According to MedlinePlus, "the average person has 5 million hairs." There is hair growing everywhere on the human body, except the soles of the feet, the palms of the hands and the lips. Hair has a few different functions on the body. It helps prevent body heat from escaping and keeps small particles from entering the ears, nose and eyes. Different types of hair grow at different speeds. For example, head hair grows at a faster rate than eyelashes.
Hair Properties
Hair is made up of hard keratin, which is approximately 85 percent protein. The type of keratin that hair is made up of is slightly denser than other forms of keratin found in the body, such as in callouses. Hair cells are flat and bound tightly together to form each individual hair. In the dermis, each hair is surrounded by a follicle. Hair follicles have two main components that act to elevate the hair and simultaneously lubricate it.
Rate of Hair Growth
The rate of hair growth varies greatly, depending on where the hair is on the body. According to Dr. Robert Baratz, the average rate of growth for head hair is 0.34mm to 0.36mm each day, or 1cm each month. However, it can vary between individuals, from as little as 0.6cm per month to as much as 3.36cm per month. The average hair growth for eyelashes and eyebrows is 0.16mm a day. Men's beard hair grows at an average rate of 0.38mm a day, faster than the average person's head hair.
Growth Cycles
Hair growth occurs in cycles. Each cycle contains four phases -- the anagen, catagen, telogen and resting phases. The first phase is the anagen phase, or the growing phase. This phase lasts from two to six years. Generally, at any given time, 90 percent of a person's hair is in the anagen, or growing, phase. The second phase is called the catagen phase. In this phase, the hair has stopped growing and the hair follicle begins to move toward the surface of the skin. During the next phase, the telogen phase, the individual hair falls out of the follicle. Then the cycle enters a resting phase. After the resting phase, the hair follicle begins again with the anagen phase and the growth of a new hair.
Substances and Hair Growth
There is a wide variety of substances that can affect or prohibit hair growth. Some drugs, such as those used for chemotherapy, are cytotoxic and cause hair to fall out. Parathyroid hormone is a drug that prohibits hair growth. Other substances that can cause hair to immediately enter the telogen phase and fall out include heparin, L-dopa and lithium. Some substances -- such as testosterone, anabolic steroids, retinoids and insulin -- can cause hair growth in places where it previously didn't grow.
Hair Loss
Alopecia is the name for partial or total hair loss. Natural hair loss generally occurs gradually over time; however, certain drugs and other chemical substances can cause a faster rate of hair loss. Genetic baldness is the result of hair follicles closing up after the telogen phase and not beginning a new cycle, rather than the result of significantly increased hair loss. Although most hair loss is caused by aging and genetic predisposition, there are other common causes. Severe episodes of physical or mental stress -- such as childbirth, severe infection, crash diets and intense emotional stress -- can cause half or even more of your hair to fall out. After several months, hair growth should return to normal.
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