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
In Ireland, the percentage of population with red hair is estimated to be at around 10%, making it the world's highest concentrated population of red heads. According to IrelandsDNA, 34.7% of the Irish population carry the allele for red hair, although this doesn't directly translate proportionally into births of red-haired children.