Hair care
Scalp hair grows, on average, at a rate of about 1.25 centimeters per month, and shampoos or vitamins have not been shown to noticeably change this rate. Hair growth rate also depends upon what phase in the cycle of hair growth one is actually in; there are three phases. The speed of hair growth varies based upon genetics, gender, age, hormones, and may be reduced by nutrient deficiency (i.e., anorexia, anemia, zinc deficiency) and hormonal fluctuations (i.e., menopause, polycystic ovaries, thyroid disease).
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.