Rumor has it that Britney Spears may have shaved off all her hair last weekend to avoid having it drug tested during her child custody battle with Kevin Federline. How much can you learn by testing someone's hair?
A lot. A bundle of hair about the thickness of a pencil can tell chemists what specific drugs someone has used and provide a rough timeline of when she used them. Narcotics like cocaine, meth, ecstasy, and PCP introduce toxins to the bloodstream that are then incorporated into each hair as it forms in the follicle. Urine and blood only retain evidence of these toxins for a week or so, but hair can hold on to them indefinitely. (When administered correctly, hair tests are about as accurate as urinalysis.)
Head hair grows about half an inch every month, so a woman with shoulder-length hair carries around a two-year record of her drug use. To test her, scientists would cut off a strip of about three months' worth of growth and then cut it into pieces for analysis. This segmentation allows them to approach each strand the way a dendrochronologist would approach a tree trunk. Like the cross section of a tree, a strand of hair can store information for thousands of years: In the early 1990s, scientists studying the hair of Peruvian and Chilean mummies found evidence of cocaine use dating back to 1000 B.C.
Hair testing for illicit drugs started in the late 1970s, but the practice did not catch on commercially until about 15 years ago, when improved technology allowed for more-accurate results. (There are still no federal standards for hair testing.) These days the annual market is estimated at $800 million to $1 billion. Psychemedics Corp., the largest provider, sells hair-testing services to more than 2,500 corporations, including police departments, banks, hospitals, and schools.
Drug tests can be done on any body hair—armpit, leg, back, pubic—but head hair is preferred because of its length and relative cleanliness. Although pubic hair may be long enough to provide a few months' worth of information, it is less reliable because of its likely contamination from sweat and urine. Hair from any part of the body might become unreliable if it came into external contact with drugs—for example, if someone spent time in a room filled with crack cocaine smoke. Test results can also be affected by hair color or texture. Studies have shown that a dark strand will take up drug residues more readily than a light one—a peculiarity that has prompted some critics, including the American Civil Liberties Union, to deem the practice racist.
Bonus explainer: Could Britney's ex really force her to undergo a hair-based drug test? No. Drug testing is becoming increasingly common in custody battles as a way to prove a parent unfit or irresponsible, but at this point courts in California only allow urinalysis. Even if Britney lived in a state where hair-based tests were allowed, her newly shaved head wouldn't necessarily get her off the hook. * Federline could still get his hands on the trimmed tresses, which the salon has put up for auction. To use them in court, he'd just have to show a secure "chain of custody" or otherwise prove that the hair being tested did in fact belong to Britney.
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Explainer thanks Bruce Goldberger at the University of Florida and Melissa E. Murray at UC-Berkeley's Boalt Hall School of Law.