All microwave ovens made after October 1971, are covered by a radiation safety standard enforced by the Food and Drug Administration
This standard limits the amount of microwaves that can leak from a microwave oven during the lifetime of the appliance. The limit is 5 milliwatts of microwave radiation per square centimeter at approximately 2 inches from the oven surface. This is far below the level known to harm people. Furthermore, as you move away from an oven, the level of any leaking microwave radiation that may be reaching you decreases dramatically.
For example: Someone standing 20 inches from a microwave oven would receive approximately one one-hundredth of the amount of microwaves received at 2 inches.
This 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 is 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 opened 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 is much like an electric light that stops when it is turned off.
Although the FDA standards assure that microwave ovens do not present any radiation hazard when new or used, the Oregon Health Division licenses all microwave repair facilities. This licensing ensures that the repairs are done only by authorized and qualified facilities, so that repaired microwave ovens meet the same safety standards as when manufactured.