Follicles genetically vary, producing flattened hairs which are curly, or round hairs which are straight. If you flatten a straight hair by running it between two finger nails, it will become curly. Hairs vary in diameter. There are thick hairs, as in eye lashes, grading down to fine down-like hairs on different parts of the body. Hair grows on the head or in the beard at about 4 mm a week.
Hair usually contains pigmentation making the hair appear black, or brown or red, or blonde.
In aging follicles there is little or no pigmentation of the hair and the hair is then white. Oil glands in the skin can secrete a covering to the hair making it appear glossy. People differ in the extent of their hair covering. Hair can slow the cooling of the skin and also give protection against sun burn. It can also prevent abrading of the skin when parts of the body rub against each other as in walking or arm movements against the chest.