Microwave ovens use non-ionizing radiation to heat food. The radiation causes the water molecules in the food to vibrate, which in turn produces heat and makes the food cook faster. There are regulations for the amount of radiation that a microwave can emit during operation. If the microwave is not operated according to instructions, though, or is damaged in any way, excessive amounts of microwave radiation can escape, especially if a microwave door does not close properly. If your microwave door is bent or cracked, if the door seal is not tight because it is dirty, or if the door does not close properly after excessive use, you might be exposed to higher levels of microwave radiation than the FDA has allowed. Check your microwave to make sure that it is operating properly, and never lean directly against a microwave while it is heating food.
Microwaves cause water, fat, and sugar molecules to vibrate 2.5 million times per second. The vibration causes heat, but takes a while to stop. That is why instructions often say to let the food sit for a minute or two after cooking. As the molecules slow down, the food will still continue to cook even after the microwave is off.
Microwaves do not make food radioactive. The waves used in microwave cooking are similar to television and radio waves. Microwaves only penetrate food to a depth of one to one and half inches and cause vibrations that heat the food to the inner core.