Eidgenössisches Departement des Innern EDI
Bundesamt für Gesundheit BAG
Microwave ovens
Date:
12 October 2016
Microwave ovens work with powerful high-frequency radiation
(microwave radiation). This radiation is absorbed by food and
transformed into heat. Food heated by microwave ovens does not
actually contain any microwaves and therefore does not give off
radiation. The metal case of a microwave oven and a metal-wire
screen in its door ensure that most of the radiation stays inside the
appliance. A small proportion leaks out. High radiation intensities
can develop close to the surface of the housing (0-5 cm), but these fall to more than ten times below
the permitted level at a distance of just 30 cm. However, leakage radiation may be greater if the door
seals are defective or dirty.
As far as is currently known, leakage radiation from an intact microwave oven that is used correctly
poses no threat to health. Overheated food or incompletely inactivated germs in the food may constitute a danger. As with conventional cooking methods, burns also represent the major risk associated
with microwave ovens. Hot water or hot foodss are very dangerous if it are spilled or spurts onto the
body.
Food can be heated safely in a microwave oven as long as the appliance is used correctly. The following tips will help you to get the best results:
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Familiarise yourself with the instructions for use and the safety precautions contained in them. If
unsure, ask for clarification from the place of purchase or from the manufacturer.
Use microwave ovens solely for heating foodstuffs.
Keep the microwave oven out of the reach of children or use the child-proof lock built into the appliance so as to avoid the machine being inadvertantly switched on or opened by a child.
Keep the door frame and seal and the inside of the oven clean and check that the door catch, the
seal and the housing of the appliance are intact. Under no circumstances should you use an appliance with visible damage such as a defective or deformed door. Have the repairs carried out by
the manufacturer of the appliance.
Prevent liquids from overheating and ‘exploding' by placing a glass rod or a heat-resistant plastic
spoon in the cooking vessel.
In general, cook eggs on the hob, as eggs heated in the microwave oven can explode.
Food with a skin or shell (e.g. fruit, potatoes, tomatoes) must be pierced before cooking or cooked
using a different method.
Both the inside and the outside of food should be heated for 10 minutes to at least 70 °C in the microwave oven to make sure that it is cooked through and that micro-organisms are killed. Stir the
food from time to time so that it is evenly heated.
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In order to avoid burns use conventional methods to heat baby food. Take particular care if you do
use a microwave oven to heat baby food. Warm the bottle without the teat in place. The above tip
prevents explosive evaporation of liquids. Shake the bottle before you give it to the baby and
check the temperature of the baby food to make sure that no part of it is too hot. Do not hold the
baby in your arms when operating the microwave oven.
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1 Technical Data
Typical frequencies:
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0 Hz from the permanent magnets used to power the magnetron (static magnetic field)
50 Hz from the power supply (low-frequency magnetic field)
2,45 GHz from the microwave radiation (high-frequency electromagnetic field)
Output
500 – 2000 Watt (Haushaltsgeräte)
1.1 Principle
Materials which are weak conductors of electricity, e.g. water and many other components of food, absorb microwave energy. Absorption takes place when energy is transferred to electrically charged particles (ions) and dipoles. Dipoles are particles which have one positively and one negatively charged
end, which means that their electrical charge is unevenly distributed. The dipoles most commonly
found in food are water molecules.
All the dipoles and ions in a food product are in constant motion. If a food is additionally exposed to an
electrical field, the dipoles attempt to align themselves with this field according to their charge.
The electrical field (radiation) of a microwave oven changes direction extremely rapidly (five billion
times per second). The ions and dipoles attempt to align themselves with this alternating field. This intensifies their movements even further, leading to a rise in the temperature of the food.
This type of heating is the absolute opposite of the conventional way of heating food, in which heat
energy is applied to the surface of the food and transferred to the inside of the product by conduction.
1.2 How it works
In a microwave oven, the microwave radiation is produced by a high-frequency generator known as a
magnetron. A transformer generates the high voltage needed to power the magnetron. The magnetron
is switched on and off 50 times a second, producing pulses of microwave radiation. A pulse typically
lasts 10 milliseconds. The radiation is introduced into the cooking area of the oven by the magnetron.
The metal walls of the case and metal-wire screen in the door ensure that the radiation is reflected
and stays inside the oven. Opening the door switches off the radiation source immediately, and the
oven becomes radiation-free within 10 μs (microseconds).
Some of the microwaves reflected from the walls are superimposed on each other, producing a
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greater or lower intensity of radiation in certain places. This phenomenon, known as interference effects, means that food will cook faster in some parts of the oven than in others. A stirrer can distribute
the radiation more equally or a turntable in the oven can ensure that food is heated more evenly in
spite of the irregular distribution of the radiation (figure 1)
1 Gehäuse
6 Reflektorflügel
2 Transformator7 Reflektorblech
3 Kühlgebläse 8 Garraum
4 Magnetron
9 Drehteller
5 Hohlleiter
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1.3 Operation
The transformer generates a high voltage for the magnetron, which produces electromagnetic waves.
These microwaves are focussed by a metal tube, and fed into the cooking chamber by the waveguide.
The waves are distributed through the cooking chamber by a rotating reflector and reach the food on
the turntable directly or by reflection from all sides.
1.4 Suitable and unsuitable cooking vessels
Electrical isolators such as glass and porcelain are almost completely permeable to microwaves and
therefore make good materials for microwave cooking vessels. Porcelain decorated with gold should
not be used as the decoration will be destroyed.
Microwaves are reflected almost completely by electrically conductive materials such as metals, so
food in metal containers will not be heated. Another reason for not using metal cooking vessels is that
they disrupt the optimum distribution of the microwaves in the oven's cooking space. Sparking may
occur if metal parts are placed too close to the wall of the oven cavity.
2 Meauring exposure
2.1 High-frequency radiation
Leakage radiation from microwave ovens
The high-frequency radiation that escapes from a microwave oven while it is in use is called leakage
radiation. It must not exceed 5 mW/cm2 at a distance of 5 cm (this is equivalent to an electrical field
strength of 137 V/m). Three different studies have investigated the leakage radiation from new and
used microwave ovens in households and restaurants (Table 1).
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Table 1. Mean leakage radiation from microwave ovens measured at a distance5 cm from the
appliance
Average leakage radi- Number of appliances meaation (mW/cm2)
sured
Study
Age of appliances (years)
Study [2]:
0,41
106
0,1 – 14
new appliances
0,08
60
0
used appliances
0,17
103
1 - 23
< 0.062 for 50% of the
appliances, max. 0.086
130
0,5 – 18
used appliances
Study
[3]
Study [4]:
used appliances
The three studies conclude that leakage radiation is most likely to occur if the door seals are worn or
dirty, or if the door or catch is worn. The maximum permitted level of leakage radiation was exceeded
in only one case.
Specific absorption rate (SAR)
The best way to describe exposure of humans to high-frequency radiation is in terms of the SAR (specific absorption rate). The SAR (in W/kg) shows how much energy (W) is absorbed by the human body
(kg).
A study was carried out to determine the SAR caused by leakage radiation from a microwave oven[5].
The microwave oven was prepared in such a way that it emitted the maximum permitted leakage radiation. At distances of 30 cm (equivalent to whole-body exposure) and 5 cm (equivalent, for example,
to exposure of the head while looking at the cooking food) the measured SAR values were well below
the recommended thresholds (Table 2); the recommended threshold is exceeded only when the body
is in direct contact with the microwave oven [6, 7].
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Table 2: SAR values of a microwave oven with maximum permitted leakage radiation (5
mW/cm2 at a distance of 5 cm) compared with the recommended threshold for various types of
exposure
Distance (cm)
Levels measured with microwave oven
SAR (W/kg)
Recommended limit
SAR
(W/kg)
exposed parts of body
< 0.1
7.95
4
limbs
5
0.256
2
head, trunk
30
0.0056
0.08
whole body
2.2 Low-frequency magnetic fields
Low-frequency magnetic fields are created by the transformer, the motor that rotates the turntable and
the powerful transformer in the magnetron. The low-frequency magnetic fields of 34 microwave ovens
were determined and the mean was calculated (Table 3) [8]. The measured values were all below the
recommended threshold of 100 µT (microtesla). The magnetic fields decreased greatly with increasing
distance from the appliance.
Table 3: 50 Hz magnetic fields of 34 measured microwave ovens, mean ± standard deviation
Distance (cm) Magnetic field (µT)
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5
27,3 ± 16,7
50
1,66 ± 0,63
100
0,37 ± 0,14
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3 Effects on health
3.1 The effects of electromagnetic fields
High-frequency radiation
The possible effects of the radiation emitted by microwave ovens on the eyes have been investigated.
The lenses in the eyes do not have an extensive blood supply, making it difficult for heat generated in
them to be dissipated. This means that the lenses in the eyes are sensitive to high temperatures,
which can lead to permanent damage (cataracts). When a person looks at cooking food through the
window in a microwave oven, the eyes are brought very close to the appliance and to within the range
where leakage radiation is present. However, studies have shown that the radiation exposure is too
low when the door is closed to produce any significant increase in the temperature in the eyes [9].
Overall, there is no evidence that microwave ovens have any impact on health by increasing the body
temperature.
Low-frequency magnetic fields
Microwaves are among the household appliances that tend to produce powerful low-frequency magnetic fields. However, the burden of the magnetic fields caused by such appliances is relatively low
since they are only ever operated for a short time. The studies carried out to date have not produced
reliable evidence that microwave ovens have any effect on health [10-13].
3.2 Hazards associated with incorrect use of appliances
Heating foods and objects
Microwaves typically heat food rapidly, leading to excess pressure inside items of food that are not too
large (not thicker than 3-4 cm). Foods such as eggs, tomatoes, potatoes and sausages, which are surrounded by an external shell or skin, can burst or explode in a microwave oven or after they have
been removed from it. Eggs For this reason, foods enclosed in a skin or shell must be pierced or
peeled before they are heated; it is better to boil eggs conventionally in water. heated in a microwave
oven are particularly dangerous, as they can cause mild to severe burns [14] and injure the eyes.
Airtight containers which are tightly closed, e.g. bottles, glass jars of baby food, vacuum-packed food,
must be opened before they are heated in a microwave oven. Teats must be removed from babies’
bottles since exploding bottles can cause extremely serious burns [15].
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Heating liquids and food
There is a danger that liquids may be heated beyond their boiling point in a microwave oven without
evaporating or forming bubbles. In superheated liquids (beverages, inhaled medications etc.) even the
slightest movement can cause large bubbles which explode from the container, taking liquid with them
and causing burns. This danger can be avoided by standing a glass rod or plastic spoon in the liquid.
A teaspoon can also be used, provided that it is made of a single material, e.g. stainless steel. Silverplated spoons should not be used.
Microwaves tend to heat the interior of liquids, foods and gel-like substances (e.g. hot/cold gel packs)
faster than the surface, so the container or food does not necessarily feel hot when it is taken out of
the microwave oven. There is a danger that babies, in particular, may burn their throats. For this
reason, liquids, food and other objects should always be stirred and checked to see that they are not
too hot.
Microwave ovens should be placed out of reach of children in order to avoid accidents. Even children
below 2 years old are able to open microwave ovens and remove food [16]. In addition, children represent the age group that is in most danger from injuries from microwave ovens [17].
Hygienic food preparation
Food may heat up unevenly in a microwave oven.
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Because of the way microwave ovens are designed, parts of the cooking area may be very hot
why some are cooler.
The different components of food (water, fat, protein, carbohydrates) heat up at different rates,
leading to the formation of hot spots and cold spots.
Very salty foods tend to overheat on the surface.
If food cooks unevenly, the micro-organisms contained in it may not be destroyed completely. The following tips will help to ensure that food prepared in a microwave oven is safe to eat:
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Extended heating times and standing time after heating allow heat to spread and help to equilibrate the temperature between hot and cold spots. This process can be accelerated by stirring.
Food must be heated both inside and on the outside to at least 70°C for 10 minutes.
Large quantities and thicker items (thicker than 3-4 cm) may not be cooked through properly; in
such cases a different cooking method should be used.
Changes in the chemical composition of food
Whenever food is heated, it undergoes chemical changes, new chemical substances are formed and
existing substances are partly destroyed. These changes are dependent on the cooking time and temperature. The changes that take place in food heated in a microwave oven are roughly the same as
those that occur during conventional cooking. The nutritional value, digestibility of proteins, composition of amino acids and stability of fat-soluble vitamins (A and E) are similar with both methods. Micro-
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wave cooking has less negative impact on the stability of water-soluble vitamins (C and B) than conventional cooking methods. Less oxidation of fats occurs, so no highly reactive toxic substances (radicals) are formed. When food is heated in a microwave oven, fewer undesirable substances such as
polycyclic aromatic hydrocarbons and heterocyclic aromatic amines are generally formed than when
food is fried in a pan. However, microwave cooking does not brown food.
Extensive toxicological studies have investigated food heated in a microwave oven for the presence of
novel toxic substances [16]. It was shown that there are no differences between food heated in a microwave and conventionally cooked food. Microbiological and biochemical studies have not demonstrated any toxic effect of food cooked in a microwave oven on genetic material.
4 Regulation in law
Microwave ovens are low-voltage appliances which are governed in Switzerland by the Regulation
concerning electrical low-voltage appliances [19]. This regulation requires low-voltage appliances not
to endanger either persons or objects when used correctly, where possible when used in a foreseeable incorrect manner, and when foreseeable faults occur. It also states that low-voltage appliances
may only be marketed if they comply with the essential health and safety requirements of the European Low Voltage Directive.
Manufacturers of low-voltage appliances must obtain a Declaration of Conformity for a product before
it can be brought onto the market; this declaration states that the product complies with the essential
requirements. The essential requirements for individual products are specified in technical standards;
the requirements that the electromagnetic fields created by microwave ovens have to meet are specified in EN 62233 [20] for the low-frequency range and EN 60335-2-25 [21] for the high-frequency
range. The corresponding conformity criteria are identical to those stated in the threshold recommendations of the EU [6].
Manufacturers are responsible for ensuring that their appliances comply with the conformity criteria;
there is no comprehensive oversight of the market in Switzerland. The Swiss Inspectorate for High
Current Installations (www.esti.admin.ch) checks compliance with the regulations by inspecting random samples of products on the market
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5 References
1. International Life Science Institute (ILSI): Microwave Ovens. ILSI Europe Concise Monograph Series, 2006.
2. Alhekail ZOI. Electromagnetic radiation from microwave ovens. J.Radiol.Prot. 2001;21:251-8.
3. Thansandote A, Lecuyer D, Gaijda B. Radiation leakage of before-sale and used microwave ovens. Microwave World 2000;21:4-8.
4. Matthes R. Radiation emission from microwave ovens. J.Radiol.Prot. 1992;12:167-72.
5. Bangay B, Zombolas C. Advanced Measurements of Microwave Oven Leakage. The Journal of
the Australian Radiation Protection Society 2003;20:47-51.
6. Council Recommendation 1999/519/EC of 12 July 1999 on the limitation of exposure of the general public to electromagnetic fields (0 Hz - 300 GHz). http://ec.europa.eu/health/electromagnetic_fields/docs/emf_rec519_en.pdf
7. Preece AW et al. Magnetic fields from domestic appliances in the UK. Phys.Med.Biol. 1997;42:6776.
8. Leitgeb N, Tropper K. Eye heating caused by microwave ovens. Biomed.Tech (Berl) 1993;38:1720.
9. Behrens T et al. Quantification of lifetime accumulated ELF-EMF exposure from household appliances in the context of a retrospective epidemiological case-control study. J Expo.Anal.Environ
Epidemiol. 2004;14:144-53.
10. Preston-Martin S et al. Los Angeles study of residential magnetic fields and childhood brain tumors. Am.J Epidemiol. 1996;143:105-19.
11. Kleinerman RA et al. Self-reported Electrical Appliance Use and Risk of Adult Brain Tumors. Am.J
Epidemiol. 2005;161:136-46.
12. Hatch EE et al. Association between childhood acute lymphoblastic leukemia and use of electrical
appliances during pregnancy and childhood. Epidemiology 1998;9:234-45.
13. Gurney JG et al. Childhood brain tumor occurrence in relation to residential power line configurations, electric heating sources, and electric appliance use. Am.J.Epidemiol. 1996;143:120-8.
14. Wolf Y, Adler N, Hauben DJ. Exploding microwaved eggs--revisited. Burns 2001;27:853-5.
15. Dixon JJ, Burd DA, Roberts DG. Severe burns resulting from an exploding teat on a bottle of infant
formula milk heated in a microwave oven. Burns 1997;23:268-9.
16. Robinson MR, O'Connor A, Wallace L, Connell K, Tucker K, Strickland J. et al. Behaviors of young
children around microwave ovens. J Trauma 2011, 71,5 Suppl 2; S534-S536.
17. Thambiraj DF, Chounthirath T, Smith GA. Microwave oven-related injuries treated in hospital EDs
in the United States, 1990 to 2010. In: Am J Emerg.Med 2013;31,6,958-963.
18. Jonker D, Til HP. Human diets cooked by microwave or conventionally: comparative sub-chronic
(13-wk) toxicity study in rats. Food Chem.Toxicol. 1995;33:245-56.
19. Ordinanza del 9 aprile 1997 sui prodotti elettrici a bassa tensione (OPBT; RS 734.26).
20. EN SN 62233 Electromagnetic fields around household and similar electrical appliances - Methods for evaluation and measurement
21. EN SN 60355-2-25 Household and similar electrical appliances - Safety - Part 2.25: Particular requirements for microwave ovens, including combination microwave ovens
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Specialist staff
Federal Office of Public Health FOPH
emf@bag.admin.ch
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