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

The idea that all handled evidence should be placed in plastic bags for preservation is a common misconception, often perpetuated by crime dramas and movies. On screen, it might seem logical for investigators to seal away items in airtight plastic to protect them. However, in reality, this practice can actually compromise certain types of crucial forensic evidence.

The scientific truth behind evidence preservation reveals why plastic is problematic for biological samples. Materials like clothing, bloodstains, hair, or other bodily fluids contain moisture. If these items are sealed in a non-breathable plastic bag, that moisture becomes trapped, creating a humid environment. This dampness encourages the rapid growth of bacteria and mold, which can quickly degrade the DNA within the biological evidence, making it difficult or impossible to analyze accurately in a lab. Paper bags, in contrast, are porous, allowing air to circulate and moisture to evaporate naturally, thereby inhibiting microbial growth and preserving the integrity of the DNA.

People commonly believe the myth about plastic bags because plastic is generally understood to be protective and durable. It’s used for many household items to keep them safe from external elements. Furthermore, plastic bags *are* used for certain types of evidence, such as dry items, electronics, firearms, or substances like fire accelerants, which can add to the confusion. However, for sensitive biological material, the need for air circulation to prevent degradation outweighs the perceived protective benefits of an airtight seal. This distinction is vital in forensic science to ensure evidence remains viable for analysis and can contribute to solving cases.