Pick of the Week #7: Hazards of Microwave Radiations – Review from 1960.

August 23, 2010:  Pick of the Week #7 (Hazards of Microwave Radiations – A review) gives us some insights into what was known 50 years ago (1960).

Kuo-Chiew Quan.  1960.  Hazards of Microwave Radiations – A Review.  Industr. Med. Surg. 29:315-318, July 1960 and reprinted in Occupational Medicine, Medical News Letter, Vol. 36, No. 10.  November 18, 1960, pp 29-34.

This document, written 50 years ago, discusses the hazards associated with relatively high levels of microwave exposure that might be experienced by those who work near radar installations, as radio frequency heat sealers, or with medical diathermy machines. Note this also applies to those who repair both broadcast and cell phone antennas.

The author makes several statements about energy absorption and  thermal effects of microwave radiation.

Statement 1: “any biological effects, beneficial or harmful, produced by microwaves can result only from absorption of energy by the tissues.”

Statement 2: “It is not clear whether all biologic effects of microwaves can be attributed solely to temperature increases that result from energy absorption or whether these effects are produced in part by mechanisms other than simple thermal elevation.”

Statement 3:  “at this time it is impossible to rule out completely the possibility of athermal effects of microwaves.”

So the thermal vs athermal debate was well underway in 1960.

The concept that microwaves can kill has been known for decades. Rabbits exposed to a constant 300-watt field were killed after 75 seconds.  The same power level killed a rat in 22 seconds and a hamster exposed to 400 watts died after a 20-second exposure.  Death was attributed to thermal paralysis of the respiratory center.  This makes microwaves considerably more lethal than x-rays.

Note: The power of the transmitter, measured in watts (W), and the signal intensity (power per unit area) influence exposure.  In the examples above, exposure from 300 to 400 watt fields can kill laboratory animals within seconds. How does this compare to the power of transmitters used today?

Typical combined radiated radio frequency (RF) power of microwave oven is 1000 W although they seldom work at 100% efficiency.  Good thing the microwave generator  is in a metal (faraday) cage with minimal leakage! Typical maximum output RF power from ham radio transceiver is 10 W.  Remote uplink trucks use 120 watts pointed towards satellites and transmitters for cell phone antennas use approximately 10 watts of power although this depends on their range.  Maximum output from a mobile phone ranges from 2 W (power class 1) to 0.125 (power class 4).  The older bag mobile phones were more powerful with 4 watts of power.

Exposure of body parts elevates temperature of selected organs that are unable to thermoregulate. Testis are damaged without any damage to the skin.  Cataracts occur optimally when exposed to 10 to 12 cm wavelengths (3 to 2.5 GHz frequency).  Note:  WiFi and microwave ovens as well as some digital cordless phones use 2.4 GHz frequencies. Lower power levels, at the same frequency, produce cataracts over a longer period of time so recurring exposures at lower intensities have the same effect as fewer exposures at higher intensities. Cataracts are repeated reported by radar repair workers.

Metal implants can magnify the intensity of the microwave exposure by forming standing waves and those with such implants may be vulnerable to tissue damage from microwave exposure. Swelling and pain associated with metal implants have been reported with the symptoms disappearing when exposure was stopped. Those with metal implants should be excluded from working with microwave emitting equipment.

In 1957 a fatal exposure, attributed to microwave irradiation, was still debated 3 years later.

To prevent injury it is important to point the radar beam away from people, to use metal screens to restrict the radiation beam, to use lights to indicate radar is operating, to indicate shape of radar beam with signage.  And finally those who work near radar installations should be supplied with photographic flash bulbs to warn them when they are exposed to intense microwave fields.

Note:  These precautions (minus the flash bulb) should also be applied to cell phone antennas that are placed on buildings or on towers near buildings since people are exposed to this radiation 24/7 for many years, especially since cumulative chronic exposure may have similar effects to short-term acute exposure.

Medical diathermy, which uses microwave radiation, should have certain precautions to minimize exposure of the eyes and testis and it should be used cautiously on those with metal implants.

Acute thermal exposure can kill quickly (in a matter of seconds to days) while repeated non-thermal exposure contributes to chronic ill health (as previous reports have documented).

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