Tag: microwave ovens

How do microwave ovens affect people fitted with pacemakers? — W

If a microwave oven doesn’t leak microwaves, then it won’t affect such people at all. However, if microwaves do leak from a particular microwave oven, they will cause undesirable currents to flow in the electric leads of the pacemaker. That’s because a microwave consists of electric and magnetic fields, and an electric field exerts forces on charged particles. The mobile charged particles in the pacemaker’s electric wiring will experience these forces as the microwave encounters them and they will move back and forth with the microwave’s fluctuating electric field. The pacemaker’s wiring isn’t meant to carry these unexpected current flows, and the pacemaker and/or the person attached to it may experience unpleasant effects. While such problems are very unlikely, it makes sense to warn pacemaker users whenever a microwave oven is in use.

I know that an electromagnetic wave cannot pass through the holes in a metal cage (a Faraday cage) if those holes are significantly smaller than the wavelength of the wave. But what if it is just a constant electric field? What determines the hole size now? — KBH, Logan, Utah

If the electric field isn’t changing with time, then it can’t enter a metal cage no matter how large the cage’s holes are. In effect, the constant electric field has an infinite wavelength and can’t propagate through holes of any finite size. However, the holes don’t stop an electromagnetic wave instantly—the wave does penetrate a short distance into the cage before it dwindles to insignificance. The distance over which the wave diminishes by a factor of about 3 is roughly the size of the hole through which it is trying to pass. So if your Faraday cage has holes that are 1 centimeter in diameter, the constant electric field will take several centimeters to diminish to nearly zero. If the holes are much larger than that, the electric field will penetrate far into the cage and the cage will only be an effective shield if it is extremely large. To avoid having to use a very large cage, it’s better to use small holes.

Why does food become soggy after heating in the microwave oven, particularly pastry?

A normal oven heats foods by exposing them to hot air and thermal radiation. It cooks the foods from the outside in. As a result, a normal oven tends to make the surfaces of food dry and crispy because it heats those outer surfaces first and drives the water out of them. A microwave oven heats the food by heating the water in that food. It cooks foods from the inside out. As a result, a microwave oven tends to drive water out of the middle of the food and into the outer layers of that food. The outer layers are essentially “steamed” and steaming makes everything soggy.

If you stand near a microwave oven, looking at your food, is it dangerous—tissue damage or make you blind?

Properly built and undamaged microwave ovens leak so few microwaves that they aren’t dangerous at all. Even if they did leak enough to be in violation of the safety limits, those safety limits are very conservative. While there is no reason to court disaster by holding your face right up to the microwave for hours and hours, it shouldn’t hurt you at all.

Why can you put a can of frozen concentrate juice in the microwave? The metal doesn’t spark or burn.

The microwaves in a microwave oven consist of electric and magnetic fields. Since electric fields push on electric charges, microwaves cause electric currents to flow through any metal objects they encounter. These movements of current don’t necessarily cause any problems in a microwave oven. In fact, metal objects only cause trouble in the microwave oven when they are so thin or narrow that they can’t tolerate the electric currents that flow through them or when they have such sharp ends that electric charges leap off them as sparks. A thin object like a twist-tie can’t tolerate the currents and becomes very hot. Its sharp ends also allow charges to leap out into the air as sparks. But the thick, rounded end of a juice concentrate can easily tolerates the currents sent through it by the microwaves and doesn’t have the sharp ends needed to send charges into the air as sparks. It doesn’t present any problem for the microwave oven.

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.

Is it possible isolate a room or part of it totally from microwaves? — DMJ

Because conducting surfaces reflect electromagnetic waves, you can shield a room from electromagnetic waves by enclosing it in conducting surfaces. For example, a room surrounded by metal mirrors will be completely black inside because light won’t be able to enter it. Furthermore, if the electromagnetic waves that you’re trying to exclude have reasonably long wavelengths, you can put holes in the conducting surfaces because electromagnetic waves can’t pass through holes in a conducting surface if those holes are substantially smaller than their wavelengths. So, to shield your room from microwaves, I’d suggest enclosing it in copper screening with holes that are no more than a few millimeters in diameter. Many scientific experiments are performed in such screen rooms, which are generally called Faraday cages.

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.

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.

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.