Body hair

Human bodies are covered in different types of hair that serve various biological roles. Most people have a layer of vellus hair covering almost their entire body from childhood. This hair is very fine and light in color, making it difficult to see.

There are only a few exceptions where vellus hair does not grow, such as the palms of the hands, the soles of the feet, and the lips. During and after puberty, a different type of hair called androgenic hair, or terminal hair, begins to develop. This hair is much thicker and darker than vellus hair.

The transition from vellus to terminal hair is a key part of human development.

The growth of androgenic hair is driven by a specific biological mechanism involving hormones. It depends on the levels of androgens, which are male hormones, in the body. It also depends on the density of androgen receptors located in the dermal papillae. The dermal papillae are the structures at the base of hair follicles. For hair follicle cells to multiply, both androgen levels and receptor density must reach a certain threshold.

As these levels rise during puberty, the fine vellus hair transforms into coarser terminal hair. This process happens to varying degrees across most human populations.

Androgenic hair follows a different growth cycle than the hair on the scalp. Every hair goes through an anagen phase, which is the active growth stage. For scalp hair, this phase lasts for many years. However, for androgenic body hair, the anagen phase only lasts a few months. There is also a telogen phase, which is a dormant or resting stage. The length of the telogen phase varies by location, ranging from a few weeks to nearly a year.

This shorter growing period and longer resting period explain why body hair is much shorter than head hair.

Different regions of the body show different patterns of hair distribution. Pubic hair is often the first to appear because these areas are highly sensitive to androgens. In industrialized countries, this typically begins around age 11 for females and 12 for males. Some females may begin this growth as early as age 8.

Arm hair often concentrates on the wrists and forearms. In adolescent males, terminal arm hair growth is often much more intense than in females. Men generally develop more abundant hair on the chest, back, and limbs compared to women.

Facial hair is another major area of androgenic development. Both men and women experience facial hair growth, though the patterns differ significantly. Moustaches in young men often appear around the onset of puberty. Some men may not develop facial hair until their late teens, or perhaps not at all.

Women may develop hair around the chin, sideburn area, or upper lip. In some cases, female beard growth results from a hormonal imbalance or a rare genetic disorder called hypertrichosis.

Beyond appearance, androgenic hair serves a vital sensory function. It provides tactile sensory input by acting as a bridge between the environment and the skin. When the hair shaft moves, it transfers vibrations and movement to sensory nerves within the skin. Follicular nerves specifically detect the displacement of these hair shafts.

This allows the body to detect air movements or the contact of small objects, like an insect, before they even touch the skin. This extends the sense of touch into the space surrounding the body.

Cultural practices and history have also shaped how people interact with their body hair. For example, the practice of shaving underarm hair became popular in the United States and United Kingdom around 1915. This was largely due to advertisements in magazines. The widespread use of the safety razor at the start of the 20th century made regular shaving much easier. Today, many people shave hair on their legs or arms for cosmetic reasons or for sports. Athletes like swimmers and cyclists may shave to reduce friction or highlight muscle development.