Microwave Ovens

Kelly Classic, Certified Medical Physicist

Microwaves are used to detect speeding cars, to send telephone and television communications, and to treat muscle soreness. Industry uses microwaves to dry and cure plywood, to cure rubber and resins, to raise bread and doughnuts, and to cook potato chips. But the most common consumer use of microwave energy is in microwave ovens.

The Food and Drug Administration (FDA) regulates the manufacture of microwave ovens. On the basis of current knowledge about microwave radiation, the agency believes that ovens meeting FDA standards and used according to the manufacturer's instructions are safe for use.

Microwaves are a form of electromagnetic radiation; that is, they are waves of electrical and magnetic energy moving together through space. Electromagnetic radiation ranges from the energetic x rays to the less-energetic radiofrequency waves used in broadcasting. Microwaves should not be confused with x rays, which are more powerful.

Microwaves have two characteristics that allow them to be used in cooking: they are absorbed by foods and they pass through glass, paper, plastic, and similar materials.

Microwaves are produced inside a microwave oven by an electron tube called a magnetron. The microwaves bounce back and forth within the interior until they are absorbed by food. Microwaves cause the water molecules in food to vibrate, producing heat that cooks the food-that's why foods high in water content, like fresh vegetables, can be cooked more quickly than other foods. The microwave energy is changed to heat as soon as it is absorbed by food.

Although heat is produced directly in the food, microwave ovens do not cook food from the "inside out." When thick foods like roasts are cooked, the outer layers are heated and cooked primarily by microwaves while the inside is cooked mainly by the slower conduction of heat from the hot outer layers.

Microwave cooking can be more energy efficient than conventional cooking because foods cook faster and the energy heats only the food, not the oven compartment. Microwave cooking does not reduce the nutritional value of foods any more than conventional cooking. In fact, foods cooked in a microwave oven may keep more of their vitamins and minerals because microwave ovens can cook more quickly and without added water.

All microwave ovens made after October 1971 are covered by a safety standard enforced by the FDA. The standard limits the amount of microwaves that can leak from an oven. The limit is 5 milliwatts of microwave radiation per square centimeter at a distance of two inches from the oven surface. This is far below the level known to harm people. Furthermore, as you move away from an oven, the level of any leaking microwave radiation that might be reaching you decreases dramatically. For example, someone standing 20 inches from an oven would receive approximately one one-hundredth of the amount of microwaves received at 2 inches.

The 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 addition, a monitoring system stops oven operation in case one or both of the interlock systems fail. The noise that many ovens continue to make after the door is open is usually the fan. The noise does not mean that microwaves are being produced. There is no residual radiation remaining after microwave production has stopped. In this regard a microwave oven is much like an electric light that stops glowing when it is turned off.

All ovens made since October 1971 must have a label stating that they meet the safety standard. In addition, FDA requires that all ovens made after October 1975 have a label explaining precautions for use. This requirement may be dropped if the manufacturer has proven that the oven will not exceed the allowable leakage limit even if used under the conditions cautioned against on the label. To make sure the standard is met, FDA tests microwave ovens. FDA also evaluates manufacturers' testing and quality-control programs. When FDA finds a safety problem in a certain model or make of oven, it requires the manufacturer to correct all defective ovens at no cost to the consumer.

Much research is under way on microwaves and how they might affect the human body. It is known that microwave radiation can heat body tissue the same way it heats food. Exposure to high levels of microwaves can cause a painful burn. The lens of the eye is particularly sensitive to intense heat, and exposure to high levels of microwaves can cause cataracts. Likewise, the testes are very sensitive to changes in temperature. Accidental exposure to high levels of microwave energy can alter or kill sperm, producing temporary sterility. But these types of injuries-burns, cataracts, temporary sterility-can only be caused by exposure to large amounts of microwave radiation, much more than can leak from a microwave oven.

Less is known about what happens to people exposed to low levels of microwaves. To find out, large numbers of people who had been exposed to microwaves would have to be studied for many years. This information is not available. Much research has been done with experimental animals, but it is difficult to translate the effects of microwaves on animals to possible effects on humans. For one thing, there are differences in the way animals and humans absorb microwaves. For another, experimental conditions can't exactly simulate the conditions under which people use microwave ovens. However, these studies do help to better understand the possible effects of radiation. One experiment, for example, showed that repeated exposure to low-level microwave radiation (less than 10 milliwatts per square centimeter) does not cause cataracts in rabbits.

These findings, together with the fact that many scientific questions about exposure to low levels of microwaves are not yet answered, point to the need for FDA to continue to enforce strict controls. They also underscore the need for consumers to take certain commonsense precautions to use microwave ovens only as manufacturers prescribe.

At one time there was concern that leakage from microwave ovens could interfere with certain electronic cardiac pacemakers. There was similar concern about pacemaker interference from electric shavers, auto ignition systems, and other electronic products. Because there are so many other products that also could cause this problem, FDA does not require microwave ovens to carry warnings for people with pacemakers. The problem has been largely resolved since pacemakers are now designed so they are shielded against such electrical interference, however, patients with pacemakers may wish to consult their physicians about this.

There is little cause for concern about excess microwaves leaking from ovens unless the door hinges, latch, or seals are damaged or if the oven was made before 1971. In FDA's experience, most ovens tested show little or no detectable microwave leakage. If there is some problem and you believe your oven might be leaking excessive microwaves, contact the oven manufacturer, a microwave oven service organization, your state health department, or the nearest FDA office. Some oven manufacturers will arrange for your oven to be checked. Many states have programs for inspecting ovens or they may be able to refer you to microwave oven servicing organizations that are equipped to test ovens for emissions.

To safely operate a microwave oven:

• Follow the manufacturer's instruction manual for recommended operating procedures and safety precautions for your oven model.
• Don't operate an oven if the door does not close firmly or is bent, warped, or otherwise damaged.
• Never operate an oven if you have reason to believe it will continue to operate with the door open.
• To add to the margin of safety already built into the oven, don't stand directly against an oven (and don't allow children to do this) for long periods of time while it is operating.
• Users should not heat water or liquids in the microwave oven for excessive amounts of time.