Hair drug testing
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 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.
Even if the person being tested has a shaved head, hair can also be taken from almost any other area of the body. 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.
The claim that a hair test cannot be tampered with has been shown to be debatable. One study has shown that THC
does not readily deposit inside epithelial cells so it is possible for cosmetic and other forms of adulteration to reduce the amount of testable cannabinoids within a hair sample.