It shouldn't be. Not all Styrofoam is microwavable safe. You should check before subjecting it to such heat. Non microwavable Styrofoam has a very low melting point and hot liquids and foods can easily cause it to melt. Not something you rant in your food. read more
Conclusion: If the food you microwave doesn't exceed 175 degrees, it'll survive being microwaved in styrofoam. If you have no other option to reheat your food, and you get a little styrofoam in your food, the worst that'll happen is an upset stomach. read more
Put simply, there are two basic types of Styrofoam, one is microwave safe and the other is not. If your container is marked as “microwave safe” go for it, and if not, move your food or drink to a glass, ceramic or otherwise microwave safe container. read more