Hair "Records" Where You've Been, Study Finds

Joab Jackson
for National Geographic News
February 26, 2008
The chemical makeup of your hair can reveal where in the world you've been, researchers have found.

Water molecules found in human hair closely resemble those in the tap water people drink, according to a new study by University of Utah researcher Thure E. Cerling and his colleagues.

Such a link could provide evidence of where a person has spent the last few months, thanks to water's tendency to differ slightly in its chemical makeup from one latitude to the next.

Researchers have long known that water contains different ratios of chemical signatures, called isotopes, in different regions of the world.

Since water is a principal ingredient in hair, the researchers hypothesized that the hair itself should reflect a person's geographic location.

"For years, hair records diet information with carbon and nitrogen isotopes," Cerling said. "This research shows that geography is recorded as well."

To test their idea, the researchers collected hair samples from barbershops in 65 cities in the continental United States. They also took samples of the tap water in these locations.

They found an 85 percent correlation between the isotope ratios in the hair and those in the tap water.

The researchers report their results in this week's issue of the Proceedings of the National Academy of Sciences.

Barbershop Clues

An isotope is a chemical compound with a different number of neutrons than the normal form of that compound. A "light" water isotope has fewer neutrons than what researchers call a heavy isotope.

The varying distribution of isotopes in water is so predictable that scientists have drawn up maps showing ratios of various isotopes around the world.

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.

"CSI" Applications

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.

(See an interactive map of ancient human migration.)

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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