Radiofrequency (RF) electromagnetic energy and microwave radiation is used in a variety of applications in industry, commerce, medicine and research, as well as in the home. In the frequency range from 3 to 3 x 108 kHz (that is, 300 GHz) we readily recognize applications such as radio and television broadcasting, communications (long-distance telephone, cellular telephone, radio communication), radar, dielectric heaters, induction heaters, switched power supplies and computer monitors.
High-power RF radiation is a source of thermal energy that carries all of the known implications of heating for biological systems, including burns, temporary and permanent changes in reproduction, cataracts and death. For the broad range of radiofrequencies, cutaneous perception of heat and thermal pain is unreliable for detection, because the thermal receptors are located in the skin and do not readily sense the deep heating of the body caused by these fields. Exposure limits are needed to protect against these adverse health effects of radiofrequency field exposure.
By applying an intense alternating magnetic field a conducting material can be heated by induced eddy currents. Such heating is used for forging, annealing, brazing and soldering. Operating frequencies range from 50/60 to several million Hz. Since the dimensions of the coils producing the magnetic fields are often small, the risk of high-level whole-body exposure is small; however, exposure to the hands can be high.
Radiofrequency energy from 3 to 50 MHz (primarily at frequencies of 13.56, 27.12 and 40.68 MHz) is used in industry for a variety of heating processes. Applications include plastic sealing and embossing, glue drying, fabric and textile processing, woodworking and the manufacture of such diverse products as tarpaulins, swimming pools, waterbed liners, shoes, travel check folders and so on.
Measurements reported in the literature (Hansson Mild 1980; IEEE COMAR 1990a, 1990b, 1991) show that in many cases, electric and magnetic leakage fields are very high near these RF devices. Often the operators are women of child-bearing age (that is, 18 to 40 years). The leakage fields are often extensive in some occupational situations, resulting in whole-body exposure of operators. For many devices, the electric and magnetic field exposure levels exceed all existing RF safety guidelines.
Since these devices may give rise to very high absorption of RF energy, it is of interest to control the leakage fields which emanate from them. Thus, periodic RF monitoring becomes essential to determine whether an exposure problem exists.
Workers in the fields of communication and radar are exposed only to low-level field strengths in most situations. However, the exposure of workers who must climb FM/TV towers can be intense and safety precautions are necessary. Exposure can also be substantial near transmitter cabinets that have their interlocks defeated and doors open.
One of the earliest applications of RF energy was short-wave diathermy. Unshielded electrodes are usually used for this, leading possibly to high stray fields.
Recently RF fields have been used in conjunction with static magnetic fields in magnetic resonance imaging (MRI). Since the RF energy used is low and the field is almost fully contained within the patient enclosure, the exposure to operators is negligible.
The specific absorption rate (SAR, measured in watts per kilogram) is widely used as a dosimetric quantity, and exposure limits can be derived from SARs. The SAR of a biological body depends upon such exposure parameters as frequency of the radiation, intensity, polarization, configuration of the radiation source and the body, reflection surfaces and body size, shape and electrical properties. Furthermore, the SAR spatial distribution inside the body is highly non-uniform. Non-uniform energy deposition results in non-uniform deep-body heating and may produce internal temperature gradients. At frequencies above 10 GHz, the energy is deposited close to the body surface. The maximum SAR occurs at about 70 MHz for the standard subject, and at about 30 MHz when the person is standing in contact with RF ground. At extreme conditions of temperature and humidity, whole-body SARs of 1 to 4 W/kg at 70 MHz are expected to cause a core temperature rise of about 2 ºC in healthy human beings in one hour.
RF heating is an interaction mechanism that has been studied extensively. Thermal effects have been observed at less than 1 W/kg, but temperature thresholds have generally not been determined for these effects. The time-temperature profile must be considered in assessing biological effects.
Biological effects also occur where RF heating is neither an adequate nor a possible mechanism. These effects often involve modulated RF fields and millimetre wavelengths. Various hypotheses have been proposed but have not yet yielded information useful for deriving human exposure limits. There is a need to understand the fundamental mechanisms of interaction, since it is not practical to explore each RF field for its characteristic biophysical and biological interactions.
Human and animal studies indicate that RF fields can cause harmful biological effects because of excessive heating of internal tissues. The body’s heat sensors are located in the skin and do not readily sense heating deep within the body. Workers may therefore absorb significant amounts of RF energy without being immediately aware of the presence of leakage fields. There have been reports that personnel exposed to RF fields from radar equipment, RF heaters and sealers, and radio-TV towers have experienced a warming sensation some time after being exposed.
There is little evidence that RF radiation can initiate cancer in humans. Nevertheless, a study has suggested that it may act as a cancer promoter in animals (Szmigielski et al. 1988). Epidemiological studies of personnel exposed to RF fields are few in number and are generally limited in scope (Silverman 1990; NCRP 1986; WHO 1981). Several surveys of occupationally exposed workers have been conducted in the former Soviet Union and Eastern European countries (Roberts and Michaelson 1985). However, these studies are not conclusive with respect to health effects.
Human assessment and epidemiological studies on RF sealer operators in Europe (Kolmodin-Hedman et al. 1988; Bini et al. 1986) report that the following specific problems may arise:
- RF burns or burns from contact with thermally hot surfaces
- numbness (i.e., paresthesia) in hands and fingers; disturbed or altered tactile sensitivity
- eye irritation (possibly due to fumes from vinyl-containing material)
- significant warming and discomfort of the legs of operators (perhaps due to current flow through legs to ground).
The use of personal radiotelephones is rapidly increasing and this has led to an increase in the number of base stations. These are often sited in public areas. However, the exposure to the public from these stations is low. The systems usually operate on frequencies near 900 MHz or 1.8 GHz using either analogue or digital technology. The handsets are small, low power radio transmitters that are held in close proximity to the head when in use. Some of the power radiated from the antenna is absorbed by the head. Numerical calculations and measurements in phantom heads show that the SAR values can be of the order of a few W/kg (see further ICNIRP statement, 1996). Public concern about the health hazard of the electromagnetic fields has increased and several research programmes are being devoted to this question (McKinley et al., unpublished report). Several epidemiological studies are ongoing with respect to mobile phone use and brain cancer. So far only one animal study (Repacholi et al. 1997) with transgenic mice exposed 1 h per day for 18 months to a signal similar to that used in digital mobile communication has been published. By the end of the experiments 43 of 101 exposed animals had lymphomas, compared to 22 of 100 in the sham-exposed group. The increase was statistically significant (p > 0.001). These results cannot easily be interpreted with relevance to human health and further research on this is needed.
Standards and Guidelines
Several organizations and governments have issued standards and guidelines for protection from excessive exposure to RF fields. A review of worldwide safety standards was given by Grandolfo and Hansson Mild (1989); the discussion here pertains only to the guidelines issued by IRPA (1988) and IEEE standard C 95.1 1991.
The full rationale for RF exposure limits is presented in IRPA (1988). In summary, the IRPA guidelines have adopted a basic limiting SAR value of 4 W/kg, above which there is considered to be an increasing likelihood that adverse health consequences can occur as a result of RF energy absorption. No adverse health effects have been observed due to acute exposures below this level. Incorporating a safety factor of ten to allow for possible consequences of long-term exposure, 0.4 W/kg is used as the basic limit for deriving exposure limits for occupational exposure. A further safety factor of five is incorporated to derive limits for the general public.
Derived exposure limits for the electric field strength (E), the magnetic field strength (H) and the power density specified in V/m, A/m and W/m2 respectively, are shown in figure 1. The squares of the E and H fields are averaged over six minutes, and it is recommended that the instantaneous exposure not exceed the time-averaged values by more than a factor of 100. Furthermore, the body-to-ground current should not exceed 200 mA.
Figure 1. IRPA (1988) exposure limits for electric field strength E, magnetic field strength H and power density
Standard C 95.1, set in 1991, by the IEEE gives limiting values for occupational exposure (controlled environment) of 0.4 W/kg for the average SAR over a person’s entire body, and 8 W/kg for the peak SAR delivered to any one gram of tissue for 6 minutes or more. The corresponding values for exposure to the general public (uncontrolled environment) are 0.08 W/kg for whole-body SAR and 1.6 W/kg for peak SAR. The body-to-ground current should not exceed 100 mA in a controlled environment and 45 mA in an uncontrolled environment. (See IEEE 1991 for further details.) The derived limits are shown in figure 2.
Figure 2. IEEE (1991) exposure limits for electric field strength E, magnetic field strength H and power density
Further information on radiofrequency fields and microwaves can be found in, for instance, Elder et al. 1989, Greene 1992, and Polk and Postow 1986.