Unknown hazards and mysterious phenomena in microwave heating are intimidating to most people and, in contrast to conventional heating, there is no visible heat source. Inside the oven the food is heated up to cooking temperature so the obvious query is whether radiation leaking from the device can heat anyone standing close to it. For this reason, the permissible energy density at the surface of any microwave facility is limited to 10mW/cm2 for an unlimited length of time of exposure in the USA and European countries. In some eastern European countries and the former Soviet Union the upper limit for long-term exposure to microwaves has even been set to 10 mW/cm2. This very low limit was justified because of supposed non-thermal effects attributed to interaction of microwaves with living organisms. The alleged health problems included nervousness, hormonal imbalance, malformations and anomalous brain activity. The experimental evidence, however, for such phenomena is unsubstantiated (Foster, 1992; Hileman, 1993).
The practical and achievable limit of 10mW/cm2 is justified because the microwaves that are used are identical with those used in therapy. (For therapeutic effects the energy density must be well above a level of 100mW/cm2.) There are numerous studies to determine damage thresholds and it has been observed that no permanent effects occur at levels below 100mW/cm2. For a critical organ, the eye, it was observed that cataract formation may occur at 150mW/cm2 when the microwaves are applied for more than 90 minutes. Within certain limits the body can absorb energy including microwaves and compensate for the temperature increase easily by removing excessive heat by means of blood flow. There are certain avascular structures in the body that may have a relatively poor heat exchange; this is possibly true for testicles and temporary sterility has been reported after microwave exposure. The energy flow from the sun may be considered for comparison: on a sunny day in summer the infrared portion of the spectrum may carry as much as 100mW/cm2, which is not responsible for sunburn. Hence, introducing a safety factor of 10 is considered sufficient and the limiting energy density of 10mW/cm2 is widely accepted. However, in recognising concerns about cell phones, broadcast antennas and satellites a lowering of this limit by another factor of 10 is being considered.
To enhance radiological safety of microwave appliances several features are implemented. Safety latches cut off the power as soon as the door is opened and the user is not exposed to spurious radiation. Further chokes and absorbing strips are attached to the doors as seals to eliminate any leakage of microwave radiation to the outside. Industrial facilities are equipped with energy trapping devices at the conveyor and system openings for product entry and exit. Such facilities for continuous treatment have to be operated 'open door', whereas the household oven is used batchwise and the door must always be closed. The majority of household appliances and industrial facilities that are built and installed today are well shielded. At the time of purchase, for a domestic appliance the energy leakage is usually considerably less than 1 mW/cm2 at 5 cm from the door as presently regulated in most countries. Microwave appliances have been regularly used in households for several years and no major accident has been reported. Survey studies have confirmed the radiological safety of the appliances that are on the market. However, there have been certain consumer demands regarding larger door openings and bigger transparent windows at the front that have led the industry to be more rigorous regarding the regulated leakage limits of 1 mW/cm2 at 5 cm. The statistics from such surveys have shown that an increasing number of appliances is manufactured as close as possible to the limits but this should not cause concern because the energy limits already include a reasonable safety factor.
The presence of metallic conductors in the human body, especially if they are heart pacemakers, can lead to complex effects and locally high electrical currents as does a piece of metal left in food while being heated in a microwave oven. This may lead to unforeseen interactions and so a particular note of caution must be given to people with pacemakers. Experiments with volunteers have shown that the most critical frequency is around 9 GHz which is used in radar facilities; at those frequencies commonly used for heating purposes, 2450 MHz, only small changes of the heart rhythm have been observed (the maximum energy density in such experiments was limited to 25 mW/cm2). However, such warnings are irrelevant in the case of household appliances as the regulated leakage limit is usually 1 mW/cm2 at 5 cm distance. Some precautions may be advisable where microwave facilities are installed on a food production line in industry. However, radiological safety at open-door facilities has also been generally established.
Prejudice and lack of technical understanding are causes of unfounded allegations that food heated by microwave might become toxic and exotic chemical compounds could be formed. However, microwave radiation cannot break chemical bonds and cannot cause ionisation nor create free radicals. Here there is an essential difference from ionising radiation which has a quantum or photon energy that is larger by several orders of magnitude. For this reason the possibility of induced chemical reactions other than thermal ones is nonexistent. This has been confirmed by many studies and it has been possible to attribute any par ticular chemical effect to uncontrolled heating patterns in various experiments. Microwave heating is newer than traditional methods such as broiling, roasting, frying, smoking and barbecuing so it is less understood and accepted by the public. However, in practice, microwave ovens are used in a large and increasing number of households with no concerns.
Variations in food shape, size or composition can also result in underdone products, and this is another risk in microwave cooking. Lack of understanding has resulted in illness from surviving pathogen microorganisms or the formation of microbial toxins; however, this is no different from undercooking food by traditional methods. In the same way, local overcooking can lead to the formation of unacceptable chemical changes as it does in traditional heating methods. Because there are no substantiated, special mechanisms of the interaction of microwaves with atoms, molecules, organisms and microorganisms the toxico-logical concerns regarding the consumption of food that is heated by microwave have no foundation in reality.
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