Standard drug testing measures drug by-products in the urine which move through the body quickly and for most drugs are no longer measurable after 1 month.  Drugs are deposited in the hair shaft through absorption into the hair follicle.  Testing is conducted on the hair shaft which prevents external contamination on the outside of the hair that may occur during accidental exposure, such as being in the same vicinity as someone using drugs.  Traces of drugs will remain in the hair longer than in the urine or the blood providing a much longer detection window.    Most often a 90 day detection window is requested for hair follicle drug tests.