The aboveground nuclear tests that occurred between 1955 and 1963 dramatically increased the amount of the isotope carbon 14 in the atmosphere. The levels rapidly equalized around the globe, even though the explosions occurred at only a few locations, and entered plants in the food chain through photosynthesis. By eating plants, and animals that feed on plants, humans absorb carbon 14 and exhibit levels of the benign, traceable isotope that are similar to atmospheric concentrations. What is more, carbon 14 decays with a half-life of 5,730 years, a phenomenon that scientists can exploit as a way to determine the ages of objects that contain the isotope. For the new study, Jonas Frisén of the Medical Nobel Institute in Stockholm, Sweden, and his colleagues analyzed the carbon content of tooth enamel. Because teeth do not exhibit any turnover during a person's life, the scientists can determine when a tooth formed by comparing its carbon 14 content to past atmospheric levels. In addition, adult teeth form during a distinct period of childhood development around age 12, so this information can be translated into the age of an individual.
The team tried the new approach on teeth recovered from 22 individuals and found that it gave a remarkably precise estimate for their ages. In all cases, the result projected by carbon 14 dating was within 1.6 years of the correct age. This is a marked improvement over the accuracy of other methods currently employed to determine the age of remains, which can have associated errors ranging between five and 10 years. Of course, this particular approach can only precisely identify the age of individuals born after 1943, because they would be the first group to have developed adult teeth by 1955, when widespread atmospheric nuclear testing occurred. But because exposure to carbon 14 through diet or local conditions may vary around the world, the authors caution that "the method will need to be verified on a larger scale, and perhaps on a wider geographical range of cases before it can be applied to forensic work."