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EMF Measurements, Exposure Criteria, and Dosimet~y STUDYING THE EFFECTS of electric and magnetic fields (EMFs) on organisms involves accurate assessment of exposure to these fields and of the dose that an organism receives as the result of exposure. Exposure is a measure of the field strength of an electric or magnetic field immediately outside an organism over a specific period. Dose is a measure of the induced field strength within an organism over a specific period. The first section of this chapter describes how UTR] characterized the EMFs in the vicinity of the transmitting facilities. Later sections discuss the problems inherent in estimating doses and account- ing for possible effects due to signal modulation. The term "EMF" applies to an alternating field generated by moving charged particles. EMFs are characterized by their wavelength (expressed in meters) and their frequency (expressed in hertz). The wavelength of a field multiplied by its frequency equals the velocity of propagation. The full range of frequencies of EMFs is described as the electromagnetic spectrum. The "extremely-Iow-frequency" (ELF) designation is generally reserved for fre- quencies that range from 3 Hz to 300 Hz. Most equipment used for the gener- ation, transmission, and distribution of electric power in the United States generates EMFs with a frequency of 60 Hz. The Navy's ELF Communica- tions System uses a frequency-modulation principle called minimum-shift keying. In this type of modulation, the frequency is shifted between 72 Hz 21
22 EVALUATION OF ELF ECOLOGICAL MONITORING PROGRAM and 80 Hz (with a center of 76 Hz) depending on whether a code of "one" or "zero" is to be transmitted to a submarine (Zapotosky et al. 19961. The inten- sities of the electric fields are expressed in volts per meter (V/m), and mag- netic fields are expressed in milligauss (mG). Additional information is pro- vided by NIOSH, NIEHS, and DOE (1996~. CHARACTERIZATION OF ELECTRIC AND MAGNETIC FIELDS To characterize the electric and magnetic fields near the ecological moni- toring sites, TITR] measured the spatial and temporal characteristics of the following fields: . A magnetic field in the air and the earth generated by the electric current in the antenna and ground terminals. . An electric field in the earn that is the sum of the fields induced by the magnetic field and the current flowing from the buried ground terminals. · An electric field in the air resulting from the difference in electric potential between the antennas and the earth or created as a byproduct of the electric field in the earth. . The earth's static geomagnetic field. IITRI provided the following dimensions of the EMFs near the transmit- ting facilities (see Table 2-! for instruments used): The ambient 60-Hz resultants EMFs above the earth. 2. The unmodulated 76-Hz resultant EMFs above the earth. 3. The modulated 76-Hz resultant EMFs above the earth. 4. The root-mean-square (rms) values of harmonics of the 60-Hz and 76-Hz EMFs above the earth. ~Resultant" is defined in the following way. The root-mean-square (rms) magnitudes of three rectangular components of the field are determined by either measurement or calculation. For fields that vary sinusoidally in time, the rms magnitude of each component is the zero-to-peak magnitude divided by the square root of 2. The resultant is the square root of the sum of the squares of those three rms values.
EMF MEASUREMENTS, EXPOSURE CRITERIA, DOSIMETRY 23 5. The earth potential differences in two orthogonal directions and, on the basis of this, the resulting electric field in the earth. The ability to measure low-level magnetic fields depends on, among other things, the sensitivity of the instrument used to analyze the magnetic- field probe voltage. According to TTTRT, the magnetic-field measurement equipment was calibrated by measuring the magnetic-field probe voltage output when placed in a lOO-mG magnetic field (Haradem et al. 1994~. It claims that this calibration is valid at smaller field levels on the basis of the fact that the probe, constructed solely of passive components, is known to have an output linear with respect to field intensity. The smallest full-scale sensitivity of UTRI's instnunent (a Hewlett-Packard 3581A) was 0. ~ ,uV, which corresponds to a magnetic-field level of about 0.2 mG. Measurements much lower than that (about 0.02 mG or lower) are not expected to be very accurate. Fortunately, the most-important reported levels used in establishing treatment and control sites and in analyzing ecological data were well above these levels and are expected to be accurate representations of the magnetic fields. However, reported field levels like 0.0002 mG are not expected to be accurate. To eliminate the possibility of contamination of the ecological studies by harmonics or interactions between the power-line frequencies and ELF-antenna frequencies, a spectrum, field strength versus frequency, was measured. All the unwanted signals were found to be at least 30 dB below the level of the ELF-antenna frequencies and were therefore judged not to be contaminating the ecological studies. The rms values of harmonics of the 60-Hz and 76-Hz electric and magnetic fields above the earth were reported to be either below detection levels or "so low as to not be considered a confounder." Spectra measured by ITTR] at the antenna terminals with the transmitter off and with the transmitter on yield data on the ambient 60-Hz fields and the 76-Hz fields (me. Gauger, lITRI, letter to U.S. Navy's Communications Systems Project Office, December 23, 1985~. Although the spectral data reported represent a single day's observation (December 11, 1985), they confirm the statement quoted above.2 Indications of the natural Schumann resonances in the earth's 2The power-line 60-Hz signal is 30 dB below the transmitter 76-Hz signal; and the strongest harmonic of the power-line frequency (at 300 Hz) is at least 60 dB below the transmitter signal. The spectra show harmonics of the ambient power-line frequen- cy out to the 17th harmonic and of the transmitter frequency out to the 11th harmonic. The harmonics of the transmitter are down from the fundamental by 35 dB at 216 Hz and 240 Hz, by 50 dB at 144 Hz and 160 Hz, and by 55 dB at 360 Hz and 400 Hz.
24 EVALUATION OF ELF ECOLOGICAL MONITORING PROGRAM atmosphere and the measured behavior of the harmonics also lend credence to the quality of the observations. A few obvious errors in the labels on the spectra are consistent with the problems of field observations and were easily removed. However, such errors were relatively small. TABLE 2-! Instruments Used by TITR} to Measure EMFs 1 1 An IlTRI-constructed single-ax~s magnetic-field probe with flat frequency response. The output of the sensor was a voltage proportional to the magnetic field. A s~ngle-ax~s electric-field probe that consisted of a spherical sensor with an optical-fiber link to a receiver. The output of this probe was a voltage proportional to the electric field. A probe to measure the electric field in the earth that consisted of two orthogonal electrode pairs. The output was the voltage difference between the electrodes in a pair. A Hewlett-Packar(1 3581A signal-wave analyzer. This device, a frequency-selective rms-calibrated voltmeter with an adjustable bandwidth, was used to measure the voltage output of the three probes mentioned above. An IlTRI-constructed 60-Hz notch filter to filter out the 60-Hz contribution from the modulated 76-Hz measurements. A Walker Scientific FGM-3D! fluxgate magnetometer for measuring the static magnetic field in the earth. An electric-field measurements EMDEX T! magnetic-field meter for long-term measurements of the resultant magnetic field in the frequency range 40-400 Hz. An IlTRI-constructet1 instrument for measuring the electric field in the earth as a function of depth.
EMF MEASUREMENTS, EXPOSURE CRITERIA, DOSIMETRY EXPOSURE CRITERIA FOR SITE SELECTION 25 Any source of electric and magnetic fields (such as the ELF transmitting facility and antennas) creates fields essentially everywhere. As one moves farther from a source, the field intensities become lower, either because of distance or because of attenuation due to obstacles (in the case of electric fields). However, one will then be moving toward other sources, and the EMFs that they generate will increase in intensity as one moves closer. It is therefore not possible to select a control site where there is no exposure to ELF EMFs generated by the Navy's transmitting facility and antennas. It is possible ordy to select sites that have different levels of exposure to the EMFs generated by the antennas and to those generated by other sources, such as power lines. UTR] helped researchers to select study sites for the ecological monitor- ing program by determining whether they were in the treatment or control category. The specific criteria used to determine whether a site was treatment or control were as follows: T(76 Hz) / C(76 Hz) > 10 T(76 Hz) / T(60 Hz) > 10 T(76 Hz) / C(60 Hz) > 10 0.l < T(60 Hz) / C(60 Hz) < 10 where T(76 Hz) is the treatment-site exposure due to the ELF communications system, T(60 Hz) is the treatment-site exposure due to power lines, C(76 Hz) is the control-site exposure due to the ELF communications system, and C(60 Hz) is the control-site exposure due to power lines. In other words, the intensities of the 76-Hz EMFs at a treatment site had to be 10 times as large as the intensities of the 76-Hz EMFs at the control site. In addition, at both treatment and control sites, the intensities of the 76-Hz EMFs due to the antennas had to be 10 times as large as the intensities of the 60-Hz EMFs due to nearby power lines. Finally, the ratio of the intensities of the 60-Hz fields at a treatment and control site had to be between 0. ~ and 10. Those criteria were applied to EMFs in the air and in the earth during
26 EVALUATION OF ELF ECOLOGICAL MONITORING PROGRAM full-power operation of the relevant transmitter antenna. However, there was no evidence to indicate, a priori, that a one-tenth reduction in exposure should lead to a reduction of one-tenth (or less) in the effect that might be observed with full exposure. Variation of field intensities with distance means that each site is exposed to a spatial gradient of intensity, rather than a uniform intensity across the site. Sites were therefore classified according to annual measurements taken at the same point each year. To isolate the effects of the ELF EMFs, paired treatment and control sites were intended to be as alike as possible with respect to ecological vari- ables, including soils, foliage, species abundance, and temperature, depending on the focus of the study. For example, the wetlands study required similar bogs for treatment and control sites, whereas the pollinating insects study required sites with similar flower abundances. Several study teams had diffi- culty in identifying pairs of sites that met both the exposure and ecological criteria, as discussed in Chapter 3. EXPOSURE DATA SUPPLIED TO RESEARCHERS UTR} made available to researchers data on magnetic fields and electric fields in the air, and electric fields in the earth. ITTR} provided extensive data on specific measurements of ELF EMFs at each site to the ecological monitor- ing teams. The purpose of these measurements was to allow the monitoring teams to determine indicators of exposure for different parts of the treatment sites and to attempt to relate indicators to appropriate measures of ecological effect. UTR} also made data about transmitter on and off times available to the ecological monitoring teams. These data could be used to determine whether a site characterized as a treatment site was actually exposed to ELF EMFs from the antenna at any particular time. That is important because the trans- mitter was not on continuously and a treatment site is exposed only when the transmitter is on. USING FORMULAS FOR PREDICTING ELECTRIC AND MAGNETIC FIELDS Electric and magnetic fields can be characterized either by physical mea
EMF MEASUREMENTS, EXPOSURE CRITERIA, DOSIMETRY 27
28 EVALUATION OF ELF ECOLOGICAL MONITORING PROGRAM structions, the formulas provided are satisfactory (provided that antenna volt- age is known). However, in a grove of trees, there is much distortion of the fields, and the formula provided by IITR] would be of no use. Of the formulas provided by IITRI, only the magnetic-f~eld formula was widely used by researchers. This formula was used only in efforts to interpo- late the magnetic fields at points within study sites. For the upland-flora study, measurements of the 76-Hz magnetic field were made at several points near the Wisconsin antenna. Table 2-2 compares measured with calculated fields for an antenna height of 13.7 m and an antenna current of 150 A. The measured and calculated values agree reasonably well. Unfortunately, no measurements of antenna-to-ground voltage that could be used to validate reported measurements of electric fields in air were re- ported by TTTRI. However, this voltage could be estimated in the following way. According to Dill (1984), one design criterion for the ground terminals was achievement of a maximal total ground resistance of 6 ohms for both grounds. On the basis of that number and the assumption that the antenna resistance is much smaller than the ground resistance, the antenna-to-ground voltage at full current (150 A) is 900 V. According to the formula provided by TTTRT, the electric field in the air directly below the antenna would be about 25 V/m. The measured electric field under the antenna for the small- mammal and nesting-bird studies was 1040 V/m. It can be inferred from this result and the theoretical estimates that the measurements are in reasonable agreement. DOSIMETRY As stated above, the EMF quantities relevant to ELF interaction with biologic systems are the exposure (field strength immediately outside the organism over a period of time) and the dose (induced field inside organisms over a period of time). The latter quantities can be expressed in terms of induced electric-field strength, magnetic-field strength and induced current or current density. Dos~metry involves assessment of the magnitude and distribu- tion of induced fields and currents within biologic organisms that are exposed to ELF EMFs. The induced fields and currents not only are functions of the externally imposed EMFs, but are determined by the EMF properties and geometry of the exposed organism and any nearby objects. Such induced fields and currents at ELF should not be expected to constitute the dose.
EMF MEASUREMENTS, EXPOSURE CRITERIA, DOSIMETRY TABLE 2-2 Comparison of Measured and Calculated Magnetic Fields 29 Horizontal Distance 1993 Measured Calculated Site to Antenna, m Magnetic Field, mG Magnetic Field, mG 4T2-13 28 10.3 9.7 4T2-12 58 5.5 5.0 Dos~metric measures of the induced fields and currents were not part of the study design. However, to provide some indications of relative strength of induced electric fields in various biota exposed to 76-Hz EMFs, the com- mittee has performed some analyses with simple models to serve as an index to the induced fields within an organism. The ELF-EMF exposure environ- ment was characterized at control and treatment sites through periodic surveys. This environment included 76-Hz fields produced by the ELF communications system, 60-Hz fields from power lines, and the earth's magnetic field. Be- cause the wavelength at 76 Hz is much longer than the longest dimension of the organism, the quasistatic-field theory can be appropriately applied to calcu- late the induced electric field inside the body of the organism (Michaelson and Lin 1987). Briefly, the calculated results suggest that induced electric fields in in- sects, birds, and small vertebrates are fairly low for exposures to external electric fields up to 5,000 mV/m and magnetic fields up to 50 mG. In con- trast, electric fields induced in hardwood stands by the same EMFs could be substantial. Calculations based on these simple models suggest that the field induced by a vertically oriented electric field in a 25-m tree could be as high as 5,000 mV/m and that induced by a horizontally oriented magnetic field as high as 29.8 mV/m. The applied or impinging electric field would decrease in strength with distance from the antenna wire and because of shielding. However, magnetic-field strength would be attenuated away from the antenna wire only by distance. Therefore, at greater distances from the antenna, the field induced in tree stands by a horizontal magnetic field could become a dominant factor in the resulting dose. (See Appendix B for more detailed information.) Researchers involved in the ecological monitoring program were not asked to estimate doses received by biota from the Navy's ELF transmitting antennas. Only the researchers involved in the upland flora study attempted to do so. Because sufficient information was not available on dosimetry, the
30 EVALUATION OF ELF ECOLOGICAL MONITORING PROGRAM committee determined that it was not possible to extrapolate the monitoring program's findings to other situations that might be comparable with exposure conditions. DIFFERENCES IN EFFECT BETWEEN UNMODULATED 60-Hz AND MODULATED 76-Hz SIGNALS As mentioned previously, the ELF electric and magnetic fields generated by the communications system antennas are frequency modulated between 72 and 80 Hz (with a dominant frequency of 76 Hz), unlike power-line EMFs, which are unmodulated at 60 Hz. Unfortunately, there is little information on the differences between the effects of modulated and unmodulated frequencies. Most of the research undertaken over the last 25 years to understand the bio- logic effects of low-frequency EMFs has focused on exposure to unmodulated EMFs at power-line frequencies of 50-60 Hz (see, for example, Anderson 1990; ORAU 1992; Tenforde 1996; OTA 1989; NRC 1997~. There has been little research on the effects of the modulated 76-Hz signals produced by the ELF communications system. CONCLUSIONS REGARDING EMF MEASUREMENTS UTR! has done a good overall job of characterizing the ELF electric and magnetic fields in the vicinity of the treatment and control sites. In cases in which it became obvious that more information was needed, ITTR] was respon- sive and performed additional measurements. Specific conclusions are as follows: · Although there were some minor questions about instrument design, it appears that the associated errors were small and would lead to no changes in TITRI's conclusions about the measured data. . The spatial and temporal variation of the magnetic fields above the earth near the treatment and control sites has been well characterized. · The spatial and temporal variation of the electric fields above the earth in open areas near the treatment and control sites has been well charac
EMF MEASUREMENTS, EXPOSURE CRITERIA, DOSIMETRY 31 terized. In shielded areas, such as near trees, more extensive measurements were necessary. When requested, these were provided. · The electric fields in the earth depend on local earth conductivity, so they require a more-thorough measurement survey to characterize them. When requested, UTR] provided engineering support for these measurements. · The electric fields in the earth have been examined in light of an- nual changes in earth electric characteristics. Most variations were modest, but there were daily and annual changes in these fields.
