Usually, the treatment consisted of warming up the oil of your choice, completely saturating your hair with it, and spending a few minutes sitting under the hairdryer. The theory: Heat helps the oil penetrate the hair shaft better, giving dry hair a quick infusion of moisture.
"Technically, steam causes cuticle swelling, and whatever is applied to the hair during the time the cuticle is swollen can absorb into the shaft," Yolanda Lenzy, MD, of Lenzy Dermatology in Massachusetts, tells Refinery29. Hence, why some believe that adding heat to your hair while it's drenched in oil or applying heated oil will allow it to better penetrate. But that isn't always the case.
"Whether or not oil actually absorbs depends on the molecular weight of the oil," Lenzy says. In a study published in the International Journal Of Trichology, researchers compared the molecular weight of coconut, sunflower, and mineral oils. They found that coconut oil has a low weight, which allows it to penetrate the hair shaft. But others, like sunflower and mineral, latch to the surface of the cuticle, which could leave the hair with an extra-greasy feeling even after you shampoo.
According to celebrity stylist Ursula Stephen, this is one reason that hot oil treatments aren't a hot commodity in her salon. "Hot oil treatments are supposed to infuse oil into the hair," says "But it isn't in high demand like before because, sometimes, it can make your hair feel too oily."
While Lenzy explains that there isn't anything wrong with treating your hair with oil before you shampoo, there are a few precautions to take. First, she says it's not a good idea to heat up the oil before application, since that could cause burns. Instead, apply the oil and then add heat. And when you shampoo, make sure all the oil is removed — that way you get slip and shine, minus the extra-greasy feeling.
Lenzy also explains that there are some hair types that should avoid oil treatments altogether. If you're struggling with an itchy, flaky scalp, oil can potentially worsen your condition. "Generally, flaking is caused by yeast on the scalp and putting oil on the scalp is like providing food for the yeast," Lenzy explains. "So initially, oil can make it look like flakes are gone, but they come back soon after and can be way worse."
To give your hair the benefits of shine, hydration, and manageability, Lenzy recommends reaching for a conditioning treatment that is water-based over a hot-oil treatment. "Water hydrates, so if your hair is dry a water-based conditioning treatment will deliver optimal moisture," says Lenzy. "Overall, oil works best as a finisher in your hair-care routine to smooth the cuticle."