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Health Tips
Microwave Oven
About Microwaves
United States Food and Drug Administration - Center for Devices and Radioactive Health
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?
United States Food and Drug Administration - Center for Devices and Radioactive Health
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
United States Food and Drug Administration - Center for Devices and Radioactive Health
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.
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 Ovens and Health
United States Food and Drug Administration - Center for Devices and Radioactive 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.
Are Microwave Ovens Safe?
Government of British Colombia - Ministry of Health Services
Microwave ovens used in homes generate large amounts of microwave energy (500 to 1000 watts) at a frequency of 2,450 MHz. There are some ovens that operates at 915 MHz but these are rare and mostly in industrial facilities. This amount of microwave energy is so large that it will boil water, cook hamburger and set some materials on fire. So it is obvious that being inside the oven is not safe, but what about the outside?
The maximum microwave radiation leakage permitted by the Radiation Emitting Devices regulations for microwave ovens operated with a load, is 1.0 milliwatts per square centimeter measured at 5 cm from any external surface of the oven. This level of microwave power density is about 1,000 times less than what is present on the other side of the door and is not harmful. Microwaves can not penetrate metal and are contained inside the oven because they are always enclosed by metal. The oven is a metal container. The walls inside of the oven are usually plastic but behind the plastic there is metal. If you look carefully at the plastic viewing window you will see that it has a metal mesh or screen. This screen is enough material to keep most of the microwaves inside the oven. To stop leakage of microwaves between the door and the door frame, the frame or the door is fitted with quarter wavelength choke. Unless your microwave is physically altered or damaged it will not leak more microwave energy than it did when it was new.
Microwave Oven Safety Standard
United States Food and Drug Administration - Center for Devices and Radioactive Health
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.
Erupted Hot Water Phenomena in Microwave Ovens
United States Food and Drug Administration - Center for Devices and Radioactive Health
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?
United States Food and Drug Administration - Center for Devices and Radioactive Health
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.
Ovens and Pacemakers
United States Food and Drug Administration - Center for Devices and Radioactive Health
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.
What Materials are Safe to Use in a Microwave Oven?
Government of British Colombia - Ministry of Health Services
All containers and materials that are identified by the manufacturer as safe to use in a microwave oven are suitable to use in the oven. This identifying information may be on the packaging or on the material itself. If a container does not specifically state that it is microwave safe it still may be useable in the oven. To test the material, place it in the oven along with a cup of cool water and operate the oven on high for 1 minute. If the container is lukewarm or cool it is safe to use. If the container is warm or hot it is not safe to use.
In general suitable materials tend to be made of plastic, glass, china cookware, plastic dinnerware, etc. Do not use dishes with metallic rims or cookware with metal parts and avoid using twist-ties. Although aluminum foil is a metal it may sometimes be used safely but only when used as directed by the microwave cookbook or microwave oven instruction book. If recycled plastic containers do not state that they are microwave safe do not use them. Some plastics when heated release plastic components which may migrate into the food. Remember that plastic containers that are not heated by microwave energy can still be heated from being in contact with the food that is being heated.
Safety Tips for Operation of Microwave Ovens
Canadian Centre for Occupational Health and Safety
- Do not operate oven when empty.
- Check to see that door seal and inside surfaces of door and oven cavity are clean after each use.
- Keep out of the reach of children. Do not permit young children to operate the oven.
- Do not put face close to door window when oven is operating.
More Tips on Safe Microwave Oven Operation
United States Food and Drug Administration - Center for Devices and Radioactive Health
- 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.
Precautions For Radiation Safety in the use of Microwave Ovens
Canadian Centre for Occupational Health and Safety
- Follow the oven manufacturer's instructions on recommended operating procedures and safety precautions.
- Never tamper with or inactivate the interlocking devices.
- Never use the oven without the trays provided by the manufacturer unless specifically allowed in the manufacturer's instructions.
- Never operate the oven without a load (ie. an absorbing material such as food or water) in the oven cavity unless specifically allowed in the manufacturer's instructions.
- Never rest heavy objects such as food containers on the door while it is open.
- Clean the oven cavity, the door and seals with water and a mild detergent at regular intervals (do not use abrasive cleaning pads).
- Supervise children using microwave ovens
Other Precautions for the Safe Use of Microwave Ovens
Canadian Centre for Occupational Health and Safety
- It is sometimes possible to super-heat some liquids or foods beyond their natural boiling point. Such super heated liquids may boil suddenly and violently when jolted or stirred after removal from the oven - it is therefore a good idea to take precautions such as covering the food or liquid or allowing it cool before removal from the oven.
- When thawing frozen foods in the microwave oven it is important to thaw the food thoroughly before cooking.
- Do not use microwave ovens for sterilising baby bottles or other food utensils.
- Ensure that all food prepared in a microwave oven is stirred and/or left to stand for a few minutes before consuming. In particular, care must be exercised when heating a baby's bottle. Even though the glass bottle may be cool to the touch, the milk could be very hot. Cases have been reported where babies have received severe burns from drinking liquids taken directly from a microwave oven. To avoid problems do not heat baby's liquids in a microwave oven. If there are no alternative means, thoroughly shake the bottle to mix the contents and test it's temperature against the skin after heating.
- Use only cooking containers designated as suitable for microwave cooking.
Inspection By The User
Canadian Centre for Occupational Health and Safety
A microwave oven should only be used if an inspection confirms all of the following points.
- The surface of the door is not damaged.
- The door fits squarely and securely and opens and closes smoothly.
- The door hinges are in good condition.
- The oven is clean and in particular the door edges and interior surrounds are not covered with food or burnt material.
- No corrosion is evident on the door, the door hinges or the oven interior.
Safety Tips for Installation and Maintenance of Microwave Ovens
Canadian Centre for Occupational Health and Safety
- Take special care to ensure that no damage occurs to the part of the oven making contact with the door or door seals.
- Ensure that the microwave is unplugged or disconnected from electrical power before reaching into any accessible openings or attempting any repairs.
- Ensure that the adjustment of applied voltages, replacement of the microwave power generating component, dismantling of the oven components, and refitting of waveguides are undertaken only by persons who have been specially trained for such tasks. The services of a qualified repairman should be sought when any malfunction is suspected.
- Do not by pass the door interlocks.
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