Hair marks the spot: Maps of the U.S. showing predicted average hydrogen and oxygen isotope ratios in human hair across the states.

Credit: PNAS/National Academy of Sciences

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SYDNEY: It might not look like much, but a single strand of your hair is now enough for forensic scientists to work out where you live. The new isotopic analysis technique could prove invaluable for criminal investigations.

The method is detailed today in a study in the U.S. journal, the Proceedings of the National Academy of Sciences, which shows that the ratio of oxygen and hydrogen isotopes in a person's hair typically match those found in their local water supply.

"This will certainly be useful in forensics and law enforcement," said co-author Thure Cerling, biologist and geochemist at the University of Utah in Salt Lake City.

Varying ratios

Because these ratios vary across and between regions, the discovery represents a way to back-trace a person's movements and origins, be they living or dead. Though this is similar to the way in which bone and teeth samples are currently used in forensics, the ease of a single-hair test would offer a distinct advantage.

To make the discovery, Cerling's team visited 65 towns across 18 U.S. states, collecting barbershop clippings and water samples along the way. They analysed both, and observed roughly an 85 per cent correlation between their isotopic ratios – strongly linking the drinking water, and thus geographical locale, to hair composition.

Even in the face of imported foods, made from differing water supplies, the local water remains fairly prominently recorded in the timeline of the hair, Cerling said.

The team also used the information collected to develop a predictive map of the U.S., illustrating the expected hydrogen and oxygen isotope ratios in the hair of people across 48 states. This map itself may be an important future tool, the researchers said. A hair sample from an unidentified body, for example, could be analysed and its isotopic readout matched to a general area on the map.

"Phenomenal method"

"It's a phenomenal method," Salt Lake City police detective Todd Park told the AFP news service. "Every little bit helps… you put pieces of the puzzle together to get a whole picture. And this is definitely something that will give us a piece of the puzzle."

Park's police force has already used the method to identify a murdered woman by finding that she moved around between Idaho, Montana and Wyoming during the last two years of her life.

Ivan Kempson, from the Ian Wark Research Institute at the University of South Australia, in Adelaide, said the new work offers "powerful potential" not only in forensics, but also in the anthropological and archaeological analysis of hair as well.

The water isotope ratios fill "an interesting niche for hair analysis," said Wark, who is an expert on forensic isotopic analysis. "Other isotopes can reveal a persons dietary habits, for example if they eat red meat, fish or vegetables… but the oxygen and hydrogen can give a locality for a persons origin or geographical movements independent of dietary variables."