Your hair is naturally coated in sebum, an oily barrier that protects it from environmental damage.

Sebum is a complex mixture of lipids produced by the sebaceous glands, which are located in the dermis layer of the skin and usually attached to hair follicles. It consists of approximately 41% triglycerides, 26% wax esters, 12% squalene, and 16% free fatty acids. This unique chemical composition is found only in humans and serves as a critical hydrophobic barrier.The primary function of sebum is to maintain the structural integrity of the hair cuticle. According to dermatological studies, hair is hygroscopic, meaning it can absorb up to 30% of its weight in water. Without the fatty acid coating of sebum, water would penetrate the cortex too easily, causing the hair to swell and leading to hygral fatigue, which eventually breaks the hair fiber.Beyond physical protection, sebum plays a vital role in the skin's innate immune system. Research published in the Journal of Lipid Research highlights that the free fatty acids in sebum, specifically sapienic acid, have potent antimicrobial properties against pathogens like Staphylococcus aureus. This creates an 'acid mantle' on the scalp with a pH between 4.5 and 5.5, inhibiting harmful bacterial growth.Historically, the study of sebum intensified in the mid-20th century as cosmetic chemistry evolved. Scientists at institutions like the University of Pennsylvania helped map how sebaceous activity is regulated by androgens. This explains why sebum production peaks during puberty and gradually declines with age, directly affecting hair texture and scalp health over a person's lifetime.