“Handled evidence must be placed in plastic bags to preserve it”

It's a common scene in crime dramas: forensic experts meticulously collect evidence, often placing items into clear plastic bags. This ubiquitous portrayal has led many to believe that all handled evidence, especially items like clothing or those bearing bloodstains, must be sealed in plastic to preserve it. However, in the world of forensic science, this practice is largely a misconception, particularly when dealing with biological materials.

The scientific reality is that biological evidence, such as garments, swabs containing bodily fluids, or other items with potential DNA, should ideally be stored in breathable containers like paper bags or cardboard boxes. Plastic bags, while useful for some types of evidence, trap moisture. This trapped moisture creates a humid environment that is highly conducive to bacterial growth and mold formation. Bacteria and mold rapidly degrade organic material, including the precious DNA evidence that forensic scientists rely on for identification and linkage to a crime.

The belief in using plastic for all evidence likely stems from a combination of factors. For non-biological items, plastic can indeed be effective at preventing loss or contamination. Furthermore, the visual clarity of plastic bags makes for more dramatic and understandable storytelling in television and film, which may inadvertently perpetuate the myth. In reality, the priority for biological evidence is to allow it to air dry and then package it in a way that prevents moisture buildup, thus safeguarding the integrity of potential DNA for analysis.