Styrofoam, also known by generic name polystyrene, is a type of plastic that is made of expanded polystyrene beads. Urban legends abound on the potential side-effects and health risks associated with microwaving styrofoam products. However, studies have show that microwaving a styrofoam cup (along with some other styrofoam products) can be safe.
When any plastic container or cup, including one that's made of styrofoam, is microwaved, it has the potential to release chemical substances into the food or drink. This is a fairly normal side effect, and can even happen when microwaving a cup that is deemed microwave-safe. These chemicals, called plasticisers, are present in many things that we ingest from plastic containers.
According to the Harvard Medical School, the Food and Drug Administration monitors all plastic containers, including styrofoam cups, to make sure that they are safe when subjected to the conditions related to their intended use. This includes testing the amount of plasticisers released by styrofoam cups that are deemed microwave-safe. This does not mean that the chemicals will not migrate into the food or drink during the heating process. Rather, it means that the FDA makes sure the amount that is transferred falls within a safe range.
A greater risk when it comes to microwaving styrofoam cups is the danger of melting the plastic. Melted styrofoam is incredibly hot and can cause intense burns. The melted plastic can also be very difficult to clean up and, obviously, will ruin any food or drink that is contained in the cup.
When microwaving styrofoam, it's important to make sure that the cup is microwave-safe before heating it if you want to avoid the dangers. If a cup isn't microwave-safe, more dangerous levels of plasticisers could leach from the cup into the food or drink. This is because the FDA doesn't test these cups at that temperature, so there's no real way of knowing how the plastic will react in the microwave. These types of styrofoam cups are also more likely to melt in the microwave.
For your own benefit, it's smart to check the intended use of a styrofoam cup before putting it in the microwave. If a styrofoam cup is microwave-safe, that fact should be printed either on the bottom of the cup or on the packaging in which the cup is contained. For example, microwaveable noodle cups are often packaged in microwave-safe styrofoam containers. If the cup or packaging does not mention anything about being microwave-friendly, play it safe and transfer the food to another container for heating.