Human hair growth
The three stages of hair growth are the anagen, catagen, and telogen phases. Each strand of hair on the human body is at its own stage of development. Once the cycle is complete, it restarts and a new strand of hair begins to form. The rate or speed of hair growth is about 1.25 centimetres or 0.5 inches per month, or about 15 centimetres or 6 inches per year.
Red hair
Emigration from Europe has multiplied the population of red haired humans in the Americas, Australia, New Zealand and South Africa. In the United States, it is estimated that 2–6% of the population has red hair. This would give the U.S. the largest population of redheads in the world, at 6 to 18 million, compared to approximately 420,000 in Ireland and 300,000 in Scotland.