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A person's hair can reveal a lot of secrets about them, and you can uncover them with science. Crime shows will often show forensic scientists being able to extract someone's DNA from a strand of hair, but it's actually a lot harder than those shows make it seem. Most hair is dead cells, and when they die they are pressed together to form keratin. In the process, the nucleus and the DNA get destroyed, so usually to get a hair sample with any useful DNA, it has to have the root attached.
But even without the root, hair can be full of secrets. Keratin has sulfur atoms that bond to trace elements. These elements can give away what you've been putting in your body. Hair can be used to find out if someone's been using marijuana, opiates, amphetamines, or PCP. And since those elements bind to your hair when the hair is forming, the longer your hair, the farther back the test goes. Shaving your head bald won't help either, because body hair can be used in a drug tests, and since body hair doesn't grow like the hair on your head, it could actually represent a longer timeframe, possibly up to a year, Keratin's tendency to grab trace elements can be used to tell us what someone's diet is. This fact makes it useful for studying animals. Researchers have actually developed a new technique that runs a laser down the length of a single bear hair and vaporizes it. The gas is then analyzed by a mass spectrometer. The old method involved analyzing bunches of hair which gave them a general idea of what they had been eating. This new method is more precise, telling biologists if their diets have changed and can even monitor how much mercury they're getting from fish.
Challenges in DNA Testing and Forensic Analysis of Hair Samples (Forensic Magazine)
"People often assume hair samples make ideal samples for DNA testing. This notion might perhaps be due to the way hair samples are so often seen collected in detective crime fiction series and how solving the crime invariably hinges on finding such a sample."
Hair Drug Testing: Facts & Myths (Health-street.net)
"So, your boss asked you to get a hair drug test. Or maybe your ex. Or your parent. Or your future employer. And you have heard all kinds of rumors about what this test can detect, and for how long."