My original post had a good dose of sarcasm - I get that way when I see someone putting others in harms way with no good proof.
Here are a few more resources:
http://www.articlesbase.com/technolo...ld-495648.html
http://www.lni.wa.gov/Safety/Basics/HazAlerts/902.asp
http://erd.dli.mt.gov/safetyhealth/b...dgassafety.pdf
Here are some wonderfully instructive videos on what happens:
http://www.crazybuilders.com/forum/showthread.php?p=29
http://www.astm.org/Standards/D1785.htm
http://www.ppfahome.org/pvc/index.html
If, by now, this discussion is not at an end, I the person here said it best:
http://www.garageofevil.com/faq/showdoc.php?id=35
If that does not answer the question as to why you should not use it for this purpose, you should add yourself as a corpse prop to your haunt now.