Some Frequently Asked Questions
The Australian Radiation Protection and Nuclear Safety Agency maintains a close watch over potential radiation hazards arising from technical and consumer products and evaluates any possible public health risk arising from their use. Ovens which utilize microwave radiation for cooking of foodstuffs either in the home or in commercial or other premises have a potential to create radiation hazards if they are incorrectly used or are not maintained in good working order. Some features of microwave ovens and precautions in their use are described below.
Microwaves, like visible light are a part of the electromagnetic radiation spectrum and are extremely high frequency radio waves. Microwaves travel in straight lines and may be either reflected, transmitted or absorbed by matter in their path. Metallic materials totally reflect microwaves. Non-metallic materials such as glass and plastics are partially transparent to microwaves. Materials containing moisture such as foods absorb microwave energy and produce heat.
Some of the more common uses of microwaves include satellite communications, radar, carphones, air and sea navigational aids. Other applications include use in industry for thawing and drying materials and in medicine for diathermy treatment. The use of microwave ovens in industrial, commercial, domestic and other premises has increased substantially over recent years.
In the microwave oven an electronic tube called a magnetron is used to produce the microwaves. The microwaves then pass through a wave guide into the metal oven cavity where they are reflected around the oven walls. Uneven reflections may cause localized hot and cold "spots" in food. This is minimized by the use of a mode stirring fan and rotating carousel. The microwaves penetrate into the food and cause water molecules within the food to vibrate at the frequency of the microwaves (2450 MHz). The vibration causes considerable molecular friction which produces heat and results in a rapid rise in temperature. The cooking time is therefore much shorter than in a conventional oven. The rate of heating depends on the moisture content, shape, volume and mass of food present. This can produce uneven heating with some foods where the outside may be only warm while the inside may be close to boiling (jam filled donuts are an example). The oven walls and most cooking utensils are not directly heated by microwaves because they do not absorb microwave energy. However, they frequently get warm from being in direct contact with the hot food.
Microwaves generated in microwave ovens, like visible light from light globes, cease to exist once the electrical power to the magnetron is turned off. They do not remain in the food when the power is turned off and they cannot make the food or the oven radioactive. Therefore, food cooked in a microwave oven presents no radiation hazard.
All microwave ovens have at least two safety interlock switches which stop the generation of microwaves immediately the door is opened. The design of microwave ovens is such that the microwaves are contained within the oven, but it is possible for some leakage to occur around the door. However, the design of oven door seals limits this leakage to a level well below that recommended by the National Health and Medical Research Council.¹
In Australia, all State Electricity Authorities have gazetted domestic microwave ovens as prescribed items. That is, before any domestic microwave oven is sold, a sample of each model has to be tested and approved to the particular requirements for microwave ovens as per the Australian Standard². Such testing and approval requires the oven to be surveyed for microwave leakage.
There are a number of microwave oven leakage detectors, designed for household use, marketed within Australia of which unfortunately few work satisfactorily. Only detectors showing that they comply with the Australian Standard³ for microwave oven leakage detectors for household use should be used. Unless these detectors are used strictly in accordance with instructions, they may indicate a false result. Surveys by testing authorities have shown that microwave oven leakage levels in excess of the recommended limits are very rare and an oven in good condition and used correctly is safe. However, when an oven appears to have deteriorated or is damaged, then a qualified serviceman should inspect the oven before it is used and check the leakage to ensure that it does not exceed the recommended level.
Modern pacemakers are not susceptible to microwave radiation interference from microwave ovens having leakage within the limits of the Australian Standard. Therefore, there is no reason for concern. People who are unsure or experience any discomfort in the vicinity of a microwave oven should contact the doctor who implanted the pacemaker.
Plastic containers considered suitable for holding foods at room temperature may not necessarily be suitable for use in a microwave oven. The high cooking temperatures may cause the plastic’s chemistry to break down and thereby contaminate food in the container. Since it is difficult to determine the composition of plastic from its appearance, it is recommended that plastic containers or wraps not be used in a microwave oven unless clearly designated for such use by the manufacturer Research shows that ceramics, glass-ceramics, some plastics and papers are satisfactory. Dishes with metallic glazes should not be used. If fast food foil containers and aluminium foil are used, the oven manufacturer’s directions should be carefully followed. Do not let fast food foil containers or aluminium foil touch the sides of the oven as this may cause sparking.
A microwave oven should only be used if an inspection confirms all of the following points.
The National Health and Medical Research Council recognises that some microwaves may escape from ovens and has laid down a standard of safety for leakage radiation. It believes that compliance with this standard will ensure the public health is not affected by the use of microwave ovens. This standard of safety states that the power flux density of microwave radiation from any microwave oven shall not exceed 5 milliwatts per square centimetre at any point 5 centimetres or more from the external surface of the oven.