Hair and fingernails are made from a strong protein called keratin. This hard protein is also present in animal hooves and skin. With both hair and nails, the only living, actively growing part is located underneath the skin.
Hair and fingernails both grow out from a matrix. In hair, this matrix is known as the hair follicle. Cells grow out from the matrix, and older cells eventually are pushed out by newer cells growing in. The newer cells cause the older cells to become compacted, and as the cells are further removed from their blood supply, they form keratin proteins and begin to harden into the hair fiber, or nail, that we see.
Fingernails grow approximately 1/8 inch, per month. Toenails grow slower than fingernails,
at a rate of about 1/16 inch per month. Hair, on the other hand, grows at a much faster rate: roughly ¼ to ½ inch per month, or up to 6 inches per year.
For both hair and fingernails, growth rates are affected by season, gender, age and heredity. Hair and nails grow faster in the summer months than at any other time of the year. Men's hair and fingernails typically grow faster than women's hair and fingernails; pregnancy and old age are the exceptions to this rule. Fingernails grow fastest on the hand that is used most often. Pinkie nails grow the slowest; the index fingernail grows the fastest.
Several factors can slow hair and nail growth, including disease, hormonal imbalances (typically menopause, in women) and natural aging. Additionally, poor blood circulation, malnutrition and some medications can slow down the growth rates of the hair and nails.