Pick of the Week 25: Review of International Microwave Exposure Guidelines from 1957 to 1968.

April 13, 2011.  Swanson and colleagues from the International Labour Office (Geneva, Switzerland) and the Bureau of Occupational Safety and Health, Public Health Services (Cincinnati, Ohio) reviewed guidelines for microwave radiation and published their review in the American Industrial Hygiene Association Journal, Vol. 31:  623-629 (1970).  Click here to download a pdf of this article.

Below is some information from this article.  My comments appear in square brackets. To convert from mW/cm2 to microW/cm2 multiple by 1000.

United States

1.     From 1940s to 1970s the use of microwave emitting equipment had increased considerably.

2.     In the United States radio frequencies (RF) from 10 to 10,000 MHz were classified as microwave radiation, while in Europe the range was from 300 to 300,000 MHz. [NOTE:  We now use the European range to delineate the microwave part of the radio frequency spectrum.]

3.     By 1970, scientists recognized that parts of the body that are unable to dissipate heat are the most vulnerable to microwave radiation.  This includes the lens of the eye (cataracts) and the reproductive organs (sterility or degenerative changes).

4.     Depth of penetration of radiation into tissue is a function of frequency with greater penetration at lower frequencies.

5.     In the United States the first guidelines were established during the Tri-Service conference, held in 1957.  Below is a quote about the guidelines:

It was the opinion of those participating in the Conference that there were not sufficient data to determine safe exposure levels for each frequency, or ranges of frequencies, within the microwave region; therefore, a level of 10 mW/cm2 [10,000 microW/cm2] was selected for all frequencies. The U.S. Air Force, in adopting this exposure level in May 1958, applied it to the frequency range of 300 to 30,000 MHz and established it as a maximum permissible exposure level, which could not be exceeded. The only factor considered in this criterion is the power density level. Such factors as time of exposure, ambient environmental temperatures that could have an increased or decreased effect on the body’s thermal response, the frequency of the microwave energy, effects of multifrequency exposures, differing sensitivity of various body organs, and effect of air currents on cooling the body are not considered, although they are all recognized as factors that might affect biological response.

 

[NOTE:  It was clear in 1970 that the US guidelines were somewhat arbitrary, were based on thermal effects only, and did not include other factors that influence biological and health consequences. This guideline has since been lowered from 10 to 1 mW/cm2 but is still 100 to 1000 times higher than guidelines in other countries.]

UK, West Germany, France and Netherlands

 

6.     Guidelines in the UK and in West Germany allowed citizens to be exposed to 10 mW/cm2 (same as in U.S).

7.     In France only military personnel during working hours were allowed to be exposed to 10 mW/cm2.  In rest areas and in public areas the guidelines were 1 mW/cm2.

8.     In the Netherlands the guidelines were at 1 mW/cm2.

Poland, USSR, Czechoslovakia

 

9.     Guidelines in the eastern European Block countries were much more protective than those in western countries.

 

Poland

10.   Polish guidelines, established in 1961 and 1963, were as follows:

  • 10 microW/cm2 [0.01 mW/cm2]  – no limitation for time of work or sojourn in this field.
  • 10 and 100 microW/cm2 [0.01 and 0.1 mW/cm2]- cumulative time of work or sojourn not to exceed 2 hours in every 24 hours
  • 100 and 1000 microW/cm2 [0.1 and 1 mW/cm2]- cumulative time of work or sojourn not to exceed 20 minutes in 24 hours.

11.  The Polish regulation  requires an annual medical examination for exposed workers including neurological and ophthalmological examinations; safe placement of microwave generating installations; protective screening; personnel protection; site surveillance; and safety education.

12.  The Polish regulation forbids work with microwave radiation for young people (age not provided), pregnant women, and other people suffering from certain diseases, which are listed in the regulation.

USSR

13.  The USSR standards were based on time of exposure as follows:

  • 10 microW/cm2 [0.01 mW/cm2] for a working day
  • 100 microW/cm2 [0.1 mW/cm2] for 2 hours daily
  • 1000 microW/cm2 [1 mW/cm2] for 15 minutes daily [so at 1000 microW/cm2 the Soviets could be exposed for only 15 minutes, the Poles for only 20 minutes but the Americans could be exposed for 24 hours each day!

14.  The U.S.S.R. is also one of the first to propose exposure standards for intermediate-frequency electromagnetic radiation [dirty electricity], which heretofore had been considered as having no effect on the human body. These levels are:

  • Medium wave (100 kHz – 3 MHz) – 20 volts/ meter [29 microW/cm2]
  • Short wave (3 MHz- 30 MHz)- 5 volts/ meter  [1.8 microW/cm2]
  • Ultra short wave (30 MHz- 300 MHz)- 5 volts/ meter [1.8 microW/cm2]

[NOTE:  The WHO has recently recognized the importance of intermediate frequencies (IF) and the information they provide is severely limited].

15.  Medical examinations are regulated in the Soviet Union for persons exposed to electromagnetic radiation. Medical counter indications are enforced so that workers are not allowed to be exposed to microwave radiation if specified diseases exist. Heavy emphasis is placed on blood disorders, neurological disturbances, and chronic eye diseases.

16.  Preventive measures of an engineering nature are used by Soviet health and epidemiological centers to ensure compliance with their health regulations. Decreasing the amount of radiated energy, reflective and absorptive screening, and personnel protection measures are widely used for personnel operating microwave equipment.

Czechoslovakia, 1965, above 300 MHz:

17.  The following values are considered for the general population and other workers not employed in generation of electromagnetic energy as tolerable doses of radiation not to be exceeded at the person’s location during one calendar day :

  • for continuous generation in the microwave frequencies- value = 60 where the energy is expressed in microwatts per square centimeter and the time in hours [(microW/cm2) X t (hours) < 60 ; therefore twenty-four hours exposure time corresponds to an average energy flow of 2.5 microW/cm2].
  • for pulsed generation in the microwave frequencies- value = 24 where the energy is expressed in microwatts per square centimeter and the time in hours [(microW/cm2) X t(hours) < 24; therefore twenty- four hours exposure corresponds to an average pulsed energy flow of 1 microW/cm2].

18.  The final point that is worth noting is the authors’ recommendation that “in applying the concept of a time-weighted exposure the health specialist must consider how far the dose- time relationship can be extrapolated.”

Extrapolation of the dose-time relationship.

Both cell phones and WiFi routers use pulsed microwave radiation and it is well known that pulsed microwave radiation is more harmful than continuous wave radiation.  If we apply the Czech time-weighted concept for pulsed radiation we get the following (see last four rows in table 1).  These values begin to approach the Salzburg recommended guidelines for outdoor (0.1 microW/cm2) and indoor (0.01 microW/cm2) exposure.

Table 1.  Comparison of time-weight exposure guidelines in selected countries.

Clearly guidelines that differ 4 orders of magnitude (from 10,000 to 1 microW/cm2) need to be addressed.

 

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