text: Thure Cerling and James Ehleringer, a biology professor at the University of Utah, founded Isoforensics in 2003, a company with the aim of interpreting the stable isotope composition of various biological and synthetic materials. This was the first step for the discovery they made which was first published on February 25, 2008, by the "Proceedings of the National Academy of Sciences" with the title "Hydrogen and oxygen isotope ratios in human hair are related to geography".
  To know where people have been and where they lived for a while are information that became available by analyzing the stable isotope composition of their scalp hair. Cerling discovered that a strand of hair could provide valuable clues about a person's travels by studying the variation of hydrogen-2 (δ2H) and oxygen-18 (δ18O) isotopes and comparing them to the ones in the drinking water. The extent of the information that can be deduced depends on the length of the hair: the longer is the hair, the greater is the extraction of information. The variation with geography of isotope concentrations is linked with precipitations, cloud temperatures and with the amount of water that evaporates from soil and plants. When clouds move off the ocean towards inland the ratios of oxygen-18 to oxygen-16 and hydrogen-2 to hydrogen-1 tend to decrease because of the rain water with oxygen-18 and hydrogen-2, being heavier, tends to fall first.
  Samples of tap water were collected from more than 600 cities across the United States as well as hair samples from the barbershops in 65 cities in 20 states. The comparison showed that both hair and drinking water samples had the same isotopic variations. In order to display these information, the scientists produced color-coded maps based on the correlation of the isotopes in hair to those in drinking water. This maps show how ratios of hydrogen and oxygen isotopes in scalp hair vary in different areas of the United States. It was so proved that the water drank by a human being leaves in the hair an evidence which contain oxygen and hydrogen isotopes equal to the ones in the tap water.
  This technique would have been a new tool for policemen, anthropologists, archaeologists and doctors.