Hair, a filamentous, often pigmented, outgrowth from the skin, is found only on mammals and often in a high-density of filaments per unit area.
These threadlike fibers protrude from the outer layer of the skin, the epidermis, and grow from hair follicles in the inner portion of the skin, the dermis. Each fiber comprises nonliving cells whose primary component is long chains (polymers) of amino acids forming the protein keratin.
The keratinized cells arise from cell division in the hair matrix at the base of a hair follicle and are tightly packed together. Keratins also are a principle part of the cells in the nails, feathers, hooves, horny tissues, and tooth enamel of mammals.
In humans, hair, with its variety of colors, textures, shape, length, density, and other qualities, adds to individual uniqueness and provides an aesthetic quality for others to see and appreciate.
The hair of non-human species is commonly referred to as fur when in sufficient density. The effectiveness of fur in temperature regulation is evident in its use in such mammals as polar bears, and its perceived beauty is evident not only in its historical use in fur coats, but also in the popularity of pet grooming.
There also are breeds of cats, dogs, and mice bred to have little or no visible fur.
Although many other life forms, especially insects, show filamentous outgrowths, these are not considered “hair” according to the accepted meaning of the term.
The projections on arthropods, such as insects and spiders are actually insect bristles, not hair. Plants also have “hairlike” projections.
Head hair is a type of hair that is grown on the head (sometimes referring directly to the scalp). This is the most noticeable of all human hair, which can grow longer than on most mammals and is more dense than most hair found elsewhere on the body.
The average human head (an average scalp measures approximately 120 square inches or 770 cm²) has about 100,000 hair follicles (Gray 2003). Each follicle can grow about 20 individual hairs in a person’s lifetime (About 2007).
Average hair loss is around 100 strands a day. The absence of head hair is termed alopecia, commonly known as baldness.
Anthropologists speculate that the functional significance of long head hair may be adornment. Long lustrous hair may be a visible marker for a healthy individual. With good nutrition, waist length hair—approximately 1 meter or 39 inches long—would take around 48 months, or about 4 years, to grow.
Hair density is related to both race and hair color. Caucasians have the highest hair density, with an average growth rate, while Asians have the lowest density but fastest growing hair, and Africans have medium density and slowest growing hair.