It takes approximately 18 months for an African-American individual to grow 6 inches of hair, as African hair grows about 4 inches per year. Asian hair grows the fastest at nearly 6 inches per year, while European or Caucasian hair grows at 5 inches per year, according to LiveScience.
Hair is believed to grow very slightly faster during the summer months as a result of diet, hormonal shifts and increased UV radiation exposure. Human hair shedding differs from that of other mammals in that isn't affected by changing seasons. The hair shaft, or the hair that is visible above the skin, is composed of a protein called keratin. This same protein also makes up fingernails, which grow at about 3 millimeters per month.