Back in the past, i used to warm my sandwich in the microwave and then put them into a clear transparent bag and hold the sandwich to eat them. I used to think that it was not a good idea to warm the sandwich directly inside a sandwich plastic bag. So i used to put the sandwich on top of some brown paper and then microwave the sandwich, after which i put it into the plastic sandwich bag. But later on, i realized that this too wan't a good idea because as soon as the hot sandwich touches the plastic bag, the plastic bag tends to start melting slightly. I found the taste of sandwich a bit off but it never bothered me much until recently when i read an article online mentioned the problems of using plastic bag in the microwave.
So i took a different approach to solving the problem. What i did was to use a brown paper sandwich bag to put the sandwich into, and then microwave it. The brown paper sandwich bag doesn't melt or cause health problem i believe. It is a use and throw solution and it is a quite cost effective method as well. You can buy those brown paper sandwich bag from Dollarama store or any other dollar store, although not a lot of smaller dollar store carry them.