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Hair is a filamentous outgrowth of protein, found only on mammals. It projects from the epidermis, though it grows from hair follicles deep in the dermis. Although many other organisms, especially insects, show filamentous outgrowths, these are not considered "hair". So-called "hairs" (trichomes) are also found on plants. The projections on arthropods, such as insects and spiders are actually insect bristles. The hair of non-human mammal species is commonly referred to as fur. 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 primary component of hair fiber is keratin. Keratins are proteins, long chains (polymers) of amino acids.

The hair turns gray as a result of pigment  reduction, while white hair has no pigment. It is a normal 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 gray hair, and in general men tend to become gray at younger ages than women.

Parents often cite having teenagers as the cause of gray hair. This is a good theory, but scientists continue to investigate why hair turns gray. In time, everyone’s hair turns gray. Your chance of going gray increases 10-20% every decade after 30 years.

Initially, hair is white. It gets its natural colour from a type of pigment called melanin. The formation of melanin begins before birth. The natural colour of our hair depends upon the distribution, type and amount of melanin in the middle layer of the hair shaft or cortex.

Hair has only two types of pigments: dark (eumelanin) and light (phaeomelanin). They blend together to make up the wide range of hair colours. Melanin is made up of specialized pigment cells called melanocytes. They position themselves at the openings on the skin’s surface through which hair grows (follicles). Each hair grows from a single follicle.

The process of hair growth has three phases:

· Anagen: This is the active growth stage of the hair fiber and can last from 2- 7 years. At any given moment 80-85% of our hair is in the anagen phase.

· Catagen: Sometimes referred to as the transitional phase, which is when hair growth begins to “shut down” and stop activity. It generally lasts 10- 20 days.

· Telogen: This occurs when hair growth is completely at rest and the hair fiber falls out. At any given time, 10-15 % of our hair is in the telogen phase, which generally lasts 100 days for scalp hair. After the telogen phase, the hair growth process starts over again to the anagen phase.

As the hair is being formed, melanocytes inject pigment (melanin) into cells containing keratin. Keratin is the protein that makes up our hair, skin, and nails. Throughout the years, melanocyctes continue to inject pigment into the hair’s keratin, giving it a colourful hue.

With age comes a reduction of melanin. It should be noted however, that gray hair in itself is not actually gray; the gray head of hair is a result of a combination of the dark and white/colorless hair forming an overall 'gray' appearance to the observer. As such, people starting out with very pale blond hair usually develop white hair instead of gray hair when aging. Red hair usually doesn't turn gray with age; rather it becomes a sandy color and afterward turns white. In fact, the gray or white appearance of individual hair fibres is a result of light scattering from air bubbles in the central medula of the hair fibre.

Dr. Desmond Tobin, professor of cell biology from the University of Bradford in England, suggests that the hair follicle has a “melanogentic clock” which slows down or stops melanocyte activity, thus decreasing the pigment our hair receives. This occurs just before the hair is preparing to fall out or shed, so the roots always look pale.

Moreover, Dr. Tobin suggests that hair turns gray because of age and genetics, in those genes regulate the exhaustion of the pigmentary potential of each individual hair follicle. This occurs at different rates in different hair follicles. For some people it occurs rapidly, while in others it occurs slowly over several decades.

In a February 2005 Science article, Harvard scientists proposed that a failure of melanocyte stem cells (MSC) to maintain the production of melanocytes could cause the graying of hair. This failure of MSC maintenance may result in the breakdown of signals that produce hair colour.

There are other factors that can change the pigmentation of hair, making it lighter or darker. Scientists have divided them by intrinsic (internal) and extrinsic (external) factors:

Hair – raising facts:

· An average scalp has 100,000-150,000 hairs.

· Hair is so strong that each hair can withstand the strain of 100 grams (3.5 ounces). An average head of hair could hold 10-15 tons if only the scalp was strong enough!

· Human hair grows autonomously, that is each hair is on its own individual cycle. If all our hair were on the same cycle, we would molt!

· Hair has the highest rate of mitosis (cell division). An average hair grows 0.3 mm a day and 1 cm per month.