Types of hair

On most adult humans there are two main types of hair: terminal hair, and vellus hair. A third type, lanugo hair, is present in the fetus, and some newborn babies.

Terminal hair grows thick and long, and is what grows on the head, armpits and pubic area, as well as on the face, chest, arms and legs (better evident in men).

Vellus hair is a very soft and short hair that grows most places in the body in both sexes. In Caucasians it is often colourless, or blonde. It is best seen in women and children, as they have less terminal hair to obscure it.

Hair change with ageing

Older people tend to develop gray hair (actually colorless) because the pigmentation in the hair gets lost and the hair becomes colorless. The age on which this occurs varies from person to person, but in general nearly everyone from 75 years or older, has gray hair, and in general men tend to become gray at younger ages than women.

The older a person is, the more the chance is that he or she has gray hair, and above 85 almost nobody has the original hair color. Gray hair is considered to be a characteristic of normal ageing.

Androgenic hair

The hair follicles on much of the body respond to androgens (primarily testosterone and its derivatives). The rate of hair growth increases and the heaviness of the hairs increases. However, different areas respond with different sensitivities. As testosterone level increases (normally at puberty), the sequence of appearance of sexual (androgenic) hair reflects the gradations of androgen sensitivity. The pubic area is most sensitive, and heavier hair usually grows there first in response to androgens. The following regions also respond to androgens, in order of decreasing sensitivity: axillary and perianal areas, sideburns, above the upper lip, periareolar areas, chin and beard areas, center of chest, arms and legs, across the chest, shoulders, buttocks, back, and abdomen.

It is the hair in these areas that appears earlier or grows to excess in disorders of excess androgen (e.g., precocious puberty, late-onset congenital adrenal hyperplasia, and polycystic ovary syndrome).