Microwave ovens used in homes generate large amounts of microwave energy (500 to 1200 watts) at a frequency of 2,450 MHz (2.45 GHz); some industrial microwave ovens operate at 915 MHz. This amount of microwave energy (500 to 1200 watts) is so large that it will boil water, cook hamburger and set some materials on fire.
Microwaves cannot penetrate metal and are contained inside the oven. Inside of the oven one sees plastic walls, but behind the plastic there is metal. If you look carefully at the front plastic viewing window you will see that it has a metal mesh or screen. This screen reflects microwaves and keeps them inside the oven because the wavelength of the microwave is about 120 times the size of the holes. To stop leakage of microwaves between the door and the doorframe, the frame or the door is fitted with a quarter wavelength choke.
Unless your microwave has been physically altered or damaged, it will not leak more microwave energy than it did when it was new.
EHS-RPS has prepared a Q&A regarding microwave oven safety.
Cardiac pacemakers and microwave ovens
Implanted pacemakers are not easily susceptible to interference from microwave ovens. All modern pacemakers have shielded circuitry, highly selective filters, and the ability to recognize and reject interference. Tests have demonstrated that a normally functioning microwave oven will not affect an implanted pacemaker. There is no requirement to post warning signs near microwave ovens to alert persons with pacemakers.