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Microwave Radiation

Microwave radiation is electromagnetic radiation in the frequency range of from 30MHz to 300GHz. The frequency range is further subdivided into the microwave bands of VHF (30 – 300MHz), UHF (0.3 – 3GHz), SHF (3 - 30 GHz) and EHF (30 – 300 GHz). Television, microwave ovens and meteorological radar equipment operate in the UHF; satellite communication and navigational radar operate in the SHF bands and radio astronomy in the EHF band.

Microwave generators include radar, radio transmission equipment, jammers, RF induction and dielectric heating equipment and microwave ovens.

Before any type of microwave generator is installed a risk assessment must be undertaken. This does not apply to domestic microwave ovens.

A maximum exposure limit is set at 10mW.cm-2 for all microwave sources (including ovens). The present state of research suggests that no injuries of any kind should occur below 1mW.cm-2.

Microwave ovens must be checked for leakage if damage is suspected (contact the Safety Office). There are no specific UK regulations governing microwave ovens used in food preparation, but it is recommended that the current USA regulations are adopted. These state that all ovens must have two interlocks switches, and that the maximum permitted leakage radiation shall not exceed 1mW.cm-2 for new ovens or 5mW.cm-2 for existing ovens.

Harm (biological effects) can be caused by overexposure to microwave radiation usually a consequence of heat generated in tissues by the radiation. The most heat sensitive body organs are the lens of the eye, the gall bladder and the testicles. In the case of the eye, visible signs of damage are apparent in the form of white flecks due to coagulation of lens protein. Temporary sterility due to damage to the spermatic duct can also occur and if radiation density is high and/or prolonged the sterility can become permanent.

Radiofrequency radiated energy is more harmful in pulse form than in uniform mode. Normal clothing offers little protection to microwaves. The eyes can only be protected by glass or plastics covered with a thin film of metal, e.g. gold.

Reflections are a major problem for microwaves, but these can be reduced by wire netting or perforated metal sheet. Walls, partitions etc. can be lined with absorbent material such as graphite-impregnated mats.

Ovens can be damaged in several ways. The use of metal objects inside the oven may result in high-voltage arcing, damaging the enclosure or causing a fire. Interlock connectors or switches may fail in an unsafe condition through abuse of the door, allowing microwave leakage. Finally the oven may be dropped, damaging the door seal, enclosure, switches or power supply