Add yet another way for the government to track you to the list. A University of Utah team led by geochemist Thure E. Cerling and ecologist James R. Ehleringer have shown that the Hydrogen and Oxygen isotopes in human hair correlate with those isotope ratios in local tap water. (doi: 10.1073/pnas.0712228105)

We all expect that a persons diet greatly affects the concentration of certain commonly found isotopes. 13C, 15N, 34S, 40K are all present in our bodies, though in such small quantities that they don’t appear to have a significant effect on our health. Ideally you could measure the concentration of these isotopes, or their relative isotopic ratio, and learn the dietary habits of an individual. By taking into account small differences caused by fractionation events during metabolism and the larger differences in 15N values, the isotopic ratios of 13C, 15N, and 34S provide limited geographic based information about the origins of a food source.

Cerling and Ehleringe, however, chose to look at a more unlikely source for traceable isotopes that turns out to be more useful and easily detectable.

Hydrogen (²H) and oxygen (¹⁸O) isotope ratios of organic matter are more useful, because ²H and ¹⁸O values of precipitation and tap waters vary along geographic gradients (10, 11). Although differences in the ²H and ¹⁸O values of scalp hair have been noted in humans (12), less is known about diet–organism patterns of ²H and ¹⁸O values. Four potential sources can be important: dietary organic molecules, dietary waters, drinking waters, and atmospheric diatomic oxygen.

The ²H and ¹⁸O values of keratin in human hair should be influenced by a number of factors during synthesis within the hair follicle, including all dietary and atmospheric sources of H and O. Bear in mind, however, that their discussion was limited to the ²H that was not subject to postsynthesis isotopic exchange. Cerling and Ehleringe hypothesized that variations in the nonexchangeable ²H and ¹⁸O values in human keratin could provide insights into water and human diet across geographical regions if the hydrogen and/or oxygen isotopes from these sources were recorded in human hair.

After developing a model to account for the difference in the isotopic ratios between drinking water, body water and the actual scalp hair, Cerling and Ehleringer tested the model by attempting to predict the geographic region of origin of individuals based on the isotope composition. They obtained ²H and ¹⁸O values in hair sampled from 65 cities in 18 states. Their model which was a function of drinking water, bulk diet, and protein isotope ratios, explained more then 85% of the observed variation and strongly reproduced the relationship of the isotopic composition of hair samples to that of local drinking water.

Plots of the relationships between mean H isotope ratios (Upper) and mean O isotope ratios (Lower) of human scalp hair and tap water for samples randomly acquired in cities representing 18 states across the United States. The lines through the data in each plot represent model-predicted values based on local tap water and a continental supermarket diet.

The really interesting part, though, is this:

Based on the geographical distributions of the isotope ratios of tap waters and the assumption of a “continental supermarket” dietary input, we constructed maps of the expected average H and O isotope ratios in human hair across the contiguous 48 states. Applications of this model and these observations are extensive and include detection of dietary information, reconstruction of historic movements of individuals, and provision of region-of-origin information for unidentified human remains.

So now using a sample of a persons hair, not only can we find out exactly who they are (DNA) we can find out where they are from…as long as they are drinking local tap water.

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