Despite the label on many plastic containers claiming that they are "microwave-safe," it is prudent to use either glass or ceramic bowls for heating foods in a microwave oven. Heating plastics can make chemicals used in their manufacturing to leach into your food. Even so-called "microwave safe" containers have been shown to leach Bisphenol A.
Laboratory studies with rats indicate that exposures to BPA, especially during prenatal through early adolescence, predispose an individual to increased risk for developing breast cancer. Most supermarkets now sell Pyrex or other glass food storage containers that are easy to heat, allow you to freeze, thaw and heat (in oven or microwave) food safely, and can be reused for years. When you do microwave your food, whether in glass or ceramic containers, cover the food with a piece of kitchen parchment paper, or other non-dyed, non-bleached paper product. Or just put a ceramic plate on top as a cover.
See 10 more "microwave safe" myths.