Human hair growth
The three stages of hair growth are the anagen, catagen, and telogen phases. Each strand of hair on the human body is at its own stage of development. Once the cycle is complete, it restarts and a new strand of hair begins to form. The rate or speed of hair growth is about 1.25 centimetres or 0.5 inches per month, or about 15 centimetres or 6 inches per year.
Drug test
Hair drug testing is a method that can detect drug use over a much longer period of time, and is often used for highly safety-critical positions where there is zero tolerance of illegal drug use. Standard hair follicle screen covers a period of 30 to 90 days. The growth of head hair is usually at the rate of 0.5 inches per month. The hair sample is cut close to the scalp and 80 to 120 strands of hair are needed for the test. In the absence of hair on the head, body hair can be used as an acceptable substitute. This includes facial hair, the underarms, arms, and legs or even pubic hair. Because body hair grows at a different rate than head hair, the timeframe changes, with scientists estimating that drug use can be detected in body hair for up to 12 months. Currently, most entities that use hair testing have prescribed consequences for individuals removing hair to avoid a hair drug test.