Help me convince my DH not to keep doing this at work!
If the styrofoam does not say it is microwave safe then do not use it in the microwave.
Microwave safe containers still have some chemicals migrate to food but that level has been deemed acceptable. http://www.health.harvard.edu/fhg/updates/update0706a.shtml
I personally wouldn't do it if I had another container available even the styrofoam is labeled microwave safe.