Published online 25 February 2008 | Nature | doi:10.1038/news.2008.617

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Your history is printed in your hair

Chemical imprint from local water leaves an identifiable 'signature'.

You are what you drink: local water leaves an isotopic mark in your hair.Digital Vision

The tap water that you drink leaves a 'signature' in your hair that can provide a history of where you’ve lived, according to researchers. Using these imprints to trace people's past movements may eventually become a common tool for anthropologists and law-enforcement officials.

Tell-tale isotope signatures of what we eat and drink are found throughout our bodies in bones, fingernails and hair. Tests of bones in archaeological sites can show whether a person lived and ate locally for years or migrated from elsewhere1, and have been used in forensics to help determine where an unknown victim came from. But hair has been of particular interest because it grows steadily: depending on its length, a strand can yield several years of chronicled information about a person's whereabouts.

“Bone represents a long-term average and it’s constantly being remoulded,” says study author James Ehleringer, a biologist at the University of Utah in Salt Lake City. “Hair is produced sequentially and once it’s produced it’s never modified.”

That feature, says Ehleringer, allows researchers to trace a person's geographic history — to an extent. While the technique cannot pinpoint specific locations in which a person has lived, it can narrow the possibilities. The results will be published this week in Proceedings of the National Academy of Sciences2.

Elementary analysis

Isotope ratios of rainwater vary around the United States with distance from shore and temperature.courtesy of PNAS/National Academy of Sciences

Most tap water comes from local precipitation, and the 'isotopic signature' — the ratio of heavy to light elements — of local rainwater is heavily influenced by geography. In general, heavier isotopes like deuterium (hydrogen with one extra neutron) and oxygen-18 (oxygen with two extra neutrons) tend to be 'rained out' of a cloud first, as the heavier isotopes condense more easily. So, as rain gets carried inland from where it is created over the oceans, the heavy isotopes tend to rain down first on the coastal areas. Inland areas tend to have lower isotope ratios, as do colder climes.

Local water is the primary ingredient in much of the liquid we drink, including soft drinks, which tend to be bottled regionally in the United States. Of course people also consume a lot of non-local food and drink, but Ehleringer and his colleagues started with the assumption that most people have the same 'global diet', and looked to see if they could detect the remaining signal from local water in the hair.

They collected hair clippings from 65 barber-shops across the United States. They compared the hydrogen and oxygen isotopic ratios in the hair with tap water from each location, and found that they matched.

The team also collected tap water samples from 431 other locations around the United States to develop a map of deuterium and oxygen-18 levels. Texas and Florida, being warm and close to the coasts, have the highest relative levels of both isotopes.

Crossing borders

A centimetre length of hair corresponds to roughly a month of growth, the team reports. To test the ability to produce a history of travel, they chopped up lengths of a single strand of hair and found isotope ratios that were consistent with a recent move from Beijing, China to Salt Lake City. For hair to register a new location, a person must live in a new place for three or more months, they say.

The technique has already been used to assist law enforcement, says Ehleringer. In once case, the researchers were able to help confirm that a murder victim had travelled around western states including Utah, Montana and Wyoming over two years.

The technique "will never be able to give you a precise location", says Ehleringer, although it can help exclude potential areas of residence, or help confirm other evidence of a person's whereabouts.

On a diet

More work may be needed to weed out complications in the analysis before it becomes an established tool. Individual metabolism and diet may influence isotope ratios in hair, for example. “There is quite a lot of biological noise from one individual to another and that has to be taken into account,” says Robert Hedges of the University of Oxford, UK. "I suspect that they might get two different lines if they looked at American meat-eaters and compared them with American vegetarians."

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Ehleringer admits that their assumption that most people share a 'global supermarket' diet might not withstand additional testing.

Co-author Thure Cerling, also based at the University of Utah, sees additional potential in using the technique to study animal migration. The long hairs on the tails of African elephants, Cerling says, may contain up to two years of history. 

  • References

    1. Price, T. D., Knipper, C., Grupe, G. & Smrcka, V. Eur. J. Arch. 7, 9-40 (2004). | Article |
    2. Ehleringer, J. R. et al. Proc. Natl Acad. Sci. USA advanced online publication, doi: 10.1073/pnas.0712228105 (2008).
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