Head hair grows approximately 1.3 cm or a ½ inch per month. The standard length of hair tested by the laboratory is the first 3.9 cm or 1½ inches from the root end. Therefore, a hair analysis of 3.9 cm covers a time span of approximately 90 days and detects a pattern of drug use over this timeframe.
Many employers find it useful to test both hair and urine for pre-employment purposes. Urine is useful for detecting recent or new drug use (the last 1-3 days except Marijuana, which is longer) and hair for providing an approximate three-month drug history of repetitive use.