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A fixture in office break rooms, convenience stores and homes for decades, the microwave oven has been heating frozen foods, leftovers and even more elaborate meals for decades. In fact, some hip urban restaurants employ the familiar device to cook all their meals, from apps to entrees. Not only does this save energy and allow the restaurants to cope with small square footages in space-constrained districts, but it also offers a new retro-novelty, giving a wow factor to those who aren't familiar with the appliance's true versatility.

Yet Google "are microwave ovens safe," and you'll get a barrage of hits from concerned mothers and others who are worried that the handy device might have a dark, even dangerous side. Of course, the prevailing consensus among scientists, public health experts, government agencies and the general public is that microwave ovens are overwhelmingly safe when used as directed. However, it's also true that there may be some legitimate questions about the safety of certain aspects of the technology, beyond the paranoia of the tin-foil hat crowd.

Let's take a closer look at some myths, facts and misconceptions about microwave ovens, which are estimated to be used in at least 90% of American homes.

1. Microwave Ovens Were Discovered Accidentally

Status: Fact

Apparently no one thought of cooking food with microwaves until the 1940s, when a self-taught engineer named Percy Spencer was building radar equipment in a lab for Raytheon, and noticed that a chocolate bar he had in his pocket started to melt. He had been building magnetrons, and realized that microwaves can be directed at food to heat it up rapidly. He tested his idea by popping popcorn and exploding an egg. Not long after we were all happily scarfing down TV dinners.

2. There Is Dissent Over How Microwaves Actually Heat Food

Status: Fact

Microwave radiation is a form of non-ionizing radiation (meaning it can't directly break up atoms or molecules) that lies between common radio and infrared frequencies. So it is not thought to damage DNA of living things, the way X and gamma rays do. Still, microwaves can obviously cause heating effects, and can harm or kill at high energies. That's why microwave ovens on the market must operate at or below strict limits set by the federal government.

Most microwave ovens hit food with microwaves at a frequency of 2.45 gigahertz (GHz) (a wavelength of 12.24 centimetres (4.82 in)). The prevailing belief is that molecules in the food, particularly water, absorb energy from the waves through dielectric heating. That is, since water molecules are polar, having a positive end and negative end, they begin to rotate rapidly as the alternating electric field passes through. That rotation is thought to add heat to the food.

However, there are some scientists who have dissented with this view, suggesting that other interactions between the particles may be responsible for the heating.

3. Microwave Ovens Cook Food from the Inside Outside

Status: Myth

Although many people believe this to be the case, microwaves actually work on the outer layers of food, heating it by exciting the water molecules there. The inner parts of food are warmed as heat transfers from the outer layers inward. This is why a microwave can only cook a big hunk of meat to a depth of about one inch inward.

4. Metals Get Dangerously Hot in Microwaves

Status: Myth

Metals reflect microwaves, whereas plastic, glass and ceramics allow them to pass through. That means metals don't appreciably heat up in a microwave oven. However, thin pieces of metal, such as foils or the tines of a fork, can act as antenna, and the waves can arc off them, forming dramatic sparks.

5. Microwave Ovens are Energy Efficient Ways to Cook

Status: Fact

A complete analysis of cooking efficiency depends on a number of factors, including what you are trying to prepare and the cost -- and greenness -- of your local supply of electricity, gas or other fuel. Typically though, a microwave uses less energy to heat food than conventional ovens or ranges, because it works faster and more of the energy is focused directly on the food, versus heating containers or surrounding air. In fact, Energy Star calculated that cooking or re-heating small portions of food in the microwave saves as much as 80% of the energy needed for an oven.

Don't get too excited, however. Consumer advocate Michael Bluejay pointed out to Earth Talk that even for someone who bakes three hours a week, using the cheapest cooking method would save only an estimated $2.06/month compared to the most expensive method. "Focusing on cooking methods is not the way to save electricity [at home]," says Bluejay. "You should look at heating, cooling, lighting and laundry instead."

6. You Can't Heat Oils in a Microwave

Status: Fact

Oils such as olive oil do not heat well in microwaves because their molecules lack the polarity found in water. It's also true that frozen butter is hard to thaw in a microwave, because the bulk of the substance is oil, and the portion of water present is in the form of ice, which keeps the molecules locked up in crystal form, making oscillation more difficult.

7. Heating Plastics in a Microwave Can Be Dangerous

Status: Fact

The safest course of action is to avoid putting any plastics in the microwave. When the Milwaukee Journal Sentinel tested plastics labeled microwave-safe and advertised for infants, even those were found to release "toxic doses" of Bisphenol A when heated in a microwave. "The amounts detected were at levels that scientists have found cause neurological and developmental damage in laboratory animals," the paper reports.

In fact, the term "microwave safe" is not regulated by the government, so it has no verifiable meaning. According to the Journal Sentinel's testing, BPA "is present in frozen food trays, microwaveable soup containers and plastic baby food packaging." It is often found in plastics marked No. 7, but may also be present in some plastics labeled with Nos. 1, 2 and 5 as well, according to the report. Better to stick to glass or ceramics.

8. Boiling a Cup of Water in a Microwave Can Cause It to Explode

Status: Fact

One potential danger of microwave ovens is getting scalded by over heated water. What can happen is that when plain water is heated in a microwave in a clean ceramic or glass container for too long, it can prevent bubbles from forming, which normally cool it down. The water can become superheated, past its boiling point. So when it is disturbed, say by moving it or dropping something in it, the heat releases violently, erupting boiling water out of the cup.

To avoid this risk heat water only the minimum amount of time needed. Or place a wooden spoon or stick in it (you should be fine with a metal spoon too, as we discussed above. Don't use a metal fork though, which could spark.)

9. Microwave Cooking Can Be Unsafe Because It Doesn't Heat Evenly

Status: Fact

As we learned from Jim Gaffigan, microwaves don't always heat food evenly, sometimes leaving cold pockets next to hot pockets. If you're working with raw meat, this can be dangerous, since it could leave harmful bacteria.

The Center for Science in the Public Interest has warned that consumers should follow heating instructions carefully, including the standing time needed for additional cooking (in other words don't try to cool it off before you touch it).

10. Microwaves Leak Unsafe Levels of Electromagnetic Radiation

Status: Myth (at least most of the time)

For decades scientists and consumers have been debating over the possible effects of non-ionizing electromagnetic radiation on living tissue. Since we can't very well grow people in controlled lab experiments, it's very difficult to sort out the various risks we might get from fields emitted from power lines, cell phones, airplane flights, computers, clock radios, and of course microwave ovens. We know strong fields raise cancer rates and other problems, but what about the cumulative effect of small exposure, or effects on children?

No one knows, although we can take heart that the FDA limits the amount of microwaves that can leak from an oven throughout its lifetime to 5 milliwatts (mW) of microwave radiation per square centimeter, at approximately 2 inches from the oven surface. According to the agency, "This limit is far below the level known to harm people." It's also true that microwave energy decreases dramatically as you move away from the source of radiation. A measurement made 20 inches from an oven would be approximately one one-hundredth of value measured at 2 inches. The federal standard also requires all ovens to have two independent interlock systems that stop the production of microwaves the moment the latch is released or the door opened.

In an interview with TDG, mechanical engineer Mark Connelly, the deputy technical director of Consumer Reports, said that the vast majority of microwave ovens his group has tested have shown "very little leakage of radiation." Connelly echoed the advice of the FDA, which is that if people are concerned, they can simply step away from a microwave oven when in use.

Asked if people should avoid looking into a working microwave, since the eyes are known to be the most sensitive to that form of radiation, and are known to develop cataracts at high field strengths, Connelly said he didn't think it mattered, "since the window is shielded, and there shouldn't be leakage through that."

"If you are concerned, then go out and spend $20 on a testing kit to reassure yourself that there isn't any radiation leaking from your microwave," Connelly added. He said his testing of consumer-grade kits has shown them to be reasonably reliable, despite some press accounts to the contrary. "Microwaves can wear over time, with gaskets wearing or trouble developing in the door. So I think it's prudent to spend a little money to test them," he said.

11. Microwaves Alter Food in Undesirable, Possibly Unsafe, Ways

Status: Undetermined but Unlikely

It's a fact of life that any type of cooking changes the chemistry of food. It can reduce the levels of some nutrients, just as it can increase the levels of others (e.g. lycopenes), or make them more or less available to the body for use. (Raw food anyone?) The prevailing view is that microwaves do not alter foods in ways that are any more deleterious or harmful than other types of cooking. In fact, some have argued that the faster cooking time may actually preserve more nutrients versus other methods.

Still, we know sufficiently little about nutrition and the cumulative effects of food science that some aren't so convinced (of course, there is also the threat of any harmful substances present getting released upon cooking, such as the diacetyl blamed for "popcorn lung.") In a recent article E Magazine pointed out that popular holistic health expert Dr. Andrew Weil has written, "There may be dangers associated with microwaving food... there is a question as to whether microwaving alters protein chemistry in ways that might be harmful." According to the magazine, Dr. Fumio Watanabe of Japan's Kochi Women's University found that heating samples for six minutes degenerated 30 to 40% of the milk's vitamin B12. This kind of breakdown took about 25 minutes of boiling with conventional heat. In a 1992 Stanford Medical School study often cited by microwave opponents, researchers reported a "marked decrease" in immune-boosting factors in microwaved human breast milk. In the late 1980s Swiss scientists reported decreases in hemoglobin and white blood cells in rats that had eaten microwaved food.

It's also much reported on the Internet that microwaving human blood renders it unsafe for transfusion -- though medical professionals point out that rapidly heating blood via any method can have the same negative result.

The conclusion made by government agencies and mainstream organizations is that microwaved food is safe, as well as convenient. There's a limited number of studies that may suggest otherwise, but given the lack of large-scale or compelling evidence it's hard to feel that tossing our your microwave is a particularly smart step. Everyone interviewed for this piece pointed to other issues as more pressing, from ubiquitous exposure to cell phones to more serious threats from radon, or bigger energy users like heating and cooling. That doesn't mean microwaves aren't worth thinking about, however.

And what are microwaves good for?

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