The folks over at PopSugar asked me whether or not it was safe to microwave food in plastics was safe. Some are concerned about dioxins, but plastics don't have those anymore. However, there are other substances like BPA's that could cause problems.
If a container says "microwave safe" then it is OK. However, for all other containers, it is probably not a good idea to microwave food in them. See the full article here.