Out of all the drug testing methods employed by courts and square workplaces across the nation -- hair testing has the daunting reputation of being the most thorough/menacing.
While urine testing is known to detect THC from a few days to more than a month (for everyday smokers), hair testings claims to be able to detect marijuana use for months, even years after going cold turkey. There have been some serious objections to hair testing, but it is still being used in child custody cases, professional screenings, and other high-stakes situations that can make or break a person's entire life. Obviously, it'd be much cooler if it wasn't.
But now, a group of German scientists are striving to change the general consensus on hair testing, as they claim a positive hair test can be triggered by a hair's external exposure to second-hand smoke, or even by someone with cannabis "dust" on their hands, touching the hair in question. This is the problem: you could test positive on a hair test (and face the serious implications that come with it) without ever actually partaking, personally.
The major study on detection of cannabinoids in hair follicles, conducted by researchers at the Institute of Forensic Medicine in Freiburg, Germany, showed that only a scientifically irrelevant amount of THC was passed from a person's bloodstream to hair, rendering the tests useless. And as previously mentioned, the worst part is that you are more likely to test positive from external exposure to marijuana, than from actually ingesting it. The researchers came to this conclusion by testing subjects that use marijuana frequently, and those who don't, but had been exposed to marijuana externally.