Dear Dr. Prescott,
It seems police are now routinely solving “cold cases” by using DNA evidence from crimes committed long ago. How is it they’re still able to use DNA that’s been around for a decade or more? Doesn’t it go bad?
Regina Buckley, Bala Cynwyd, PA
Like all biological materials, human DNA has a shelf life. But that expiration date varies wildly, depending on storage conditions. The keys are protecting it from heat, water, sunlight and oxygen.
For example, if a body remains exposed to the elements, its DNA will be useful for testing only for a few weeks. On the other hand, if it’s buried a few feet below ground, it will last 1,000 years or more. And if it’s kept cold and protected, it can last much, much longer: A sample of Neanderthal DNA found in a Belgian cave dates back 100,000 years.
Since long before the advent of DNA sequencing technology, law enforcement investigators have been collecting biological evidence from crime scenes in ways that would preserve it. For example, that means gathering dry samples (hair, dried blood or other bodily fluids, cigarette butts with traces of saliva) and placing them in separate containers. This avoids cross-contamination with other samples and, when stored in cool, dry spaces, prevents spoliation caused by the growth of organisms like mold or mildew.
With certain pieces of wet evidence, forensics experts dry the samples, then store them in a similar way. For tissue or fluids, they’re typically collected in vials and stored in refrigerators or freezers.
Obviously, sample storage procedures vary among police departments and have changed over time. Still, so long as those methods keep samples dry, relatively cool, out of the sunlight and uncontaminated by other substances, that evidence should remain “good” for DNA testing longer than anyone reading this will be around.