Hair care isn’t one-size-fits-all. We asked dermatologists to explain the science of hair—and what all those shampoo ingredients really do.
A marker of beauty, pride, health, even political beliefs—today our hair takes center stage in our formation of identity, and it’s always been a part of what makes us human.
Our earliest ancestors were covered from head to toe in hair. It was important for regulating body temperature and protecting the skin from sun exposure. Scalp hair might have even provided important padding for our skull, says Brett King, an associate professor of dermatology at Yale University.
However, over time humans evolved to grow less hair on our limbs and bodies. While some body hair serves important health functions––nose hair protects against bacteria getting in our airway––the most prominent hair, on our scalp, is primarily aesthetic.
“Right now, the way we use hair, it’s really a form of self expression and empowerment,” says Loren Krueger, an assistant professor of dermatology at Emory University.
It’s also one of the first functions that break down when health declines. Cancer patients who undergo chemotherapy lose their hair quickly because the treatment targets growing cells.
“If you’re nutritionally deficient or have lost a lot of weight because you’re sick, your hair is one of the first things that kind of shuts down,” says George Cotsarelis, chair of the dermatology department at University of Pennsylvania.
Hair care is a booming industry, worth billions. While countless products market themselves as promoting hair growth, it can be hard to figure out what your hair actually needs. Experts explain the science behind hair and what consumers should consider when buying hair products.
The science of hair
Hair is composed of a shaft, the visible part, and the root, which connects to the skin. The root is surrounded by a sheath called a hair follicle below the skin, which promotes hair growth.
Scalp hair goes through three phases: a growing phase (the majority of its life), a transition phase, and the last phase, when it falls out.
“In order to grow hair, we need to lose hair,” explains Shilpi Kheterpal, an assistant professor of dermatology at the Cleveland Clinic.
There are different curl patterns, porosity, and thicknesses of hair. Scalp hair differs between person to person, with genetics, especially ethnicity, playing a major role in its characteristics.
The idea of healthy hair is a bit tricky and not easy to define. Oma Agbai, an associate clinical professor of dermatology at the University of California, Davis School of Medicine, sees thickness, length, shine, and strength as signs of healthy hair. Beyond excessive shedding, other signs of poor hair health can include thin-appearing hair with visible scalp, dull hair, and damaged hair with split ends.
With age, follicles produce less melanin, making hair lighter until it becomes white. Hair loss increases and growth slows, and strands become thinner. With increased brittleness, hair can be more sensitive to environmental factors like wind, UV light, humidity, and certain chemicals.
According to Agbai, wearing tightly-pulled hair styles can induce damage to the hair follicle leading to a form of hair loss called traction alopecia. Excessive coloring of the hair to lighten the color can cause damage to the hair shaft leading to dullness, increased hair fragility, and breakage.
What does your hair actually need?
It might surprise you to learn nothing is actually needed to care for your hair, though regular washing is important.
“I don’t necessarily think that you have to consider it a bare minimum, it’s more so ‘what society views as acceptable now’,” Krueger says. Regular washing “allows you to have optimal moisturization and the appearance you want to have.”
Washing does more than clean. It helps the hair get rid of the dead hairs which can stimulate new growth, says Cotsarelis. Our follicles are attached to sebaceous glands, which make oil called sebum to add moisture to the hair. If people with straight hair don’t wash it frequently, the sebum can build and attract fungus (usually from the genus Malassezia). This can cause inflammation and potentially, dandruff.
If you have itching, flaking, rashes in the scalp, Krueger says that may be a sign you may need to wash more frequently. Still, too much washing can dry out the hair. “It’s all about balance,” says Kheterpal. “You want to make sure you’re washing your hair and taking care of it but not overdoing it.”
People who produce less sebum can get away with showering less. People who sweat a lot and or exercise frequently should wash their hair accordingly–– showering after working out or producing excessive sweat.
Black hair is often dry and coarse, so it doesn’t need to be washed as often as other people’s hair. Kheterpal generally recommends a separate line of products specifically formulated for Black hair, like Head & Shoulders Royal Oils. These are known as cowashes, using conditioner to wash the hair rather than shampoo to add moisture back into the hair.
Be warned though, that the only part of your hair that’s “alive” is the follicle, not the strand. That means unless a treatment is topical, you likely won’t see changes in your hair.
Look to the label
You may have heard that certain hard-to-pronounce chemicals should be avoided for use on hair. Like most professional recommendations, it’s not one-size-fits-all.
Many shampoos use sodium lauryl sulfate, a powerful cleanser. While people with fine hair may benefit from sulfate shampoos––without it the hair can look flat and dull––the harshness can dry out the hair. Shampoos with sodium lauroyl sarcosinate and sodium cocoyl glycinate can be good, milder, alternatives, says Agbai. Sulfate-free shampoos are ideal for people with curly hair or those who color their hair, because sulfates make color fade faster.
Parabens are sometimes added to help prevent the growth of harmful bacteria and mold. They are generally ok to use unless the person is sensitive and has an allergy, Kheterpal says. The same is the case with fragrances––they can be fine, but for some they can be irritating.
Agbai also cautions against formaldehyde, as it can cause irritation and is also classified as a carcinogen. Krueger notes that biotin, a common hair product additive, isn’t that important for hair health and can interfere with lab tests.
Many of these ingredients have not been tested in clinical trials as the FDA doesn’t need to approve over the counter hair products. Zinc pyrithionine, which is found in dandruff shampoos, is one ingredient that does have proven benefits. Even if you don’t have dandruff, it can decrease microscopic inflammation and promote hair growth, says Kheterpal.
“Hair is incredibly personal,” Krueger emphasizes. You may be able to get clues on what works for your hair from family members or others of the same age, ethnicity or, but ultimately, experts say you should experiment to see how often your hair needs care and what products work for you.
Source: National Geographic (22 August 2023)