Other Types of Hair
Aside from scalp hair, humans have four other primary categories of hair:
Lanugo: This is a thick, downy hair, whose name comes from the Latin “lana”, meaning “wool”. It is present on all fetal humans between approximately 5 and 8 months gestation, and is shed several weeks prior to birth. When a baby is born prematurely, it often has much of its lanugo still on its body. The hair present on the bodies of full-term babies is the much finer and less-insulating vellus hair. Lanugo is also common in the malnourished, making it a key diagnostic in anorexia nervosa.
Vellus Hair: The fine, nearly-invisible, and ubiquitous hair that covers all humans on almost all parts of the body (aside from the lips, palms, and soles of the feet) develops shortly before birth, and continues to cover the parts of the body not covered by androgenic or terminal hair throughout life. Vellus hair is less than 2-4 mm long, and is not connected to a sebaceous gland. This hair also surrounds the scalp hair on the forehead, temples, and neck.
Androgenic Hair: Beginning in puberty, thick, bushy hair begins to develop in place of the vellus hair, in the pubic and axillary (armpit) regions of both genders. In addition, it also develops on the face, chest, and stomach, to varying degrees, depending upon sex and genetics. Androgenic hair follows the same growth cycle as scalp hair, but has a shorter anagen (growth) phase, and much longer telogen (resting) phase.
Terminal Hair: This is the second of the two types of androgen-influenced hair, but it is less “bushy” and dense than what is traditionally considered “androgenic hair”. It’s colloquially known as “body hair”, and develops during puberty, but does not include facial, chest, pubic, or axillary hair.
On the legs, arms, and back, thicker, stronger hair grows beneath the vellus hair of childhood and pushes it out, replacing it completely in some parts of the body, and only partially in other parts. In women, the area covered by terminal hair is much smaller, whereas some men (particularly those with Scandinavian, Mediterranean, or Aboriginal Australian/New Zealand backgrounds) can be up to 70% covered in thick, insulating hair.
Triplets with Lanugo - Diseases of Infancy and Childhood. Henry Koplik, 1910.
Young Japanese boy, covered in vellus hair - despite the fact that it’s nearly invisible, each of us is covered with as many hairs per square inch as our apparently-hairier primate cousins! - Scenes From Every Land. Edited by Gilbert Grosvenor for National Geographic, 1907.
German boxer Max Schmeling, displaying highly developed terminal hair on the arms, as well as androgenic hair on the chest. Library of Congress digital archives. Original from 1938.
George F. Bond and Cyril Tuckfield after a rapid buoyant ascent of over 300 feet, after the USS Archerfish bottomed in 1959. Both men have significant terminal hair on the arms and legs.
Mark Twain shirtless, displaying androgenic hair - mustache and chest hair.