What containers are not safe to use in a microwave? I am particularly concerned about Styrofoam containers as I use them to make TV dinners for my family. Is it OK to heat directly in these containers?
The two critical issues with containers in a microwave are (1) that they do not absorb or reflect microwaves and (2) that they tolerate high temperatures. Concerning the first issue, a container that absorbs microwaves will become extremely hot and may be damaged or destroyed. Most plastics (including Styrofoam) don’t absorb microwaves and are fine. Glazed water-free ceramics and glasses are usually also fine, as long as they don’t have any metallic trim. Metal dishes are a poor choice because they reflect microwaves and lead to uneven heating. Unglazed ceramics absorb water and will overheat.
Concerning the second issue, many plastics melt or soften below the temperature of boiling water. Polystyrene, the plastic from which Styrofoam is made, has a glass transition temperature of almost exactly 212° Fahrenheit (100° Celsius). That means that it will begin to soften at just about the temperature of boiling water. While pure water will boil without much problem in Styrofoam, water containing dissolved solids such as sugar or salt will boil at a higher temperature and may melt the Styrofoam. You’ll know when this happens…it’s not really a health issue, just a potential for a messy oven. I’ve only encountered the problem once myself, when a Polystyrene gravy separator melted in the microwave and let the gravy spill.