"Testing of hair rather than urine is often promoted because it is less invasive and can detect drug use over longer time periods. Hair tests cannot detect very recent drug use but do detect use that has occurred between (approximately) 10 and 90 days prior to the test (depending on the length of the hair). In addition to being more expensive than urine testing, however, hair testing raises several important concerns. As compared with urine drug tests, hair testing may more frequently result in positive results because of external (i.e. passive) exposure to drugs or chemicals. Hair treatments, such as coloring or straightening, can also affect the results of hair tests, making it more difficult to detect drug use. In addition, hair testing is not used in some Federal criminal justice proceedings because there is some evidence that naturally dark hair (e.g. that of African Americans and Asians) is more likely to test positive than lighter hair, leading to concerns of racial bias in the effects of testing programs."


"Drug Testing Welfare Recipients: Recent Proposals and Continuing Controversies," Office of the Assistant Secretary for Planning and Evaluation (Washington, DC: October 2011), p. 4.