The shell or case of the microwave oven is built of metal, which reflects microwave radiation. The glass viewing window is lined with a metal mesh screen with holes too small to allow passage of microwave radiation. The door, when closed, is sealed to prevent microwave radiation from escaping. Microwave units have an automatic switch that discontinues power to the magnetron (radiation-emitting device) when the door is opened.
Observable causes of radiation leakage include a cracked or broken viewing screen, a door that does not close and latch properly, or an oven that continues to run when the door is opened. Microwave leakage can also be caused by a faulty door seal, sometimes caused by jarring or dropping the microwave oven. This situation can be especially harmful because the damage may not be detected by observation.
As of April 2011, no known microwave radiation exposure injuries have been verified by the U.S. Food and Drug Administration. Reported injuries related to microwave oven use have included scaldings and burns, common also to traditional cooking appliances.
If you suspect that your microwave oven has been damaged, have the unit inspected before continuing its use. Most home appliance repair specialists have microwave-radiation detectors and charge only a few dollars to tell you if the unit is operating safely.