I have seen on the Discovery Channel that whenever detectives are at a crime scene they always use paper bags and not plastic bags. What is the reason for using paper only? Do the chemicals in the plastic bag somehow change the evidence?
Dunno what detective shows you watch; I’ve seen them use plastic bags quite often.
Both are used…
Paper bags are great for pretty much all of the big stuff: clothes, sheets, ropes, etc.
Plastic baggies are better for hair, biohazard evidence and little tiny things. A knife full of blood might fall right through a paper bag once the blood soaked through. Better put it in in plastic.
These are not you only two options, BTW. You have plastic jars, tubes, boxes and all sorts of other things. Here is a list of some:
http://www.evidentcrimescene.com/cata/evid2/evid2.html
::shiver::
I love it when Bear gets all law enforcement 
If you mean bags put over a victim’s hands:
It’s not the chemicals in the plastic bags so much as the moisture they would allow to collect on the deceased’s hands. The moisture could then mess up the collection of trace evidence.
–tygre
I’ve been a volunteer rape crisis counselor for 7 years. A big part of what I do is get them through the rape kit (evidence collection.) When a victim undresses (his/her clothing is part of evidence), I have her stand on a large paper sheet while undressing, to catch any hairs, fibers, dirt & so on. This stuff sticks to the paper, but would slide off plastic. All other evidence - hair samples & swabs collected by a doc, go into paper envelopes because condensation could impair lab results, particularly with swabs.
That’s a partial answer, anyway. But I’ve seen plastic baggies used on those shows frequently. And, what Bear said.
Paper bags are used when there is a chance that moisture might be sealed in a plastic one.
Bear in mind that evidence collected might be in storage for many months and items left damp all this time might deteriorate.