Microwave Oven Radiation

About Microwaves
What is Microwave Radiation?
Cooking with Microwaves
Microwave Oven Safety Standard
Microwave Ovens and Health
Have Radiation Injuries Resulted from Microwave Ovens?
Ovens and Pacemakers
Checking Ovens For Leakage
Tips on Safe Microwave Oven Operation
Erupted Hot Water Phenomena in Microwave Oven
What Can Consumers Do to Avoid Super-Heated Water?
Other Tips for Microwave Oven Use

 

About Microwaves

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. That use has soared in the past decade.

The Food and Drug Administration (FDA) has regulated the manufacture of microwave ovens since 1971. On the basis of current knowledge about microwave radiation, the Agency believes that ovens that meet the FDA standard and are used according to the manufacturer's instructions are safe for use.

What is Microwave Radiation?

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 radio frequency waves used in broadcasting. Microwaves fall into the radio frequency band of electromagnetic radiation. Microwaves should not be confused with x-rays, which are more powerful.

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

Cooking with Microwaves

Microwaves are produced inside the oven by an electron tube called a magnetron. The microwaves bounce back and forth within the metal 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. Thus, it can not make food radioactive or "contaminated."

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 adding water.

Glass, paper, ceramic, or plastic containers are used in microwave cooking because the microwaves pass through them. Although such containers can not be heated by microwaves, they can become hot from the heat of the food cooking inside. Some plastic containers should not be used in a microwave oven, they can be melted by the heat of the food inside. Generally, metal pans or aluminum foil should also not be used in a microwave oven, as the microwaves are reflected off these materials causing the food to cook unevenly and possibly damaging the oven. The instructions that come with each microwave oven indicate the kinds of containers to use. They also cover how to test containers to see whether or not they can be used in microwave ovens.

FDA recommends that microwave ovens not be used in home canning. It is believed that neither microwave ovens nor conventional ovens produce or maintain temperatures high enough to kill the harmful bacteria that occur in some foods while canning.

Microwave Oven Safety Standard

All microwave ovens made after October 1971 are covered by a radiation safety standard enforced by the FDA. The standard limits the amount of microwaves that can leak from an oven throughout its lifetime. The limit is 5 milliwatts of microwave radiation per square centimeter at approximately 2 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 in commercial establishments, dealer and distributor premises, manufacturing plants, and its own laboratories. FDA also evaluates manufacturers' radiation testing and quality control programs. When FDA finds a radiation 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.

Although FDA believes the standard assures that microwave ovens do not present any radiation hazard, the Agency continues to reassess its adequacy as new information becomes available.

Microwave Ovens and Health

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. On the other hand, some animals display an avoidance reaction when exposed to low levels of microwaves - that is, they try to get away from the microwaves. Other effects noted in experimental animals include a decreased ability to perform certain tasks, genetic changes and an "immune response" (the body acts as if it were responding to protect itself from a disease). While these and similar effects have been observed in animals, their significance for human health remains unclear.

These kinds of 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 radiation controls. They also underscore the need for consumers to take certain common sense precautions.

Have Radiation Injuries Resulted from Microwave Ovens?

There have been allegations of radiation injury from microwave ovens. The injuries known to FDA, however, have been injuries that could have happened with any oven or cooking surface. For example, people have been burned by the hot food, splattering grease, or steam from food cooked in a microwave oven.

Ovens and Pacemakers

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.

Checking Ovens For Leakage

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 excessive emission. A limited number of ovens are also tested in homes by FDA as part of its overall program to make sure that ovens meet the safety standard.

A word of caution about the microwave testing devices being sold to consumers: FDA has tested a number of these devices and found them generally inaccurate and unreliable. If used, they should be relied on only for a very approximate reading. The sophisticated testing devices used by public health authorities to measure oven leakage are far more accurate and are periodically tested.

Tips on Safe Microwave Oven Operation

• 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.

Erupted Hot Water Phenomena in Microwave Ovens

The FDA has received reports of serious skin burns or scalding injuries around people's hands and faces as a result of hot water erupting out of a cup after it had been over-heated in a microwave oven. Over-heating of water in a cup can result in superheated water (past its boiling temperature) without appearing to boil.

This type of phenomena occurs if water is heated in a clean cup. If foreign materials such as instant coffee or sugar are added before heating, the risk is greatly reduced. If superheating has occurred, a slight disturbance or movement such as picking up the cup, or pouring in a spoon full of instant coffee, may result in a violent eruption with the boiling water exploding out of the cup.

What Can Consumers Do to Avoid Super-Heated Water?

Users should follow the precautions and recommendations found in the microwave oven instruction manuals, specifically the heating time. Users should not use excessive amounts of time when heating water or liquids in the microwave oven. Determine the best time setting to heat the water just to the desired temperature and use that time setting regularly.

Other Tips for Microwave Oven Use

• Some ovens should not be operated when empty. Refer to the instruction manual for your oven.
• Clean the oven cavity, the outer edge of the cavity, and the door with water and a mild detergent. A special microwave oven cleaner is not necessary. Do not use scouring pads, steel wool, or other abrasives.

For more consumer information on microwave oven radiation, write to Food and Drug Administration, Center for Devices and Radiological Health, HFZ-220, 1350 Piccard Drive, Rockville, MD 20850, or to your state health department or your local FDA office.

Updated March 8, 2000