caress or someone stroking your hair can feel pleasant. By contrast, waxing or plucking your eyebrows is quite painful, since the hair is ripped out of the follicle.
Surprisingly, hair follicles also contain smell and bitter-taste receptors that seem to influence hair growth, Dr Paus said. Early data suggests that certain fragrances seem to stimulate hair growth while others might inhibit it.
Hair follicles tell time, too. They have an internal clock, like the one inside your brain, helping to keep hair growing, winding down, resting and shedding in a repeating cycle, said Dr Luis Garza, a dermatologist at Johns Hopkins Medicine, US. It’s probably why hair tends to grow faster in the morning than in the evening (which is why it makes little difference whether you shave at night or the next day). In the future, experts believe that a few hair samples may be all it takes to monitor certain sleep disorders, since the clock genes inside the follicles are synchronised to your body’s 24-hour clock.
Signals problems
Hair is the second-fastest growing tissue in the body. (Bone marrow is the first.) It can serve as an unofficial archive of your health, said Dr Antonella Tosti, a dermatologist at the University of Miami, US. In fact, a centimetre of hair captures about a month’s worth of biological data, so doctors can test hair for drug use, poisonings, chronic stress and even medication adherence.
“Hair follicles are basically like little kidneys,” Dr Paus said. “They suck up toxic compounds and deposit them in your dead hair shaft.”
Rapid hair loss often serves as a sign of illness or stress, Christiano said. Nutrient deficiencies, thyroid dysfunction and high fevers can all cause sudden hair loss, as can sudden weight loss, childbirth and major surgery. “When it happens it can be really distressing,” she added, but the hair usually grows back within three to six months.
Some experts believe that when your body is under strain, it shuts down less essential processes like hair growth and maintenance, to redirect nutrients and energy to other parts.
Still, some hair shedding is inevitable — we lose about 50 to 100 strands of hair each day. Hair often starts off as peach fuzz when you’re young, turning longer, darker and thicker as you grow and then becoming wiry, unruly and whiter with old age.
“We take it for granted because we see it every day — but scientifically, hair is awe-inspiring, a true marvel of nature’s engineering,” Plikus said.
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