A bit of research shows BPA isn't present in the polyethylene used in envelopes for sous vide cooking. (BPA is only used in plastics labeled with a "7" in their recycling symbol apparently.) It also doesn't look like plasticizers are used in non-chlorinated polyethylene at all. Unreacted polyethylene is acetylene, a gas, and short strings of it shouldn't be as biologically reactive as carbon ring structures. So at least in this case I wouldn't worry too much. <shrug>