Scientists have devised a way to determine roughly where a person has lived 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 people’s hair. “You’re what you eat and drink, and that’s recorded in your hair,” said Thure Cerling, a geologist at the University of Utah. While U.S. diet is relatively identical, water supplies vary. The differences result from weather patterns. The chemical composition of rainfall changes slightly as rain clouds move. Most hydrogen and oxygen atoms in water are stable, but traces of both elements are also present as heavier isotopes ( 同位素 ). The heaviest rain falls first. As a result, storms that form over the Pacific deliver heavier water to California than to Utah. Similar patterns exist throughout the U.S. 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’s team collected tap water samples from 600 cities and constructed a map of the regional differences. They checked the accuracy of the map by testing 200 hair samples collected from 65 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 ( 精确定位 ),” Cerling said. “It’s good for eliminating many possibilities.” Todd Park, a local detective, said the method has helped him learn more about an unidentified woman whose skeleton was found near Great Salt Lake. The woman was 5 feet tall. Police recovered 26 bones, a T-shirt and several strands of hair. When Park heard about the research, he gave the hair samples to the researchers. Chemical testing showed that over the two years before her death, she moved about every two months. She stayed in the Northwest, although the test could not be more specific than somewhere between eastern Oregon and western Wyoming. “It’s still a substantial area,” Park said. “But it narrows its way down for me.”