Hairy Forensics: Isotopes can identify the regions where a person may have lived

Judging people by their hair isn’t shallow, it’s sound science: The proportions of certain chemical isotopes in someone’s tresses can help detectives pin down that individual’s region of origin and recent movements, a new study suggests. The finding could be particularly useful in identifying the victims of crimes or mass disasters and in poking holes in the alibis of suspected criminals.

HOME RANGE. New research predicts the average concentrations of hydrogen-2 (top) and oxygen-18 (bottom) that would be found in the hair of U.S. residents. Red depicts the highest concentrations of each isotope, blue depicts the lowest. PNAS/National Academy of Sciences

Water, made of hydrogen and oxygen, makes up more than half of an adult human’s body weight. Via various metabolic processes, some of that water is broken apart and the constituent atoms are incorporated into body tissues, fingernails, and hair. Hair is made of keratin, a remarkably stable protein, so most of its hydrogen and oxygen atoms aren’t readily lost to the environment. Because much of the water that people consume comes from the area where they reside, any variations in the concentrations of hydrogen and oxygen isotopes in that water should be recorded in the hair, says James R. Ehleringer, an environmental chemist at the University of Utah in Salt Lake City.

To test that idea, he and his colleagues collected random hair samples from barbershops in 65 cities in 18 states. Those cities are located in regions that span the full range of concentrations of hydrogen and oxygen isotopes found in tap water throughout the lower 48 states, says Ehleringer. Hair samples were presumed to have come from local residents.

Analyses suggest that about 27 percent of the hair’s hydrogen and 35 percent of its oxygen come from local tap water. Overall, about 86 percent of the variations in the hair samples’ hydrogen and oxygen isotopes derive from the isotopic signature of the local water, the researchers found. Although consumption of bottled water is increasing, much of the water in the liquids that a person drinks—including milk, sodas, beer, and reconstituted juices—are probably derived from local sources. Also, most food is cooked in local tap water.

The team’s findings, presented last week in Washington, D.C., at a meeting of the American Academy of Forensic Sciences, also appear in the Feb. 26 Proceedings of the National Academy of Sciences.

Using a database of how isotopes vary in tap water across the continental United States, Ehleringer and his colleagues developed a model that estimates the concentrations of hydrogen and oxygen isotopes that would be present in the hair of people from various regions.

Concentrations of hydrogen and oxygen isotopes in hair aren’t definite proof of a person’s region of residence, because the concentrations may represent the isotopic signature of groundwater in several regions. Nevertheless, “this work could be especially valuable for helping law-enforcement authorities in limiting the search areas for the origins of unidentified human remains,” says Jurian A. Hoogewerff, an isotopic chemist at the University of East Anglia in Norwich, England.

Wolfram Meier-Augenstein, an analytical chemist at Queen’s University Belfast (Northern Ireland), agrees: “This [technique] doesn’t allow you to find the needle in a haystack, but it reduces the size of the haystack.” A similar technique enabled forensic anthropologists to narrow down the country of origin for many of the victims of the tsunami that struck southern Asia in December 2004 (SN: 1/8/05, p. 19).

Hair analysis also may offer a way to refute the alibis of suspected criminals. One hair sample that Ehleringer and his colleagues analyzed came from a donor who had recently moved from Beijing to Salt Lake City, a move chronicled in the hair that grew in the 6 to 8 weeks following the change in residence.

Hoogewerff’s advice: “If you’re a criminal, shave.”

More Stories from Science News on Anthropology