Nails are composed of a protein called keratin, woven together by sulfur containing the amino acid cysteine. The fingernails protect the fingertips from injury, growing on average .05 to 1.2 millimeters a week.
Fingernails grow faster than toenails, and also grow faster in the summer and during pregnancy.
Certain environmental factors can damage the nail. These include exposure to strong cleaning fluids, excessive submersion in water, and dryness caused by indoor heat. Brittleness may occur as a result of frequent use of nail polish remover. Nail biting and picking can also damage the nail and surrounding skin. There are various internal causes of nail abnormalities.