Oh… Please stay away from microwaving Styrofoam and Plastic containers in the darn microwave. Protect your precious body and don’t take the chance.
According to the Mayo Clinic, the Environmental Protection Agency suggests that dioxins are a "likely human carcinogen." Since dioxins are found in some plastics, there's a chance plastics can release dioxins when you cook your food. However, the Mayo Clinic also cites the FDA's finding that "there is no evidence that plastic containers or wraps contain dioxins." Even if dioxins aren't an issue, plastics contain a number of chemicals that could have adverse effects on your body. Avoid taking chances entirely by cooking in containers meant for the microwave.
According to environmental organization Ecology Center, plastics may contain chemical compounds that can be released into your food over time, even outside of the microwave. Because of this, you can never be sure of the effects of any plastic on the food it contains.
Heating Styrofoam Link