Regional temperatures play a major role in this variance, Cerling explained.
"As you extract more and more moisture from [a] water source, you have less and less of the heavy isotope," Cerling said.
"And one way to extract more water from something is to make the system colder and colder. You condense more water from a cloud if you make it colder."
Repeated cycles of rainfall and evaporation wrings the heavy isotopes from water, noted Stanley Ambrose, an archaeologist at University of Illinois who was not involved in the research.
The correlation between hair and water isotopes found in the new study appears to be very strong, he added, especially considering that other factors that affect hair composition, such as diet, were not controlled in the study.
The new findings could help in criminal investigations, Cerling said.
A water-isotope test of a person's hair could identify where he or she was while the hair was growing, for example.
Police may not pinpoint the exact city in which a person lived, but it could allow forensic experts to identify the general region.
"If you're living someplace, you are most likely going to be using the local water," Cerling said.
(Read related story: "Bottled Water Isn't Healthier Than Tap, Report Reveals" [February 24, 2006].)
Water isotope analysis could also be used by anthropologists to better track the movements of ancient peoples, should hair samples be preserved.
Hair stays molecularly stable for thousands of years, Illinois' Ambrose noted.
"You are what you eat. And you are what you drink, in the case of this hair," Ambrose said.
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