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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