Hair

Hair is a protein filament that grows through the epidermis from follicles deep within the dermis. The fine, soft hair found on many nonhuman mammals is typically called fur; wool is the characteristically curly hair found on sheep and goats. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class. Although other non-mammals, especially insects, show filamentous outgrowths, these are not considered "hair" in the scientific sense. So-called "hairs" (trichomes) are also found on plants. The projections on arthropods such as insects and spiders are actually insect bristles, composed of a polysaccharide called chitin. There are varieties of cats, dogs, and mice bred to have little or no visible fur. In some species, hair is absent at certain stages of life. The main component of hair fiber is keratin.

The hair can be divided into three parts length-wise, (1) the bulb, a swelling at the base which originates from the dermis, (2) the root, which is the hair lying beneath the skin surface, and (3) the shaft, which is the hair above the skin surface. In cross-section, there are also three parts, (1) the medulla, an area in the core which contains loose cells and airspaces (2) the cortex, which contains densely packed keratin and (3) the cuticle, which is a single layer of cells arranged like roof shingles.

Hair types

Human beings have three distinct types of hair:

• Lanugo is fine hair that covers nearly the entire body of a fetus. Unless born prematurely, the fetus loses this layer of hair before birth. Lanugo also sometimes returns in cases of malnutrition or extreme anorexia nervosa, as the starved body attempts to insulate itself.
• Vellus hair is extremely short, fine, scarcely noticeable hair that covers most of the human body in both sexes.
• Terminal hair is fully developed hair, which is generally longer, coarser, thicker, and darker than vellus hair.

Characteristics

Balding and greying

The tendency of older people to develop grey hair is due to "a massive build up of hydrogen peroxide due to wear and tear of our hair follicles ...[which] winds up blocking the normal synthesis of melanin, our hair's natural pigment." Grey hair is considered to be a characteristic of normal aging. The age at which this occurs varies from person to person, but in general nearly everyone 75 years or older has grey hair, and in general men tend to become grey at younger ages than women.

It should be noted however, that grey hair in itself is not actually grey; the grey head of hair is a result of a combination of the dark and white/colourless hair forming an overall 'grey' appearance to the observer. As such, people starting out with very pale blond hair usually develop white hair instead of grey hair when aging. White hair is a result of both the lack of melanin, and the accumulation of air bubbles in the shaft. Red hair usually doesn't turn grey with age; rather it becomes a sandy colour and afterward turns white. Some degree of scalp hair loss or thinning generally accompanies aging in both males and females, and it's estimated that half of all men are affected by male pattern baldness by the time they are 50. The tendency toward baldness is a trait shared by a number of other primate species, and is thought to have evolutionary roots.

Distribution rate

Human scalp hair normally grows at a rate of 0.4 mm/day. One’s hair (or nails) does not continue to grow after death; the appearance of growth is actually caused by the retraction of skin as the surrounding tissue dehydrates, making nails and hair appear longer.

Texture

Hair texture is described as fine, medium, coarse or wiry, depending on the hair diameter. Within the four texture ranges hair can also be thin, medium or thick density and it can be straight, curly, 'kinky' (tightly coiled), or wavy. Hair conditioner also affects hair texture. Hair can be healthy, normal, oily, dry, damaged or a combination. Hair texture can also be affected by hair styling equipment such as straighteners, crimpers, or curlers. Also, a hairdresser can change hair texture with chemicals.

Hair is genetically programmed to be straight, wavy, curly, or "kinky", and it can change over time.

For many years, it was believed that the shape of a person’s hair was determined by the individual hair shafts, and that curly and "kinky" hair get their shape because the cross-section of the hair shaft was flatter and had more intertwined layers than straight hair, which was round. But scientists have determined that whether hair is curly, "kinky", or straight is determined by the shape of the follicle itself and the direction in which each strand grows out of its follicle. Curly and/or 'kinky' hair is shaped like an elongated oval and grows at a sharp angle to the scalp. This growth pattern, in turn, determines the cross-section of the shafts.

Curly and/or "kinky" hair has a different biological structure from straight hair. It tends to be much drier than straight hair because the oils secreted into the hair shaft by the sebaceous glands can more easily travel down the shaft of straight hair. People with very curly hair may find that this hair type can be dry and often frizzy.

Hair, whether it is curly or straight, is affected by the amount of humidity in the air. It serves as a restoring force for the hair, forcing water back into the hair fiber and forcing hair shaft to return to its original structure. This may be more noticeable in somebody with curly hair because it tends to get frizzy when the humidity rises.

Dimensions

According to The Physics Factbook, the diameter of human hair ranges from 17 to 181 µm. It varies slightly with ethnicity, where Europeans generally have 57-90 µm and Asians around 120 µm.

By using hair-water partitioning, the density of hair is estimated to 1.32 kg/L. Thus, considering hair as cylindrical, the weight of a single hair is between 3 and 340 microgram per cm, for a diameter of 17 and 181 µm respectively.

The diameter of an average human eyelash ranges from 0.05 mm to 0.20 mm.

Pathology

Drugs used in cancer chemotherapy frequently cause a temporary loss of hair (alopecia universalis), noticeable on the head and eyebrows, because they kill all rapidly dividing cells, not just the cancerous ones. Other diseases and traumas can cause temporary or permanent loss of hair, either generally or in patches. Patients with Hyperthyroidism or Hypothyroidism can experience hair loss until their hormone levels are regulated.

Hair analysis

Hair shafts may store certain poisons for years, even decades, after death, which can sometimes be detected with the use of an atomic absorption spectrophotometer.

In recent years, a University of Toronto laboratory pioneered the use of neonatal hair to show exposure to drugs during pregnancy. Drugs such as cocaine, methamphetamine, heroin, etc., accumulate in the neonatal hair during its growth and thus serve as a long term “memory” of what happened during fetal life. The hair analysis method used by Dr. G. Koren's research group revolutionized the diagnosis of in utero exposure to drugs.

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