Hair composition reflects geography

Scientists have devised a way to determine roughly where a person has lived by using a strand of hair, a technique that could help track the movements of criminal suspects or unidentified murder victims.

The method relies on measuring how chemical variations in drinking water show up in the hair of people. “You are what you eat and drink, and that is recorded in your hair,” said Thure Cerling, a geologist at the University of Utah and lead author of the study published this week in the Proceedings of the National Academy of Sciences.

While the U.S. diet is relatively homogenous, water supplies vary. The differences are a result of weather patterns. The chemical composition of rainfall changes slightly as rain clouds move.

Most of the hydrogen and oxygen atoms in water are stable, but traces of both elements are also present as heavier isotopes. The heaviest rain falls first. As a result, storms that form over the Pacific Ocean deliver heavier water to California than to Utah. Similar patterns exist throughout the United States.

By measuring the proportion of heavier hydrogen and oxygen isotopes along a strand of hair, scientists can construct a geographic timeline. Each inch of hair corresponds to about two months, Cerling said.

Cerling’s team collected tap water samples from 600 cities and constructed a map of the regional differences. The researchers checked the accuracy of their map by testing 200 hair samples collected from 65 small-town barber shops. They were able to accurately place the hair samples in broad regions roughly corresponding to the movement of rain systems. “It’s not good for pinpointing,” Cerling said. “It’s good for eliminating a lot of possibilities.”