You are actually exposed to less radiation if you stick your hand in a properly functioning microwave oven right after it turns off. First of all, the microwaves emitted by a microwave oven are not harmful beyond their ability to heat you. Microwaves are non-ionizing, meaning that they do not have enough energy per photon to rip electrons off of atoms or break chemical bonds, which is what leads to cancer and radiation sickness. In fact, microwaves have far less energy per photon than the light from a candle or even the infrared thermal radiation from your hand. Microwave ovens emit electromagnetic waves in the frequency range of gigahertz (GHz). These are the same types of waves used by radars, cell phones, and WIFI routers. Microwaves can burn you if they are powerful enough and hitting you for long enough, but this is fundamentally no different from being burned by a campfire's thermal radiation. Sticking your hand in the microwave while it is on (which would require breaking the oven's safety features) and leaving it there is a bad idea because you will get burned. Also, a properly functioning microwave oven automatically turns off the moment you open the door. The last bit of microwaves emitted by the oven bounce around inside and are absorbed within microseconds, long before you have even finished opening the door. By the time you stick your hand in the oven, the last bits of microwaves are long gone. Microwaves are a form of electromagnetic waves, just like visible light. The microwaves in the oven disappear when the door opens just as quickly as a room gets dark when you turn off the light. The walls of a microwave oven are metal, which keep microwaves inside from leaking out. The oven is constructed to avoid leaking microwaves not because they cause cancer, but because that would be a waste of energy. The oven's job is to cook food; a job it would not do very well if its energy were leaking out into the room. Interestingly, the metal walls of a microwave oven also block a lot of external radiation (from the sun, stars, rocks, storms, etc.) from getting in. Because of this shielding effect, your hand inside a non-running microwave actually receives less radiation than your hand out in the open air. Either way, the radiation you are exposed to is so low-energy that there is nothing to be concerned about.