The researchers found that certain soft tissues — notably blood, nails and hair — had radiocarbon levels identical to the contemporary atmosphere.
Therefore, the radiocarbon level in those tissues post-mortem would indicate the year of death.
The researchers found that year-of-death determinations based on nails were accurate to within three years.
Radiocarbon levels in teeth formed before then contained less radiocarbon than expected, so when applied to teeth formed during that period, the method was less precise.
To determine year of death, the researchers used radiocarbon levels in soft tissues.
Now, new applications for the technique are emerging in forensics, thanks to research funded by NIJ and other organizations.
In recent years, forensic scientists have started to apply carbon-14 dating to cases in which law enforcement agencies hope to find out the age of a skeleton or other unidentified human remains.
The researchers wanted to find out if they could identify a person's year of birth or year of death using precise measurements of carbon-14 levels in different post-mortem tissues.
They measured carbon-14 levels in various tissues from 36 humans whose birth and death dates were known.
Archaeologists have long used carbon-14 dating (also known as radiocarbon dating) to estimate the age of certain objects.
Traditional radiocarbon dating is applied to organic remains between 500 and 50,000 years old and exploits the fact that trace amounts of radioactive carbon are found in the natural environment.
All the people whose tissues were tested for the study were residents of the United States.
Atmospheric dispersion tends to create uniform levels of carbon-14 around the globe, and researchers believe that these would be reflected in human tissues regardless of location.
Thus, pupal case radiocarbon content would serve as a decay-resistant proxy for the tissues, yielding the year of death.