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
The hair closest to the root indicates where a person has been most recently. The longer the hair, the more recorded history the researchers have to work with. Hair grows one millimeter every three days, so if the hair is 20 centimeters long, that represents about 20 months’ worth of history, says Ehleringer. “I only have about six months of history,” he says of his own locks.
Joe Berry, a staff scientist in the department of geology at the Carnegie Institute of Science, in Washington, DC, says that Ehleringer’s lab is the world leader in studying isotopic compositions and that the work represents a real breakthrough because it provides a practical tool for law enforcement.
Researchers are currently using the technology to help detectives in the Salt Lake County Sheriff’s Office identify a murder victim found near Utah’s Great Salt Lake in 2000. For the past eight years, she has been a “Jane Doe,” says Todd Park, a detective working on the case. Now, using the new technique, detectives have analyzed strands of the victim’s hair and have been able to determine where she spent the last two years of her life. Park says that this is a huge help and that it narrows his search dramatically.
Ehleringer says that the map can’t define an exact location–only regions or an expansive narrow band, like the one that stretches from Oklahoma to Illinois. Fortunately, some regions, such as Northern Montana and Wyoming, are very small, with a distinct set of isotopic signatures.
This tool will be a “huge breakthrough for law enforcement” not just in helping identify murder victims, but also in tracking the movements of a serial killer, says Park. In 2003, Ehleringer and Cerling cofounded IsoForensics, a company that uses stable isotope analysis of forensic substances to find slight chemical variations.
Stephen Macko, a professor in the department of environmental sciences at the University of Virginia, who was not involved in the research, says that the work is very exciting. But he notes that the researchers still need to better define how foods from different areas impact the isotopic makeup of hair. “If you drink orange juice fresh from Florida or eat meat from a cow in Texas, you are getting the isotope signals of that state’s water,” says Macko. So how does a person get his local signature? In their study, Ehleringer and Cerling note that since things such as soup, coffee, coke, and cooking pasta use the local water, the isotope signals are incorporated into hair. The researchers are currently conducting subsequent studies to further explore such questions, but they’re unwilling to discuss any details.