CSIs now have a new tool in their belt: the chemical signatures left by local drinking water in human hair. Scientists at the University of Utah have found that the ratios of different forms of hydrogen and oxygen in local drinking water vary from region to region across the country. These elements are incorporated into the hair as it grows. Traces in the hair can show where a person has recently lived or traveled, and could help police track the recent movements of criminals. "You are what you drink — and that is recorded in your hair," said lead researcher Thure Cerling. Changing isotopes Water is of course composed of two elements: hydrogen and oxygen. But these elements can come in different forms, called isotopes, which are versions of the same chemical element but with different numbers of neutrons in their nuclei. Most hydrogen atoms have one proton and no neutrons (hydrogen-1), but some have one neutron (hydrogen-2). Similarly, oxygen commonly has eight protons and eight neutrons (oxygen-16), but can have 10 neutrons (oxygen-18). Cerling and his colleague James Ehleringer found a strong link between hydrogen and oxygen isotope levels in hair and drinking water. These different levels of isotopes can be pinned to different locales because of how rain patterns change over the country. As clouds move off the ocean and over the land, the water with heavier isotopes generally falls first, so drinking water near the coast has more oxygen-18 and hydrogen-2 than water farther inland. Cerling and Ehleringer's team collected hair samples from barbershops and tap water samples in 65 cities in 18 states across the United States. From tests run on the samples, they created color-coded maps that show how isotope ratios change between regions. While they can't pinpoint an exact location, they can give a general idea of where a person has been. "You can tell the difference between Utah and Texas," Ehleringer said. But Cerling added, "You may not be able to distinguish between Chicago and Kansas City." Tracking movements The method developed by Cerling and Ehleringer, detailed in the Feb. 25 online issue of the journal Proceedings of the National Academy of Sciences, is already being used by police to learn where unidentified murder victims might have been prior to their deaths. Salt Lake City Police Department detectives contacted the researchers after facial reconstructions of a female murder victim found in 2000 failed to turn up any identification. Samples of the victim's hair told detectives where she had traveled prior to her murder. "The samples I gave to Jim told me her approximate location for the last two years of her lifetime," said Detective Todd Park. "She moved around within the Northwest — mainly in the Idaho-Montana-Wyoming area and maybe into Oregon and Washington." Police could also use this technique to check an accused criminal's alibi as to his whereabouts at the time of a crime. Anthropologists and archaeologists may also be able to use the method to analyze hair samples to show how ancient groups migrated.