Radiation in Teeth Can Help Date, ID Bodies, Experts Say

Stefan Lovgren
for National Geographic News
September 22, 2005
Radioactive traces from nuclear tests in the 1950s and '60s are allowing forensic scientists today to date, and to help identify, human remains found after accidents and natural disasters.

The new technique has already been used to help identify victims of last year's tsunami in Southeast Asia. It may prove useful in identifying the victims of Hurricane Katrina on the U.S. Gulf Coast.

The method, developed by Swedish and U.S. researchers, measures radioactive carbon-14, a byproduct of nuclear testing, in human tooth enamel.

The process can determine the age of a tooth to within 18 months, far more precise than other methods of evaluating skeletal remains.

"Tooth enamel preserves a permanent record of the amount of radioactive carbon present in the atmosphere when the tooth was formed," said Jonas Frisén, a forensic scientist at the Karolinska Institute in Stockholm, Sweden. "By determining the amount of radioactive carbon-14 in the tooth, we can then calculate the age of the tooth, and its owner."

The research was reported last week in the journal Nature.

Locked In

Radiocarbon, or carbon-14, is produced when cosmic radiation hits the Earth's atmosphere, and it exists at low levels everywhere in the world.

The amount of radiocarbon in the atmosphere remained fairly stable until 1955, when above-ground nuclear testing caused it to rise dramatically. Although nuclear tests were conducted in only a few places, the radiocarbon spread evenly in the atmosphere.

Radiocarbon reacts with oxygen to form carbon dioxide, which plants incorporate through photosynthesis. Humans then eat the plants and animals that eat them, which transfers the plants' concentration of radiocarbon to the human body.

The radiocarbon gets into human teeth, but it remains locked inside the tooth enamel from the time the enamel is formed during childhood.

"Dental enamel is a mineral, not a live tissue … so it doesn't have constant turnover like proteins," said Bruce Buchholz, a scientist at the Lawrence Livermore National Laboratory in California. "Whatever carbon got into the enamel during the formation of the teeth is then locked in there."

Pinpoint Accuracy

Using mass spectrometry technology, scientists analyzed radiocarbon levels in 33 teeth from 22 dead individuals who lived in northern Sweden. These people, whose ages were known, all had similar diets.

The researchers then compared the radiocarbon levels in the teeth with the levels of atmospheric radiocarbon recorded in various years to determine when the teeth formed in each person.

"When you think of the strategy, it's not surprising that it works," Frisén said.

The researchers were able to pinpoint within 18 months the age of the teeth. Other methods, including the analysis of wear on teeth, are accurate only to within five to ten years.

"It turned out to be much better than we expected," Buchholz said. "It's a very powerful technique."

However, it does not work for individuals born before 1943. That's because atmospheric radiocarbon levels did not vary before nuclear testing began in 1955, and the final formation of enamel is for wisdom teeth at 12 years of age.

Tsunami Victims

The researchers emphasize that enamel dating cannot identify a specific person—that requires DNA analysis—but the method can be used to narrow down the number of people on a list of missing people in a disaster, for example.

"Knowing the age of an individual allows [us to] narrow in on fewer potential identities," Frisén said. "This is a strategy that has been used for ages, but with less precision than what can now be done."

Swedish forensic scientists found the technique to be particularly useful in narrowing their search for victims of the Southeast Asia tsunami. When bodies are left in water, as was the case with tsunami victims, they quickly deteriorate and become very difficult to identify.

"DNA analysis is favored, but [it] calls for … access to DNA," Frisén said. "The current technique can aid when DNA identification cannot be done."

Buchholz says his laboratory has made the technology available to authorities dealing with the aftermath of Hurricane Katrina.

"It's available if they want it," he said. "They will determine if they need it."

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