Hi badbedgirl,
Have you read the tutorial post here? There may be other online options available to you as well (that are linked in the tutorial post) which use similar materials that may also be well worth considering as well as some local options that you can test in person. If you let me know of your city or zip code I’d be happy to let you know of any local options I’m aware of.
Post #2 here and the other posts and sources of information it links to may also be useful to help you answer the question of “how safe is safe enough for me?” which may have a very different answer for different people.
I don’t have any personal experience in using this but the first part of post #2 here along with posts #3 - #5 here has some information that may be helpful. While it will probably sleep much warmer because it would be completely impermeable … if you use clear polyethelyne that is 5 mils or more it should block all the VOC’s from escaping the mattress.
If they are food grade polyethylene or a similar non toxic material and are thick enough to completely prevent the escape of VOC’s then they would work yes … at the tradeoff of temperature regulation and “comfort”.
Phoenix