Identifying murder victims and tracking serial killers are daunting tasks for detectives when there is minimal scientific evidence available. Now, a strand of hair could provide valuable clues about a person's travels. Researchers at the University of Utah say that they are able to determine a person's recent travel history by comparing the isotope ratios of oxygen and hydrogen in a strand of his or her hair. The extent of the information that can be deduced is dependent on the length of the hair.

Hair is like a tape recording of your diet, says Thure Cerling, a co-leader of the study and a professor of geology, geophysics, and biology at the University of Utah.

The study, which was published February 25 in the Proceedings of the National Academy of Sciences, found a strong correlation between the isotopes in the water that a person drinks and the isotopes in her hair.

"Hair isotopes reflect body water, and, in turn, body water reflects drinking water," says Jim Ehleringer, a professor of biology at the University of Utah, who is co-leading the study with Cerling.

Ehleringer and Cerling developed a model to predict the geographic region of origin and travel history of humans based on the stable isotope composition of their hair. The researchers collected samples of tap water from more than 600 cities across the United States, as well as hair samples from the "trash clippings" of barbershops in 65 cities in 20 states, says Cerling. "We chose barbershops in smaller cities, where travelers are less likely to be," he adds.

Using a mass spectrometer, the researchers measured the levels of hydrogen isotopes (hydrogen-2 and hydrogen-1) and oxygen isotopes (oxygen-18 and oxygen-16) in the water and hair.

Based on the correlation of the isotopes in hair to those in drinking water, the researchers generated maps (see image below) that indicate the isotopic makeup of a person's hair with different regions in the country.

"Based on the map, we can ask whether or not the isotopes in hair are consistent with or not consistent with the region where the hair was found," says Ehleringer. If they're inconsistent, scientists can try to deduce what region the hair appears to come from and the travel history of that individual, he says.

Researchers have generated maps that show the predicted average hydrogen (top map) and oxygen (bottom map) isotope levels in human hair across the United States. The ratios of hydrogen-2 to hydrogen-1 are highest in the red and orange areas of the top map, and lowest in the blue and darker green areas. The ratios of oxygen-18 to oxygen-16 are highest in the red and orange areas of the bottom map, and lowest in the blue and darker green areas.
Credit: University if Utah