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Rinsing your hair with cold water after washing will make it shinier—this claim, which has circulated for decades, was recently refuted face-to-face by several dermatologists.

According to the popular explanation, cold water can tighten the hair's cuticle, making it smooth.

Once the surface is smooth, light reflects more uniformly, creating what appears to be shine.

From the magazine era to the short-video era, this claim has been repackaged in several ways, but the core has always been cold water.

Dr. Paradi Mirmirani, a dermatologist at Kaiser Permanente in Northern California, put it more bluntly.

She said that whether it's cold or hot water, there is no solid research proving that water temperature makes hair shinier.

Dr. Teresa Song, a dermatologist in New York, was equally direct: cold water probably won't make much difference to shine.

Her assessment of the claim was that it is absolutely not valid.

Dr. Lilia Correa Selm, an associate professor of dermatology at the University of South Florida, recommended lukewarm water.

She believes the ideal water temperature is close to body temperature.

If the water temperature is too low, its cleansing power decreases, making it easier for oil, dirt, and hair product residue to remain on the scalp.

Dr. Jacob Beer, a dermatologist in West Palm Beach, Florida, warned about the other extreme.

Water that is too hot can dry out the scalp and easily cause irritation.

His advice is to use a silicone-based conditioner to smooth the cuticle, and shine will naturally emerge.

As for how often to wash, according to the American Academy of Dermatology, it depends on hair type and scalp.

People with straight hair and an oily scalp can wash once a day without issue.

Those with dry, thick, curly, or coarse hair can go two to three weeks between washes.

Returning to shine itself, Dr. Song said what really works is daily hair care habits, especially conditioner.

A cold water rinse is not on that list.

Dermatologist Dr. Hope Mitchell compared the cuticle to overlapping shingles on a roof.

This layer wraps the inside of the hair and reflects incoming light.

The so-called shine we see is this reflected light.

Fine, straight hair reflects light more uniformly, making the shine concentrated.

Thick, curly hair scatters light, presenting a soft, subtle sheen.

Mitchell reminds us that shiny hair does not equal healthy hair; the two cannot be equated.

Dr. Song added another aspect: friction damages the cuticle, and shine is compromised as a result.

Brushing too vigorously or rubbing wet hair hard with a towel are both forms of this friction.

Rather than obsessing over water temperature, it's better to use conditioner and be gentle when brushing and drying.

As for the cold water rinse step, you can skip it.