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).
Long hair
1972, it was estimated that 24% of American women wore shoulder length hair or longer (44% of women in ages 14 to 44), meaning slightly more than 12% of men and women altogether. Similar frequency was found in 2001, when it was estimated that about 13% of the US adult population, male as well as females, has hair shoulder-length or longer, about 2.4% have hair reaching to the bottom of the shoulder blades or longer, about 0.3% have hair waist length or longer, and only about 0.017% have hair buttocks-length or longer. By extrapolating the above data and the number of hair length records, the number of people with shoulder-length hair or longer in the USA is estimated to be 27 million, those with waist-length hair roughly 900,000, those with buttocks-length hair to be 40,000, with knee length hair to 2,000 and with ankle length hair to 70.