FDA's Microwave Oven Safety Standard:
The Food and Drug Administration (FDA) has the responsibility for carrying out an electronic product radiation control program mandated by the Electronic Product Radiation Control provisions of the Food Drug and Cosmetic Act. Through its Center for Devices and Radiological Health, FDA sets and enforces standards of performance for electronic products to assure that radiation emissions do not pose a hazard to public health.
A Federal standard limits the amount of microwaves that can leak from an oven throughout its lifetime to 5 milliwatts (mW) of microwave radiation per square centimeter at approximately 2 inches from the oven surface. This limit is far below the level known to harm people. Microwave energy also decreases dramatically as you move away from the source of radiation. A measurement made 20 inches from an oven would be approximately one one-hundredth of the value measured at 2 inches.
The standard also requires all ovens to have two independent interlock systems that stop the production of microwaves the moment the latch is released or the door opened. In addition, a monitoring system stops oven operation in case one or both of the interlock systems fail. The noise that many ovens continue to make after the door is open is usually the fan. The noise does not mean that microwaves are being produced. There is no residual radiation remaining after microwave production has stopped. In this regard a microwave oven is much like an electric light that stops glowing when it is turned off.
All ovens must have a label stating that they meet the safety standard. In addition, FDA requires that all ovens have a label explaining precautions for use. This requirement may be dropped if the manufacturer has proven that the oven will not exceed the allowable leakage limit even if used under the conditions cautioned against on the label.
To make sure the standard is met, FDA tests microwave ovens in its own laboratory. FDA also evaluates manufacturers' radiation testing and quality control programs at their factories.
From the FDA (fda.gov)