We are told all about DNA. But truthfully, who really understands it? We simply take the word of the scientific community.
A scientist could come up with PAP (physical air particles) and tell us that people leave particles in the air that proves they were at the scene. Within a few years people would be convicted on PAP evidence. No scientist wants to be outdone and seem to be out of touch.
Kinda like the EPA telling us of greater and greater dangers posed by smaller and smaller detectable amounts of toxic substances.