Strands of Evidence: Isotopic Analysis & Hair-Based GPS Tracking System

You are what you eat, and what you eat ends up in your hair. Scientists in the U.S. and Europe have used this basic idea—and some sophisticated isotopic analysis—to devise a sort of hair-based GPS tracking system. A single strand contains information on your whereabouts over the past few months, a fact that law enforcement agencies are now using to solve crimes.

Lesley Chesson opens a cardboard box inside her office at IsoForensics, a Salt Lake City-based company that uses science to fight crime. She pulls out a bulging manila envelope.

“That one definitely has hair in it,” she says with a chuckle.

A senior scientist at the company, Chesson reaches into the envelope and removes a mass of brunette hair. Her colleague, Luciano Valenzuela, looks over a list to see where it came from.

“It’s hair from Shawnee, [Oklahoma],” he says.

Another 2,000 envelopes and vials stored here also contain human hair, collected from across the United States and around the world. “Guatemala, Japan, Newfoundland, Thailand,” says Valenzuela, rattling off some of the countries.

In fact, the scientists have hair from every continent, even Antarctica. They are using all this hair—from regular, everyday people—to perfect a technique to help solve murder cases.


Both Chesson and Valenzuela were mentored by a professor at the University of Utah, Jim Ehleringer.

Ehleringer was trained as a plant biologist, but about a decade ago, he became curious about animals and whether he could develop a new technique for addressing a question that wildlife biologists commonly ask: where do animals eat and drink, and does the location of their watering hole, say, change over time?

“I could find out by being in the field every single minute of the day,” he says. “Or I can let nature do the recording for me.”
Ehleringer realized that what an animal eats and drinks does get recorded—in its tissues.

Every chemical element comes in different forms, known as isotopes, with some slightly heavier than others. Take hydrogen and oxygen, the atoms that make up water, H2O. Their different isotopes are found in different concentrations depending on where the water comes from. And that mixture of heavy and light atoms gets laid down in the growing tissues of the animals that drink the water. These tissues include hair.

“The hair becomes a linear tape recorder,” says Ehleringer. “So it tells us a little story about the history of what an animal was eating or drinking.”

Our hair acts as a timeline—recording where each of us has been and when we were there.

Ehleringer suspected the same thing would apply to humans and our hair. So he and his colleagues collected hair from local barbershops across the U.S. to test a hypothesis. They wanted to determine if it was possible to tell where hair came from based on an analysis of the hydrogen and oxygen in the local water supply.

Thure Cerling, a geologist at the University of Utah and another collaborator on this project, says the vast majority of the water in our diet is local.

“People often say, ‘Well, oh, I don’t drink water. I drink Coke,’” he says. “[But] where was the Coke or the Pepsi bottled?” It’s usually at a local bottling plant, using local water.

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