International EMF Project
Information Sheet

February 2005

ELECTROMAGNETIC FIELDS AND PUBLIC HEALTH
Microwave Ovens

WHAT ARE MICROWAVES?
Microwaves are high frequency radio waves (radiofrequency fields) and, like visible radiation(light), are part of the electromagnetic spectrum. Microwaves are used primarily for TV
broadcasting, radar for air and sea navigational aids, and telecommunications including mobilephones. They are also used in industry for processing materials, in medicine for diathermytreatment and in kitchens for cooking food.
Microwaves are reflected, transmitted or absorbed by materials in their path, in a similar manner
to light. Metallic materials totally reflect microwaves while non-metallic materials such as glassand some plastics are mostly transparent to microwaves.
Materials containing water, for example foods, fluids or tissues, readily absorb microwave
energy, which is then converted into heat. This Information Sheet discusses the operation andsafety aspects of microwave ovens used in the home. More details about the nature ofelectromagnetic fields and health effects of radiofrequency and microwave fields are available in
WHO Fact Sheets 182 and 183.

ARE MICROWAVE OVENS SAFE?
When used according to manufacturers' instructions, microwave ovens are safe and convenient
for heating and cooking a variety of foods. However, several precautions need to be taken,
specifically with regards to potential exposure to microwaves, thermal burns and food handling.
Microwave safety: The design of microwave ovens ensures that the microwaves are contained
within the oven and can only be present when the oven is switched on and the door is shut.
Leakage around and through the glass door is limited by design to a level well below thatrecommended by international standards. However, microwave leakage could still occur arounddamaged, dirty or modified microwave ovens. It is therefore important that the oven is
maintained in good condition. Users should check that the door closes properly and that thesafety interlock devices, fitted to the door to prevent microwaves from being generated while it isopen, work correctly. The door seals should be kept clean and there should be no visible signsof damage to the seals or the outer casing of the oven. If any faults are found or parts of theoven are damaged, it should not be used until it has been repaired by an appropriately qualified
service engineer.
Microwave energy can be absorbed by the body and produce heat in exposed tissues. Organs
with a poor blood supply and temperature control, such as the eye, or temperature-sensitive
tissue like the testes, have a higher risk of heat damage. However, thermal damage would only

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occur from long exposures to very high power levels, well in excess of those measured aroundmicrowave ovens.
Thermal safety: Burn injuries can result from handling hot items heated in a microwave oven, inthe same way as items heated using conventional ovens or cooking surfaces. However, heating
food in a microwave oven presents some peculiarities. Boiling water on a conventional stoveallows steam to escape through bubbling action as the water begins to boil. In a microwave
oven there may be no bubbles on the walls of the container and the water will super-heat andmay suddenly boil. This sudden boiling may be triggered by a single bubble in the liquid or by
the introduction of a foreign element such as a spoon. People have been severely burned bysuper-heated water.
Another peculiarity of microwave cooking relates to the thermal response of specific foods.
Certain items with non-porous surfaces (e.g. hotdogs) or composed of materials that heat at
different rates (e.g. yolk and white of eggs) heat unevenly and may explode. This can happen if
eggs or chestnuts are cooked in their shells.
Food safety: Food safety is an important health issue. In a microwave oven, the rate of heating
depends on the power rating of the oven and on the water content, density and amount of foodbeing heated. Microwave energy does not penetrate well in thicker pieces of food, and may
produce uneven cooking. This can lead to a health risk if parts of the food are not heatedsufficiently to kill potentially dangerous micro-organisms. Because of the potential for uneven
distribution of cooking, food heated in a microwave oven should rest for several minutes after
cooking is completed to allow the heat to distribute throughout the food.
Food cooked in a microwave oven is as safe, and has the same nutrient value, as food cooked
in a conventional oven. The main difference between these two methods of cooking is thatmicrowave energy penetrates deeper into the food and reduces the time for heat to be
conducted throughout the food, thus reducing the overall cooking time.
Only certain microwave ovens are designed to sterilize items (for example baby’s milk bottles).
The user should follow the manufacturer's instructions for this type of application.
Misconceptions: To dispel some misconceptions, it is important to realize that food cooked in a
microwave oven does not become "radioactive". Nor does any microwave energy remain in thecavity or the food after the microwave oven is switched off. In this respect, microwaves act justlike light; when the light bulb is turned off, no light remains.
HOW DO MICROWAVE OVENS WORK?
Domestic microwave ovens operate at a frequency of 2450 MHz with a power usually ranging
from 500 to 1100 watts. Microwaves are produced by an electronic tube called a magnetron.
Once the oven is switched on, the microwaves are dispersed in the oven cavity and reflected bya stirrer fan so the microwaves are propagated in all directions. They are reflected by the metalsides of the oven cavity and absorbed by the food. Uniformity of heating in the food is usuallyassisted by having the food on a rotating turntable in the oven. Water molecules vibrate when
they absorb microwave energy, and the friction between the molecules results in heating whichcooks the food.
Unlike conventional ovens, microwaves are absorbed only in the food and not in the surrounding
oven cavity. Only dishes and containers specifically designed for microwave cooking should beused. Certain materials, such as plastics not suitable for microwave oven, may melt or burst intoflames if overheated. Microwaves do not directly heat food containers which are designed for
microwave cooking. These materials usually get warm only from being in contact with the hotfood.

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2Oven manufacturers do not recommend operating an empty oven. In the absence of food, themicrowave energy can reflect back into the magnetron and may damage it.
Microwave oven users should carefully read and comply with the manufacturer’s instructions
because new ovens vary widely in design and performance. While most modern ovens cantolerate some food packaging made of metal, oven manufacturers generally recommend not
placing metal in the oven, particularly not close to the walls, as this could cause electrical arcing
and damage the oven walls. Also, because metal reflects microwaves, food wrapped in metal
foil will not be cooked, while food not in metal wrap may receive more energy than intended,causing uneven cooking.

INTERNATIONAL STANDARDS
Several countries, as well as the International Electrotechnical Commission (IEC), theInternational Committee on Electromagnetic Safety (ICES) of the Institute of Electrical andElectronics Engineers (IEEE) and the European Committee for Electrotechnical Standardization(CENELEC), have set a product emission limit of 50 watts per square metre (W/m2) at any point
5 cm away from the external surfaces of the oven. In practice, emissions from modern domestic
microwave ovens are substantially below this international limit, and have interlocks that prevent
people being exposed to microwaves while the oven is on. Moreover, exposure decreases
rapidly with distance; e.g. a person 50 cm from the oven receives about one one-hundredth ofthe microwave exposure of a person 5 cm away.
These product emission limits are defined for the purpose of compliance testing, not specificallyexposure protection. The International Commission on Non-Ionizing Radiation Protection(ICNIRP) has published guidelines on exposure limits for the whole EMF part of the spectrum.
Exposure guidelines in the microwave range are set at a level that prevents any known adverse
health effect. Exposure limits for workers and for the general public are set well below levels
where any hazardous heating occurs from microwave exposure. The emission limit for
microwave ovens mentioned above is consistent with the exposure limits recommended byICNIRP.

WHAT THE WORLD HEALTH ORGANIZATION IS DOING
The World Health Organization (WHO), through the International EMF Project, has establisheda programme to review research results and conduct risk assessments of exposure toelectromagnetic fields in the range from 0 to 300 GHz. Health risks from EMF exposure arebeing evaluated by WHO in collaboration with ICNIRP.
The EMF Project has a web site with links to WHO Fact Sheets on various aspects of EMF
exposure and health and published in multiple languages (for more information, see
http://www.who.int/docstore/peh-emf/publications/facts_press/fact_english.htm). The site also
provides information on the Project, its publications and its scientific and public information
activities.

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