A belated respose to Paul Nutteing's post of 4 Dec.
Polyethylene whether LDPE, LLDPE, HDPE etc is always subject to minute pinholes, and is always made as a multilayer product for critical containment applications.
It is also permeable to a variety of substances to a greater or lesser degree.
A good example of the state of the art is the bag inside a cardboard wine cask, which can be of 5 to 7 layers of various polymers to resist spoilage of the wine by migration of substances in or out.
How samples subject to hypersensitive DNA testing (as in the Falconio case) could be kept in plain polythene bags, probably zip sealed, and be expected to be free of contamination from handling, is a serious concern.
All the best
Ian Macmillan