Vellus hair is short, fine, light-colored, and barely noticeable hair that develops on most of a person's body from his/her childhood. Exceptions include the lips, the back of the ear, the palm of the hand, the sole of the foot, some external genital areas, the navel and scar tissue. The density of hair – the number of hair follicles per area of skin – varies from person to person. Each strand of vellus hair is usually less than 2 mm (1/13 inch) long and the follicle is not connected to a sebaceous gland.
Vellus hair is most easily observed on children and adult women, as they generally have less terminal hair to obscure it. Vellus hair is not lanugo hair. Lanugo hair is a much thicker type of hair that, in human forms, normally grows only on fetuses.
The Latin language uses the word vellus to designate "a fleece" or "wool".
Vellus hair replaces lanugo hair on a human fetus at 36 to 40 weeks of gestation. The growth cycle of vellus hair is different from the growth cycle of terminal hair. At puberty, the androgen hormone causes much of the vellus hair to turn into terminal hair and stimulates the growth of new hair in the armpit and the pubic area. In men, this change in vellus hair occurs on the face and the body.
Vellus hair can also grow in single or in small patches that usually grow 2-4cm and like to persist throughout a lifetime. This kind of hair growth is associated with genes and usually not harmful.
Vellus hair provides both thermal insulation and cooling for the body. This insulation regulates body temperature: the vellus hair functions like a wick for sweat. While a skin pore is open, sweat wets a strand of vellus hair. The sweat on the external part of the strand evaporates. More sweat wets the external part of the vellus strand and then evaporates. This process is perspiration.
The unusual growth of vellus hair is a side effect of some types of disease and of some types of mental disorders. An abundance of vellus hair can develop from an increase in the production of the cortisol hormone in a person with Cushing's syndrome. Anorexia nervosa increases vellus hair. Vellus hair can also be found in men with male pattern baldness or with hirsutism. Hormonal fluctuations in pregnant women cause foetal vellus hair to change to terminal hair. The terminal hair is usually shed after the birth of the baby upon the return of the hormones to the normal levels.