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Microwave Oven Radiation

2010-04-07 15:29:17

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.

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 are reflected within the metal interior of the oven where they

are absorbed by food. Microwaves cause 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 it is absorbed by food, and does 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 are cooked, the outer layers are

heated and cooked primarily by microwaves while the inside is cooked mainly by

the 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 whole

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 microwaves pass through these materials. 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

because 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

The Food and Drug Administration (FDA) has the responsibility for carrying out

an electronic product radiation control program mandated by the Electronic

Product Radiation Control provisions of the Food Drug and Cosmetic Act. Through

its Center for Devices and Radiological Health, FDA sets and enforces standards

of performance for electronic products to assure that radiation emissions do

not pose a hazard to public health.

A Federal standard 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. This limit is far

below the level known to harm people. Microwave energy also 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 the value

measured 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 must have a label stating that they meet the safety standard. In

addition, FDA requires that all ovens 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 its own

laboratory. FDA also evaluates manufacturers' radiation testing and quality

control programs at their factories.

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 the

5mW limit for microwave oven leakage.

Less is known about what happens to people exposed to low levels of microwaves.

Controlled, long-term studies involving large numbers of people have not been

conducted to assess the impact of low level microwave energy on humans. 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 us better understand the possible effects of radiation.

The fact that many scientific questions about exposure to low-levels of

microwaves are not yet answered require FDA to continue to enforcement of

radiation protection requirements. Consumers should take certain common sense

precautions.

Have Radiation Injuries Resulted from Microwave Ovens?

There have been allegations of radiation injury from microwave ovens, but none

as a direct result of microwave exposure. The injuries known to FDA have been

injuries that could have happened with any oven or cooking surface. For

example, many 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. Similar concerns were raised about

pacemaker interference from electric shavers, auto ignition systems, and other

electronic products. FDA does not specifically require microwave ovens to carry

warnings for people with pacemakers. The problem has been largely resolved

because pacemakers are now designed to be shielded against such electrical

interference. However, patients with pacemakers may wish to consult their

physicians if they have concerns.

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. 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.

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 and

calibrated.

Tips on Safe Microwave Oven Operation

procedures and safety precautions for your oven model.

or otherwise damaged.

operate with the door open.

don't allow children to do this) for long periods of time while it is

operating.

amounts of time.

Erupted Hot Water Phenomena in Microwave Ovens

The FDA received reports in the past 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 (water heated past its boiling

temperature), which does not appear to be boiling.

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 to the desired temperature

and use that time setting regularly.

Other Tips for Microwave Oven Use

for your oven.

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, you can contact the

Consumer Staff of the Division of Small Manufacturers, International and

Consumer Assistance (DSMICA) by electronic mail at DSMICA@cdrh.fda.gov.

Additional Information from FDA's Consumer Health Information Staff