Private Library of Simamora, Helmut Todo Tua
Environment, Research and Development Agency
Samosir Regency Government of North Sumatera Province
INDONESIA
Berikut merupakan kutipan ilmiah yang disusun dan digunakan Penulis sebagai referensi di dalam mendukung kegiatan kerja di kantor.
BASIS FOR LIMITING EXPOSURE
These guidelines for limiting exposure have been developed following a thorough review of all published scientific literature. The criteria applied in the course of the review were designed to evaluate the credibility of the various reported findings (Repacholi and Stolwijk 1991; Repacholi and Cardis 1997); only established effects were used as the basis for the proposed exposure restrictions. Induction of cancer from long-term EMF exposure was not considered to be established, and so these guidelines are based on short-term, immediate health effects such as stimulation of peripheral nerves and muscles, shocks and burns caused by touching conducting objects, and elevated tissue temperatures resulting from absorption of energy during exposure to EMF. In the case of potential long-term effects of exposure, such as an increased risk of cancer, ICNIRP
concluded that available data are insufficient to provide a basis for setting exposure restrictions, although epidemiological research has provided suggestive, but unconvincing, evidence of an association between possible carcinogenic effects and exposure at levels of 50/60 Hz magnetic flux densities substantially lower than those recommended in these guidelines.
In-vitro effects of short-term exposure to ELF or ELF amplitude-modulated EMF are summarized. Transient cellular and tissue responses to EMF exposure have been observed, but with no clear exposure-response relationship. These studies are of limited value in the assessment of health effects because many of the responses have not been demonstrated in vivo. Thus,in-vitro studies alone were not deemed to provide data that could serve as a primary basis for assessing possible health effects of EMF.
COUPLING MECHANISMS BETWEEN FIELDS AND THE BODY
There are three established basic coupling mechanisms through which time-varying electric and magnetic fields interact directly with living matter (UNEP/WHO/IRPA 1993):
c coupling to low-frequency electric fields;
c coupling to low-frequency magnetic fields; and
c absorption of energy from electromagnetic fields.
Coupling to low-frequency electric fields
The interaction of time-varying electric fields with the human body results in the flow of electric charges (electric current), the polarization of bound charge (formation of electric dipoles), and the reorientation of electric dipoles already present in tissue. The relative magnitudes of these different effects depend on the electrical properties of the body—that is, electrical conductivity (governing the flow of electric current) and permittivity (governing the magnitude of polarization effects). Electrical conductivity and permittivity vary with the type of body tissue and also depend on the frequency of the applied field. Electric fields external to the body induce a surface charge on the body; this results in induced currents in the body, the distribution of which depends on exposure conditions, on the size and shape of the body, and on the body’s position in the field.
Coupling to low-frequency magnetic fields
The physical interaction of time-varying magnetic fields with the human body results in induced electric fields and circulating electric currents. The magnitudes of the induced field and the current density are propor-
Table 1. Electric, magnetic, electromagnetic, and dosimetric quantities and corresponding SI units.
Quantity Symbol Unit
Conductivity s siemens per meter (S m21)
Current I ampere (A)
Current density J ampere per square meter (A m22)
Frequency f hertz (Hz)
Electric field strength E volt per meter (V m21)
Magnetic field strength H ampere per meter (A m21)
Magnetic flux density B tesla (T)
Magnetic permeability m henry per meter (H m21)
Permittivity e farad per meter (F m21)
Power density S watt per square meter (W m22)
Specific energy absorption SA joule per kilogram (J kg21)
Specific energy absorption rate
SAR watt per kilogram (W kg21)
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Coupling to low-frequency magnetic fields
The physical interaction of time-varying magnetic fields with the human body results in induced electric fields and circulating electric currents. The magnitudes of the induced field and the current density are proportional to the radius of the loop, the electrical conductivity of the tissue, and the rate of change and magnitude of the magnetic flux density. For a given magnitude and frequency of magnetic field, the strongest electric fields are induced where the loop dimensions are greatest. The exact path and magnitude of the resulting current induced in any part of the body will depend on the electrical conductivity of the tissue.
The body is not electrically homogeneous; however,induced current densities can be calculated using anatomically and electrically realistic models of the body and computational methods, which have a high degree of anatomical resolution.
Absorption of energy from electromagnetic fields Exposure to low-frequency electric and magnetic fields normally results in negligible energy absorption and no measurable temperature rise in the body. However,exposure to electromagnetic fields at frequencies above about 100 kHz can lead to significant absorption of energy and temperature increases. In general, exposure to a uniform (plane-wave) electromagnetic field results in a highly non-uniform deposition and distribution of energy within the body, which must be assessed by dosimetric measurement and calculation.
As regards absorption of energy by the human body,electromagnetic fields can be divided into four ranges (Durney et al. 1985):
c frequencies from about 100 kHz to less than about 20 MHz, at which absorption in the trunk decreases rapidly with decreasing frequency, and significant absorption may occur in the neck and
legs;
c frequencies in the range from about 20 MHz to 300 MHz, at which relatively high absorption can occur in the whole body, and to even higher values if partial body (e.g., head) resonances are
considered;
c frequencies in the range from about 300 MHz to several GHz, at which significant local, nonuniform absorption occurs; and
c frequencies above about 10 GHz, at which energy absorption occurs primarily at the body surface.
In tissue, SAR is proportional to the square of the internal electric field strength. Average SAR and SAR distribution can be computed or estimated from laboratory measurements. Values of SAR depend on the following factors:
c the incident field parameters, i.e., the frequency,intensity, polarization, and source– object configuration (near- or far-field);
c the characteristics of the exposed body, i.e., its size and internal and external geometry, and the dielectric properties of the various tissues; and
c ground effects and reflector effects of other objects in the field near the exposed body.
BIOLOGICAL BASIS FOR LIMITING EXPOSURE (100kHz–300 GHz)
The following paragraphs provide a general review of relevant literature on the biological effects and potential health effects of electromagnetic fields with frequencies of 100 kHz to 300 GHz. More detailed reviews can be found elsewhere (NRPB 1991; UNEP/WHO/IRPA 1993; McKinlay et al. 1996; Polk and Postow 1996;Repacholi 1998).
Direct effects of electromagnetic fields
Epidemiological studies. Only a limited number of studies have been carried out on reproductive effects and cancer risk in individuals exposed to microwave radiation.
A summary of the literature was published by UNEP/WHO/IRPA (1993).
Reproductive outcomes.
Two extensive studies on women treated with microwave diathermy to relieve the pain of uterine contractions during labor found no evidence for adverse effects on the fetus (Daels 1973, 1976).
However, seven studies on pregnancy outcomes among workers occupationally exposed to microwave radiation and on birth defects among their offspring produced both positive and negative results. In some of the larger epidemiological studies of female plastic welders and physiotherapists working with shortwave diathermy devices, there were no statistically significant effects on rates of abortion or fetal malformation (Ka¨llen et al.1982). By contrast, other studies on similar populations of female workers found an increased risk of miscarriage and birth defects (Larsen et al. 1991; Ouellet-Hellstrom and Stewart 1993). A study of male radar workers found no association between microwave exposure and the risk of Down’s syndrome in their offspring (Cohen et al.1977).
Overall, the studies on reproductive outcomes and microwave exposure suffer from very poor assessment of exposure and, in many cases, small numbers of subjects. Despite the generally negative results of these studies, it will be difficult to draw firm conclusions on reproductive risk without further epidemiological data on highly exposed individuals and more precise exposure assessment.
Cancer studies. Studies on cancer risk and microwave exposure are few and generally lack quantitative exposure assessment. Two epidemiological studies of radar workers in the aircraft industry and in the U.S. armed forces found no evidence of increased morbidity or mortality from any cause (Barron and Baraff 1958; Robinette et al. 1980; UNEP/WHO/IRPA 1993). Similar results were obtained by Lillienfeld et al. (1978) in a study of employees in the U.S. embassy in Moscow, who were chronically exposed to low-level microwave radiation. Selvin et al. (1992) reported no increase in cancer risk among children chronically exposed to radiation from a large microwave transmitter near their homes. More recent studies have failed to show significant increases in nervous tissue tumors among workers and military personnel exposed to microwave fields (Beall et al. 1996; Grayson 1996). Moreover, no excess total mortality was apparent among users of mobile telephones (Rothman et al. 1996a, b), but it is still too early to observe an effect on cancer incidence or mortality. There has been a report of increased cancer risk among military personnel (Szmigielski et al. 1988), but the results of the study are difficult to interpret because neither the size of the population nor the exposure levels are clearly stated. In a later study, Szmigielski (1996)found increased rates of leukemia and lymphoma among military personnel exposed to EMF fields, but the assessment of EMF exposure was not well defined. A few recent studies of populations living near EMF transmitters have suggested a local increase in leukemia incidence (Hocking et al. 1996; Dolk et at. 1997a, b), but the results are inconclusive. Overall, the results of the small number of epidemiological studies published provide only limited information on cancer risk.
Basic restrictions
Different scientific bases were used in the development of basic exposure restrictions for various frequency ranges:
c Between 1 Hz and 10 MHz,basic restrictions are provided on current density to prevent effects on nervous system functions;
c Between 100 kHz and 10 GHz,basic restrictions on SAR are provided to prevent whole-body heat stress and excessive localized tissue heating;
in the 100 kHz–10 MHz range,restrictions are provided on both current density and SAR; and
c Between 10 and 300 GHz, basic restrictions are provided on power density to prevent excessive heating in tissue at or near the body surface.
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In the frequency range from a few Hz to 1 kHz, for levels of induced current density above 100 mA m22, the thresholds for acute changes in central nervous system excitability and other acute effects such as reversal of the visually evoked potential are exceeded. In view of the safety considerations above, it was decided that, for frequencies in the range 4 Hz to 1 kHz, occupational exposure should be limited to fields that induce current densities less than 10 mA m22, i.e., to use a safety factor of 10. For the general public an additional factor of 5 is
applied, giving a basic exposure restriction of 2 mA m22.
Below 4 Hz and above 1 kHz, the basic restriction on induced current density increases progressively, corresponding to the increase in the threshold for nerve stimulation for these frequency ranges.
Established biological and health effects in the frequency range from 10 MHz to a few GHz are consistent with responses to a body temperature rise of more than 1°C. This level of temperature increase results from exposure of individuals under moderate environmental conditions to a whole-body SAR of approximately 4 W kg21 for about 30 min. A whole-body average SAR of 0.4Wkg21 has therefore been chosen as the restriction that provides adequate protection for occupational exposure. An additional safety factor of 5 is introduced for exposure of the public, giving an average whole-body SAR limit of 0.08 W kg21.
The lower basic restrictions for exposure of the general public take into account the fact that their age and health status may differ from those of workers.
In the low-frequency range, there are currently few data relating transient currents to health effects. The ICNIRP therefore recommends that the restrictions on current densities induced by transient or very short-term peak fields be regarded as instantaneous values which should not be time-averaged.
Sumber : ICNIRP
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