Most of your body is covered in nearly invisible 'peach fuzz' called vellus hair.

Vellus hair develops in the human fetus around the 20th week of gestation, replacing the even finer lanugo hair. Unlike terminal hair, vellus hair follicles are not connected to sebaceous glands and lack a medulla, which is the innermost core of a hair shaft. This structure makes the hair exceptionally soft and light. According to dermatological studies, these hairs are typically less than 2 millimeters long and lack significant pigmentation, making them appear almost invisible to the naked eye.The primary function of vellus hair is thermoregulation and sensory perception. By trapping a thin layer of air against the skin, it provides a minor insulation barrier. More importantly, the hair follicles are wrapped in nerves that detect displacement. This allows the body to sense external stimuli, such as a crawling insect or a light breeze, before the object actually touches the skin surface.During puberty, an increase in androgenic hormones like testosterone triggers a biological process called terminal differentiation. This causes vellus hair in specific areas—such as the armpits, face, and pubic region—to transition into terminal hair. Research published in the Journal of Investigative Dermatology notes that while the number of hair follicles remains constant throughout a person's life, the type of hair those follicles produce changes based on age and hormonal signaling.