Microwave Ovens
Page 7 of 13 (127 Questions and Answers)
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MLA Citation: Bloomfield, Louis A. "Microwave Ovens" How Everything Works 20 Jul 2018. Page 7 of 13. 20 Jul 2018 <http://www.howeverythingworks.org/prints.php?topic=microwave_ovens&page=7>.
867. Can one's health be adversely affected by the use of certain wraps, films, or containers, when heating food in the microwave?
When various plastics become hot, their molecules become more mobile. The most obvious such case is when a plastic actually melts. But even before it melts, a plastic can begin to lose molecules to objects that are touching it. However, the plastics used in cooking are pretty non-toxic, so that even eating pieces of those plastic won't cause you any significant trouble. On the other hand, I would be careful with plastics that weren't intended for cooking. Some non-food related plastics are mixed with additives called "plasticizers" that keep them softer than they would be if they were pure. These plasticizers have a tendency to migrate out of the plastics, giving such things as "vinyl" their characteristic odors. Heating a plastic containing a plasticizer can drive this plasticizer out of the plastic and into something else. I don't think that it's a good idea to eat plasticizers so I would suggest not cooking with plastics that weren't intended for use with food. Still, not all plasticizers are bad—water is an excellent plasticizer for such common plastics as hair and cotton.

868. Do microwaves have no effect on gas?
While water vapor can absorb microwaves at certain frequencies, the absorption mechanism is very different from the one that causes liquid water to become hotter. Steam isn't affected very much by a microwave oven.

869. What are the key components of a microwave oven?
In addition to the digital controller that runs the microwave, it contains (1) a power relay that allows the controller to turn on and off the microwave source, (2) a power transformer that produces the high voltage electricity needed by the magnetron, (3) a power rectifier that converts the alternating current from the transformer into the direct current needed by the magnetron, (4) a capacitor that smoothes out ripples in the direct current leaving the rectifier, (5) a magnetron that uses the high voltage direct current to produce an intense beam of microwaves, (6) a wave guide that transports the microwaves from the magnetron to the cooking chamber, and (7) a cooking chamber in which the food absorbs the microwaves and becomes hotter.

893. Is there an inexpensive device for detecting leaks from a microwave oven?
Yes. You can get one from a hardware or appliance store for about $5 to $30. ComfortHouse.com sells one on-line at www.comforthouse.com. While I have tended to downplay the leakage issue in the past, I bought a tester and found that the microwave oven in my laboratory actually leaked significantly. I had used it in many class demonstrations, so it had been abused and the door wasn't properly aligned any more. I retired it. Incidentally, the tester contains only two components: a fast diode and a current meter. It detects microwave in the same way that a crystal radio detects an AM radio broadcast. However, I should note that both the International Microwave Power Institute (IMPI) and the FDA caution against trusting those simple and not particularly accurate meters, and recommend that you take your microwave oven to a service shop for inspection with an FDA certified meter.

926. I heard recently of someone with a pacemaker who went near a microwave oven and his pacemaker faulted, with him needing urgent medical attention. How did this happen? I also know of someone currently undergoing chemotherapy, who was told by his doctor not to eat food from a microwave oven. Why?
A pacemaker contains electronic circuits and wires that can act as antennas for microwaves. If a pacemaker is exposed to sufficiently intense microwaves, currents will begin to flow in those wires and circuits, and these currents may cause computational errors to occur or they may cause the circuitry to overheat. But while a pacemaker is far more sensitive to microwave radiation than say your hand is, I'm still surprised that enough microwave radiation leaked out of the oven to cause trouble. I'd suspect a real problem with that oven.

As for the chemotherapy question, I can't think of any reason why the doctor would suggest avoiding cooking food in a microwave oven. Unless I hear otherwise, I would suspect ignorance on the part of the doctor. The doctor may not understand the difference between "microwave radiation" and "gamma radiation".

927. Why does microwave radiation affect plant seeds differently? If you microwave sunflower seeds 30 seconds, they germinate faster than if you did not microwave them at all, and yet if you microwave them for 60 seconds, the seeds do not germinate at all. If you do this same experiment with carrot seeds, the non-radiated seeds, the 30 second and 60 second seeds all germinate within 14 days. Why? Is it because the sunflower seeds are larger and absorb more radiation than the smaller carrot seeds? — ST, Mobile, AL
When you expose the seeds to microwave radiation, you are selectively heating portions of the insides of the seeds. Fats and oils don't absorb microwaves well but water does, so the parts of the seeds that become hottest are those that contain the most water molecules. Evidently, heating the water-containing portion of a sunflower seeds slightly cause that seed to germinate faster, but heating that same portion too much sterilizes the seed. That observation indicates that a moderate temperature rise causes the chemical reactions of germination to occur more rapidly while a more severe temperature rise denatures some of the critical biological molecules and kills the seed. The absence of any effect in carrot seeds may indicate that they don't have enough water in them to absorb the microwaves. It may also indicate that they can tolerate higher temperatures without undergoing the chemical reactions of germination and without experiencing damage to their critical molecules.

933. What metals and other substances are used in microwave ovens? Specifically, what is the substance on the inside of the microwave that absorbs all the microwaves? — AD, San Anselmo, CA
The walls of a microwave oven's cooking chamber are made of highly conductive metals so that they reflect the microwaves almost completely. Only a very small fraction of the microwaves inside the oven are absorbed by these metal walls and virtually none of the microwaves escape into the room. However, there is a substance inside the cooking chamber that absorbs the microwaves: water in the food! If you don't put water-containing food inside the microwave oven, there will be nothing to absorb the microwaves and they will reflect back to the magnetron and may damage it. The absence of an absorber in the cooking chamber will also increase any minor leakage of microwaves from the oven because the microwave intensity inside the cooking chamber will be much higher than normal.

952. Is it harmful for children to sit too close to microwave ovens? Is it possible to get "burned" opening the microwave oven during a cycle or too soon after a cycle? I realize the oven shuts off, but is there residual radiation? — C
As long as the microwave oven hasn't been damaged and doesn't leak excessive microwaves, there should be no harm in having children sit near it. I wouldn't hold my face right up against the door edges because that would be asking for trouble with leakage, but it's extremely unlikely that even doing that once in a while would cause injury.

As for being injured by microwave radiation after the cycle has stopped, that's essentially impossible. As soon as the high voltage disappears from the magnetron tube and it stops emitting microwaves, the microwaves in the cooking chamber begin to diminish. Even if they bounce 1000 times off the metal walls of the chamber before they're absorbed by those walls or the food in the microwave, that will only take about 2 millionths of a second. You can't open the door fast enough to let them out before they're already gone.

998. Is it true that microwaves cause cancer?
I think that it's very unlikely that microwaves cause cancer. Microwaves are not ionizing radiation—they don't directly damage chemical bonds. Instead, they heat materials, particularly those containing water. As a result, they may cause damage to proteins in the same way that cooking damages proteins (and hardens egg protein, for example). But while such protein damage can easily cause cell death, I wouldn't expect it to cause the genetic damage associated with cancer.

1019. Have you made RF leakage measurements on a sample of microwave ovens? I understand that the FDA requires that if measured 5 cm away from any of the oven's surfaces, the RF leakage must be less than 1 mW/cm2 for new ovens and less than 5 mW/cm2 over the oven's life time. I'm just curious what actual measurements reveal about a "typically used" oven. — S
I've measured several ovens and have only found one that leaks a measurable amount of microwave power. That leaker is an oven that I've used in countless demonstrations and have taken apart several times (it appears on page 514 of my book). Considering the abuse that poor oven has had, it's doing pretty well. At a talk I gave yesterday, I couldn't get it to leak more than about 1 mW/cm2 even though I was measuring microwave power directly on the edge of the oven door—the most vulnerable point in the oven. Given that this oven's door sags several millimeters as the result of its rough treatment, that's not bad. In short, I doubt that there are many leaky microwave ovens around that haven't been dropped, crushed in shipping, or otherwise suffered serious mechanical injury.

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