Possible Health Effects of Exposure to Residential Electric and Magnetic Fields (1997) / Chapter Skim
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2 Exposure and Physical Interactions
2 Exposure and Physical Interactions
Pages 21-51
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From page 21... ...
· Exposure levels of electric fields and other characteristics of magnetic fields (harmonics, transients,2 spatial, and temporal changes) have received relatively little attention in the studies of possible biologic and health effects.
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From page 22... ...
Also, the spatial patterns of the currents induced by the magnetic fields are different from those induced by the electric fields. · The endogenous current densities on the surface of the body (higher densities occur internally)
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From page 23... ...
. Frequencies associated with power lines and their common harmonics are low enough for electric fields and magnetic fields generated by them to be considered separately (i.e., uncoupled)
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From page 24... ...
The electric field at power-line frequencies produced by specific voltages on high-voltage transmission lines can be accurately evaluated by analytic or numeric methods. Similarly, for distribution lines and any other known configuration of wires and other shape conductors, it is possible to evaluate the strength and direction of the electric field in any point of the surrounding space.
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From page 25... ...
Unlike the electric field, the ELF magnetic field is not affected by the presence of humans and animals. Therefore, the measured field represents the actual exposure field.
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From page 26... ...
As a compromise, some of the more popular field-measurement devices today are able to record many samples of the magnetic field over a long period; for example, they can be set to record a sample every 10 see for 24 hr. The resulting amount of data is manageable and permits the calculation of a limited range of summary metrics (such as average rms field, peak field, median field, difference between successive measurements, and time above a specific threshold)
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From page 27... ...
Field Calculations For well-defined sources, magnetic flux densities can be calculated accurately, and measurements support the accuracy of such calculations. Electric fields can also be calculated, but because the fields are perturbed by conducting objects, calculations are often of limited value unless the perturbations by such objects can be modeled.
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From page 28... ...
Although high-voltage transmission lines produce relatively high magnetic fields directly under them, they contribute relatively little to the residential and environmental levels at distances greater than 100 m, as illustrated in Table 2-1. Background Fields The background magnetic fields in the center of the rooms (away from most appliances' of the home are most likely caused by power lines, grounding systems, or some combination of the two.
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From page 29... ...
When the main source of a magnetic field in an appliance is a coil of wire, the field decreases approximately as the inverse cube of the distance. Some of the magnetic-field values measured near household and other appliances are shown in Tables 2-3 and 2-4.
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From page 30... ...
SOURCE: EPA 1992. In addition to the appliances listed in Tables 2-3 and 2-4, magnetic fields from electric blankets might contribute a large part to the magnetic-field exposures in the home.
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From page 31... ...
Environmental Fields In the EPRI residential study (EPRI 1993b) , short-duration measurements were made in the center of the rooms and outdoors around the perimeter of the TABLE 2-5 Average and Percentile Values for Personal Exposure and Spot and Long-term Measurements of the Magnetic Field in Homes Magnetic Flux Density I1T Measurements (n = 380 homes)
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From page 32... ...
Power Lines (Transmission and Distribution J For homes near transmission rights-of-way, transmission lines can be important sources of magnetic fields. Typical values for the magnetic fields from transmission lines were illustrated in Table 2-1 (EPA 1992~.
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From page 33... ...
. In underground pipe-type transmission lines, the close spacing of the wires in the pipe and the metal pipe itself decrease the magnetic field, so that the resulting ground-level field is typically less than 0.1 AT (1 mG)
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From page 34... ...
Those differences make it difficult to make detailed comparisons between the studies. In work environments involving high currents and large distances between the current-carrying conductors, such as substations and generating stations, magnetic fields are at high levels over large areas.
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From page 35... ...
The frequency and strength of the fields are such that transients can induce currents that are larger than thermal noise; thus, magnetic fields associated with
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From page 36... ...
With few exceptions, the studies have focused on indirect estimates of the magnetic fields rather than electric fields. At low frequencies, such as 60 Hz, electric fields are substantially shielded by the shell of a house and by surrounding trees, so that residential exposure to electric fields is difficult to describe and nearly impossible to model with any accuracy.
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From page 37... ...
several of the factors used to calculate power-line magnetic fields; however, the reliability of the wire codes as a quantitative measure of exposure to 60-Hz magnetic fields is very limited. The following is a summary of some of the findings in a review of the characteristics of the wire codes as used in the epidemiologic studies: · Although the rank ordering of fields in homes is predicted reasonably well by wire codes, the wire code accounts for only 15-20% of the variance in magnetic-field measurements.
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From page 38... ...
. The field from a power line adjacent to the residence is a dominant factor for high wire codes but not for low wire codes.
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From page 39... ...
If laboratory studies had clearly indicated that 60-Hz magnetic fields were carcinogenic at such low levels as those considered in the epidemiologic studies, then there might be less concern about the limitations of the wire codes as measures of exposure. Calculations Magnetic fields from power lines can be calculated accurately if the relevant variables are known.
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From page 40... ...
A major criticism of contemporary magnetic-field measurements is that they might not reflect conditions accurately that prevailed years before during the period the disease developed. Occupational Occupational studies typically rely on job title as an indicator of a subject's magnetic-field exposure or on magnetic fields measured at representative work locations (either personal exposure or spot measurements)
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From page 41... ...
Limiting the stray fields is a challenge, as shielding magnetic fields is much more complex than shielding electric fields. Nonmagnetic metal shields provide only a small reduction in the field strength.
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From page 42... ...
Illumination and noise level in exposure and sham apparatus (only for animal studies) Applies to in vivo and in vitro exposures to electric and magnetic fields.
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From page 43... ...
Additional specifications for animal exposures to the magnetic field are listed in Table 212. The critical specifications listed, as well as those listed in Table 2-10, must TABLE 2-12 Typical Exposure Specifications for Animal and In Vitro Exposures to Magnetic Fields Critical Specifications Desirable Specifications Magnitude of stray fields in the location of the sham-exposure apparatus Means to prevent cage or culture-dish .
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From page 44... ...
In practice, this procedure is hardly ever followed, because the electric fields in the in vitro preparation produced this way are very weak, even for strong applied fields. For instance, an externally applied field of 10 kV/m at 60 Hz results in only up to a fraction of a volt per meter in the culture (Tobey et al.
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From page 45... ...
. This information is required for evaluation of induced electric fields and currents.
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From page 46... ...
. Therefore, an ELF electromagnetic field produces currents and electric fields in the exposed biologic system and causes oscillating (at ELF)
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From page 47... ...
indicate that biologic bodies produce considerable perturbation of the external electric field. The internal fields induced by exposure to 50- and 60-Hz electric fields are typically 10-6-10-7 times lower than the external fields for a conductive body, such as a culture medium or an animal.
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From page 48... ...
This is a wellestablished physical-interaction mechanism that might well aid in developing hypotheses and, eventually, in understanding the biophysical interactions involved. Magnetic-Field Exposure Induced electric fields and currents from exposure to 50-60 Hz and other ELF magnetic fields can be found by solving Maxwell's equations under the same simplifying conditions as those for the electric field (i.e., quasi-static case, large penetration depth)
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From page 49... ...
Representative data for a heterogeneous human model with calculations conducted on a grid of 1.3 cm3 cells and similar calculations for homogeneous rodents are given in Table 2-16 to provide a reference for scaling and a comparison with the currents and fields induced by exposure in the electric field. In all cases, the magneticfield orientation is selected to give maximum values of induced current densities.
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From page 50... ...
Another comparison can be made by considering the average and maximum induced electric fields. For the electricfield exposure, the reduction factor is about 10-7 for the average field and 10-5 for the maximum field as compared with the external electric field.
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From page 51... ...
magnetic fields are equivalent. In comparing the maximum values of either the induced current density or the electric fields, very close environmental levels of electric (4 V/m and 3 V/m)
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