Why does your hair turn gray?

Does a fountain of youth exist? Since science has yet to hit on a universal cure for aging, many find an antidote to the implacable march of time in a bottle— a bottle of hair dye, that is.

Covering up gray hair is a multibillion-dollar business worldwide, a beauty behemoth that promises a temporary reprieve from one of aging’s most obvious signs. But why does hair gray, anyway—and will science one day find a way to reverse it?

The answer most likely is inside our hair follicles, the place where hair pigmentation begins and, for many, fades. The cells that turn our hair gray, melanocytes, are the same ones that produce the pigment melanin responsible for our hair, skin, and eye color.

“Their sole function is to produce pigment, and they deposit it into the hair shaft as it’s growing,” explains Melissa Harris, an associate professor of biology at the University of Alabama at Birmingham. As with skin color, hair color is thought to have evolved as part of the body’s protection against the sun’s rays.

The original evolutionary purpose of hair color may be simple, but the science behind your shade is anything but. Hundreds of genes play a role, and hair color is thought to be one of our most visibly inherited traits, with up to 99 percent of hair color being genetically determined.

 Fact 1: Gray hair is full of air

Each hair on your head is at some point in a four-part growth cycle: a years-long anagen phase, in which hair cells grow from the follicle; the catagen or transition phase, when growth slows and the hair separates from the follicle; the telogen or resting phase, in which the follicle prepares to release its hair and grow a new one; and the exogen phase, during which dozens, and sometimes hundreds, of hairs are shed per day from the scalp. This regenerative cycle is ongoing, and every follicle is on its own timeline.

Hair pigmentation happens during the critical anagen phase. As the hair cycle begins, stem cells inside the bulb of a hair follicle generate melanocyte cells, which in turn produce pigment. These melanocytes die by the end of the hair cycle, and the follicle produces new melanocytes from stem cells as the growth cycle repeats.

But over time, explains Harris, the melanocytes can lose steam, producing less—and eventually no—pigmentation. “They’re no longer doing their job,” she says. “The stem cell population also declines. If those stem cells disappear, you won’t have the melanocytes during that next hair cycle.” As a result, instead of being packed with melanin, the hair shaft is filled with air, and our eyes perceive the semi-transparent shaft as fading, silvery, or white.

Fact 2: Gray hair is caused by more than the ageing process

Since graying occurs at the hair follicle, a hair’s pigment can’t be changed once it has grown from the follicle. But some processes feed a longstanding myth that stress can cause people to turn gray “overnight.” In fact, stress can trigger a condition known as telogen effluvium, essentially an increase of the number of hairs that are no longer actively growing that results in more hair loss than usual. The hairs that remain may appear more obvious, putting existing gray hairs front and center.

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Age isn’t the only factor that can cause melanocytes to lose their oomph. Genetics play a part in pigment loss, too, and race and ethnicity are associated with the age at which hair grows gray, with white people graying up to a decade before their Black counterparts. Lifestyle matters, as well, Harris notes: “There are some clear environmental factors that can increase your risk for early graying,” she says.

Smoking, UV exposure, certain nutritional deficiencies, exposure to air pollution, and excessive alcohol consumption are all associated with earlier pigmentation loss, as are diseases like neurofibromatosis, an inherited disease that causes tumors to grow throughout the body, and thyroid disease. Others, such as those with vitiligo, a rare form of albinism, or Griscelli syndrome, a genetic disease that affects skin and hair pigmentation, may develop gray hair in infancy or early in life.

Fact 3: It can make you look distinguished or haggard—depending on your gender

Despite its original evolutionary purpose, the significance and frequency of hair color—and humans’ preferences for different-colored coifs—was helped along by our reproductive drives. Though the science is split, males are thought to have developed an evolutionary preference for rarer hair colors, such as blond, and people of both sexes see hair color as an indicator of both health and age.

For better or for worse, gray hair is associated with age—and can profoundly affect the way we see ourselves and others. Though up to 23 percent of people worldwide have at least 50 percent gray hair by the age 50, discrimination is rife against those who don’t cover up their grays. And society’s perception of silvering hair can vary depending on the gender of the person sporting gray locks. Men are widely considered more distinguished and attractive as they age, a phenomenon nicknamed the George Clooney effect after the ever-handsome actor. Women, however, face bias for visible grays, and up to 75 percent dye their hair.

A 2022 study in the Journal of Women Aging found that women experience a conflict between the perceived wisdom and competence of gray hair, their desire to show their authentic selves, and societal expectations to remain youthful and age “successfully”—i.e. grow older without obtaining wrinkles or white hair. When they surveyed women who had abandoned hair dye and attempts to look younger, researchers found that “they instead covered by means of careful attention to hair styling, cosmetics and clothes, in an attempt to reduce any impression of having let themselves go.” As a result, the researchers concluded, the women ended spending as much time, money, and effort looking “competent” and “kempt” as they aged as they once did dyeing their hair.

Fact 4: We may be able to reverse the gray one day

It’s no wonder, then, that many people see their first visible gray hair as an unwelcome milestone. But science may soon be able to reverse the very process that causes hair to fade, says Harris, who has worked extensively with melanocyte-producing stem cells.

So far, she’s learned that hair graying may be connected to an immune response and is now working on potential ways to reactivate the stem cells. Harris’ work piggybacks on other research that shows these stem cells can be manipulated in the lab, along with a surprising study showing re-pigmentation in a group of lung cancer patients whose hair regained color after they took an immunotherapy treatment.

The key may be a protein called PD-LI, which suppresses the immune system and is expressed more by dormant melanocyte-producing stem cells than those in the active division stage.

“It might actually have some novel roles we haven’t appreciated yet,” Harris muses. “What is this protein doing in stem cells? Could we use it to activate [hair pigmentation]?”

For now, the work continues, and it isn’t just cosmetic: Harris thinks the pigmentation process of human hair has lessons to teach science about other ways our bodies age and respond to stress and environmental factors—with much broader implications for overall human health. If we’re lucky, discovering how to reverse graying hair could come with an even greater perk—the ability to stay healthier longer and getting more out of the lifetime that, for most, delivers a few grays along the way.