How do microwave ovens work and are they harmful in any way?

Asked by: Scott Kelley


Microwave ovens produce electromagnetic radiation of exactly the right wavelength to excite water molecules. When water molecules become excited, they heat up. Since most of our food contains a fair amount of water, we can heat up our food by selectively heating up the water inside the food.

Microwave radiation also passes through glass and plastic, which allows it to travel through tupperware and heat up the food inside. However, microwave radiation does not penetrate very deep into the food itself, so if you put something big into the microwave oven for a short amount of time, it'll be hot on the outside but still cool in the middle. To heat up something big like a turkey breast, the heat has to diffuse from the surface to the inside. Plan on putting at least a few minutes on the timer.

Microwave ovens can definitely be harmful if used improperly. Microwave radiation can pass through plastic and glass, but it'll reflect off of metal. If you put a metal object (such as a metal bowl or fork) into the microwave oven, this can cause the microwaves to reflect back to the source that produces them (called the 'magnetron'), and can result in considerable damage to the oven. (The metal wiring in the glass window of the door keeps the microwaves from leaving the oven, but doesn't reflect them back to the source.)

We have already seen that water is heated up in a microwave oven. It is an extremely unwise idea to expose live creatures, which are about 90% water, to microwave radiation. Put simply, any creature will die when exposed to microwave oven radiation long enough for water to boil.

You'll be pleased to know that the Food and Drug Administration has been enforcing strict guidelines for microwave ovens sold in the USA since 1971. These guidelines ensure that leakage is kept well below harmful doses, as well as ensuring that each microwave oven is equipped with at least two safety switches that stop microwave production the instant the door opens.

So like most of the other tools we use every day, the microwave oven can be a great convenience, but it can also be very dangerous when used unwisely. The key is to understand how it works, and to make good decisions when using it.

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Answered by: Tonio Buonassisi, B.S., Appl'd Science & Technology Grad Sdnt, UC Berkeley