Guidelines For Limiting Radiofrequency Exposure - Short Wave Diathermy - Safety Code 25

2. Biological Effects of RF Exposure

Electromagnetic radiation at the frequency of operation of short-wave diathermy (27 MHz) is known to produce various biological effects. The interactions and the biological effects produced at this frequency are not in any way frequency specific(7), and a broader base including the effects of other radiofrequencies should be considered. As a matter of fact, exposure of a human being at 27 MHz is well modelled in terms of expected biological effects by exposure of a small rodent to a much higher frequency, following the rules of physical scaling(9). Exposure to RF radiation can result in effects that are benign, beneficial (e.g., short-wave diathermy treatment) or potentially harmful. The brief review that follows is limited to effects that are potentially harmful, and against which protection is sought through adherence to exposure standards.

The interaction of RF and microwave radiation with living systems, including human beings, is a complex function of many parameters. Biological responses are due to the electromagnetic fields inside the biological body rather than to the external fields. The electrical properties (permittivity) of the living system and its geometry determine the amount of radiation reflected, transmitted and absorbed for a given exposure field. The exposure field is characterized by the frequency, intensity, polarization (direction) and type e.g., a plane wave, a fringe (leakage) field, the near-field of a radiator.

The interactions of RF fields with living systems can be considered on a macroscopic or microscopic (molecular, cellular) level. On the molecular level, two basic mechanisms govern the interactions. These are space charge polarization at lower RF frequencies and field-induced rotation of polar molecules at higher RF and microwave frequencies(5). The space charge polarization is due to travelling charge carriers, i.e., ions whole movement is affected by the applied field. Polar molecules i.e., molecules having an uneven spatial distribution of charges, such as water and proteins, experience a torque when placed in an electric field. Both of these mechanisms are of a relaxation character. In moderate fields only a relatively small number of charges or molecules is actually affected by the field (moved toward, or aligned with, the direction of the applied field). The movements are hindered by the thermal motion of molecules and charges, and the kinetic energy undergoes a conversion into the thermal energy. In these interactions the electromagnetic energy is converted into kinetic energy of molecules, and subsequently converted into thermal energy.

Quite a different mechanism underlies some specific inter-actions of extremely low frequencies (ELF) (below 100 Hz) and RF fields amplitude modulated at ELF frequencies(7), e.g., changes in calcium efflux. The fact that these interactions occur is well established, but their mechanisms are still not fully understood. These interactions are very sensitive to the frequency (ELF) and amplitude of the exposure field, as well as other variables, e.g., species(7).

Quite apart from the interaction mechanisms involved, biological effects are related to the intensities of the fields within the living body, not to the external intensity of an exposure field. The internal fields are a complex function of exposure conditions and other parameters. The internal fields are frequently described in terms of the specific absorption rate (SAR) which expresses the rate of energy absorption (e.g., at a given location, or averaged over the whole body) and is proportional to the square of the internal electric field intensity. The proportionality constant depends on the electrical properties of the tissue. The average SAR for a while body, far-field (far away from the radiator) exposure depends on the field frequency, intensity, direction, subject-to-source configuration, subject's size and shape, and presence of other objects particularly metal objects in the immediate vicinity. There are no simple methods of directly measuring the average SAR, but its approximate value can be calculated (it can be measured for animals, but the test animal has to be sacrificed). When exposure occurs in the far-field (plane-wave propagation), it has been shown(5), that for a given size and shape of a biological body, there is a frequency at which maximum amount of RF power is absorbed in the exposed body (Figure 1). This frequency is called the resonant frequency.

Figure 1- Average specific absorption rate for an average man exposed to 1 mW/cm²plane wave. E polarization denotes the electric field parallel to the body, H denotes the magnetic field parallel to the body and K denotes the wave moving from head to toes or toes to head.

Not only does the maximum absorption occur at this frequency, but in a range of frequencies around it the distribution of the absorbed power in the body is highly nonuniform. Increased absorption occurs in various places inside the body, resulting in so-called "hot spots". For human beings maximum energy absorption takes place between 30 and 100 MHz, depending on the body size and the environment. For an average man isolated from ground the frequency of the maximum absorption is about 80 MHz. For small experimental animals this frequency is much higher, e.g., for a rat, 900 MHz, for a mouse, 2000 MHz.

Very little is known about the average SAR and the SAR distribution for partial-body exposures close to the radiation source, such as may be the case for operator exposures in short-wave diathermy leakage fields. Calculations and experiments for other, not entirely dissimilar exposures, indicate that the whole-body average SAR for exposures in the near-field is less than the whole-body average SAR for exposures of the equivalent intensity in the far-field. The nonuniformity of SAR distribution is certainly further accentuated in partial-body exposures.

Biological effects of RF exposure have been investigated in various animals. There appears to be a consensus among scientists that the great majority of the effects of exposure to RF and microwave radiation, excepting those resulting from ELF modulated fields, are thermal in nature. This statement, however, should not be taken simplistically. The effects of RF induced heating are significantly different from the effects caused by other modalities of heating. Three distinctive features of RF and microwave induced heating are: various depths of penetration, the existence of internal "hot spots" and the rapidity of heating. The induction of nonuniformities in the temperature of various parts of the brain may cause alterations whole extent and implications have not yet been fully recognized.

The thermogenic effects of RF or microwave energy have been well documented and recently summarized(10):

1. biological effects due to thermoregulatory response occur when an animal is thermally loaded at a rate equal to its basal metabolic rate (BMR);
2. numerous behavioral and endocrine effects, and cardiac and respiratory changes for SARs below the BMR, are manifestations of physiological responses to mild thermal stress;
3. the thermal stress resulting from about twice the BMR, when maintained over long periods of time, leads to significant physiological effects, and
4. the responses to thermal load from pulsed fields appear to be the same as the responses to cw fields of the same average power.

The effects on two organs, the eye and the gonads, that are particularly susceptible to heat, have been extensively investigated(5). Microwave radiation at frequencies above 800 MHz can produce injury to the eye. The type of injury depends on the frequency, e.g., millimetre waves can produce keratitis. Cataracts develop after a sufficiently long exposure to power densities above 100 mW/cm². The effects on the testes result from high intensity fields and thermal injury.

Studies to investigate whether microwaves produce genetic effects have been performed on bacteria, flies, various plant and animal cells and tissue cultures. The results of the studies did not yield any reliable or systematic evidence that RF or microwaves can induce any mutation in living systems other than through induction of heat; it is known that the rate of induction of mutations increases with increasing temperature.

Studies of effects of RF exposure on mammalian teratogene-sis indicate that there is a dose-response relation between the effects and the absorbed dose. Only intense fields that result in significant heating are associated with a reliable induction of teratogenesis.

No cardiovascular disturbances occur as the result of exposure to electromagnetic fields at relatively low levels (below 10 mW/cm²). Some cardiovascular responses may result from the effects on the nervous system.

The effects of RF and microwave exposure on the central nervous system and on behaviour have been the most controversial subject in the field of biological effects of microwave radiation. In the USSR and some Eastern European countries it was asserted that exposure to low-level radiation (frequently referred to as "athermal") resulted in reversible neurasthenic disorders(5,6). These observations have not been confirmed by the Western researchers. It has been clearly shown, however, that RF exposure can disrupt animal behaviour. The threshold value of the SAR for the effect has been established. Recent studies of possible synergistic effects of RF exposure and various drugs have produced a highly inconsistent picture, with synergy for some drugs, a weakening effect for others or no interrelation.

Most of the aforementioned biological effects (except cataracts) can be caused by exposure of humans to 27 MHz, when the radiation is of sufficient intensity and duration.

These and numerous other biological effects together with their biophysical basis have been considered in establishing protection standards(5,8,10). Analysis of the scientific data base and criteria used in establishing the Canadian recommendation are given elsewhere(5).