Electromagnetic Fields - ACS Publications - American Chemical Society

Downloaded by NORTH CAROLINA STATE UNIV on March 15, 2013 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/ba-1995-0250.ch003

3

Typical Electric and Magnetic Field Exposures at Power-Line Frequencies and Their Coupling to Biological Systems Frank S. Barnes Department of Electrical and Computer Engineering, University of Colorado, Boulder, CO 80309-0425

In this chapter we reviewfivemajor sources of electric and magnetic fields. We then present some typical exposures as measured with personal dosimeters. The major sources of electric and magnetic fields to be discussed include power lines, electric railways, appli ances, ground currents, and naturally generatedfields,both by the body and the Earth's atmosphere. We also review the coupling of thesefieldsto the body in order to estimate how the externally gen eratedfieldscompare with those that are generated by the body it self.

P

ART OF THE PROBLEM OF DETERMINING i f the fields generated by the distribution of A C (alternating current) power constitute a safety hazard requires an understanding of what the sources of the fields are and how they are coupled to the body. A knowledge of the sources aids in estimating the exposure that humans receive under a variety of conditions near the sources. 0065-2393/95/0250-0037$12.00/0 © 1995 American Chemical Society

In Electromagnetic Fields; Blank, M.; Advances in Chemistry; American Chemical Society: Washington, DC, 1995.

38

ELECTROMAGNETC I FIELDS


Naturally Generated Fields The electromagnetic power radiated from natural sources as a function of frequency is shown in Figure 1. These fields can be thought of as being generated in a spherical capacitor between the surface of the Earth and the ionosphere, which begins at about 50 km above the surface. This capacitor is charged by about 100 lightning strokes per second from thunderstorms worldwide. At DC (direct current) the average value of the electric field at the surface of the Earth is about 130 V/h% and it can range up to 3000 V/hi near an active thunderstorm (/). These fields are not uniform, and local values can vary widely with temperature and humidity. The polarity of the fields can reverse in minutes. Thunderstorms as far as 50 km away can affect the local field values. The magnitude of the field generated by the atmosphere decreases rapidly as the frequency in-

ANSI 1982 USSR Occup.

USSR G. POPULATION

1

10

2

10

4

10

6

10

8

10

10

10

12

FREQUENCY (Hz)

Figure L Power emittedfromnatural sources as a function offrequency and standard exposure limits. The average atmospheric noise power density incid on the Earth (reproduced with permissionfromreference 27. Copyright 1982 Institute of Electrical and Electronics Engineeres, Inc.) is compared with th 1982 American National Standards Institute (ANSI) and USSR exposure sta dards. USSR Occup refers to the allowed occupational exposure levels; USSR Population refers to the general population exposure levels. The average sign level for 15 U.S. cities is designated by the asterisk at 10~ W/m power den and 10 Hzfrequency.(Reproduced with permissionfromreference 28. Copyright 1989 Williams and Wilkins.) 5

9


2

3.

39

BARNES Typical Electric and Magnetic Field Exposures

3

creases. A t 60 Hz the average value of the electric field is reduced to about 10 V/m. The corresponding average magnetic fields are about 5 χ 10" Τ at D C , and (5 χ ΙΟ" Τ)/(Δ/)' at 60 Hz, where Δ / i s the bandwidth. These fields are generated by the Earth's rotation around à liquid core, by the currents flowing in the ionosphere, and (particularly above about 6 Hz) by lightning activity. The most spectacular manifestation of the ionospheric currents is the aurora borealis. Both these naturally generated fields at 60 Hz are small compared with the fields that are measured in a typical home or workplace. A second major source of naturally generated fields is the body itself. The body uses electrochemical signals to control the movement of muscles and to transmit information from one part of the body to another. Typical electric signals for the heart, nerves, and muscles are shown in Figure 2 (2). The signals shown in Figure 2 also serve as a source of electric and magnetic fields that propagate through the rest of the body. Two of the most useful applications of these fields are the electrocardiogram (ECG) and the electroen5


12

/2

+50 LU

BW 0-10,000 Hz

M

LU >

-70

ω

ο

0.5 TIME (sec)

Figure 2. Transmembrane potentials (V ) innerve and muscle. Β Wis bandwidth used for measuring the signal (Reproduced with permissionfromreference 2. Copyright 1992.) m



40


cephalogram (EEG). The E C G is generated by the heart. Typical E C G signals have a peak value of about 1 mV with repetition rates from 45 to 150 beats per minute. The E E G signals are generated by the activity of brain cells. Typical E E G signals are about 30 μν between electrodes spaced a few centimeters apart with peak signals up to 50 μν at alpha rhythms around 10 Hz. Another source of electric signals is the stomach, which generates signals with frequency compo nents of 2-10 cycles per minute. A l l of these signals are typically chaotic in na ture, and this condition means that the beat-to-beat interval demonstrates sub stantial variations in time. The intervals between the beats change with the needs of the body and with changes in activity. Additionally, adaptive processes in the body lead to changes in the response of the same nerve fibers over time for re petitive activity (3). This variability is in contrast to power-line signals, which are very stable at either 50 or 60 Hz. We will compare these naturally occurring signals with externally gener ated signals in order to estimate the likelihood that the external fields will have significant biological effect.

Externally Generated Fields Power Lines. The power lines that distribute most of our electrical energy are one of the most obvious sources of externally generated electric and magnetic fields. This distribution system usually consists of transmission lines and distribution lines. The transmission lines are the high-voltage lines that are used to transport power over long distances. They operate at voltages in the range from 100 to 700 kV. The currents in these lines range from 100 to 1000 A and typically average from 300 to 400 A . A variety of configurations are used for transmission lines. Among the most common are three-wire and six-wire configurations. A typical electric field distribution for a transmission line is shown in Figure 3 (4). In Figure 3, the electric field (E) is measured at ground

Figure 3. Electricfields(E) as a function of distance from 525-kV transmission at ground level. (Reproduced with permission from reference 29. Copyright 1987 Williams and Wilkins.)


3.

BARNES

Typical Electric and Magnetic Field Exposures

Table I. Maximum Electric Field Intensities at Midspan Under Various Configurations and Voltages of Electric Power Transmission Lines


Highest System Voltage (kV) 123 245 420 800 1200

Electric Field Strength Under Line at Midspan (kV/m) 1-2 2-3 5-6 10-12 15-17

SOURCE: Reproduced with permissionfromreference 33. Copyright 1982.

level under a 525-kV transmission line. The three electric phases (i. e., conductors) of the line were located in a horizontal configuration 10 m above the ground and with a phase-to-phase spacing of 10 m. The drawing gives electric field data as a function of lateral distance measured from the centerline of the transmission line. The maximum electric fields at midspan for a number of line configurations are given in Table I (4). In most areas the maximum electric fields are limited to 10" A / m are biologically active in such functions as bone growth (15) and C a transport across membranes (16). With an applied field of 200 VAn, the peak induced current would be expected to be about 40 nA/cm or 4 χ 10^ A / m . This value is less than current densities that have been shown to be important in ex periments on bone growth and the injection locking of pacemaker cells. How ever, the modeling that has been done thus far does not take into account the full complexities of the variations in the resistivity between fat, muscle, bone, and blood vessels. If the blood in the vessels is assumed to have a low resistance, then the vessels can work like antennae to concentrate potential differences into a small region of space. At 10 VAnthe total voltage drop across a person 2 m tall


6

2

2

2+

2

2

180 kV/m

Figure 12. Induced current densities for a vertical 60-Hz 10-kV/m electric field. (Reproduced with permissionfromreference 29. Copyright 1987 Williams and Wilkins.)


52


is about 2 V . If as much as a few millivolts is concentrated across a cell mem brane, then the biological effects could be significant. Voltages of a few milli volts can change the firing rate of pacemaker cells. Sharks can detect electric fields in the range of 5 χ 10" VAn using long channels that concentrate the fields across a relatively small number of membranes (7 7). Faraday's law for induction enables the calculation of the induced currents generated by a time-varying magnetic field. This calculation has been done for simple ellipsoidal models in a uniform field and for homogeneous materials, as illustrated in Figure 13. Again, induced current densities for fields of 1 μΤ calcu lated on the basis of average conductivity are 2 or 3 orders of magnitude smaller than are known to be important. However, the calculations have not yet been attempted for complex models, in which the variations in the resistivity between fat, muscle, bone, and blood vessels are included along with their complex ge ometry. For an extreme case in which the blood vessels are short circuits and concentrate all the induced voltage across a single piece of tissue or membrane, as much as 2 mV might be generated across the membrane from a time-varying magnetic field of 1 μΤ at 60 Hz, which is just large enough to be significant for exposures on the order of 1 s in changing the firing rate of pacemaker cells. With respect to the second mechanism involving both the A C and D C magnetic fields, the evidence is much less definitive. Small magnetite particles have been discovered in leukocytes and other human tissue (18). These particles in a time-varying magnetic field could apply torque directly to membranes, af fecting the opening and closing of channels, and, thus, the behavior of cells. The


7

0.2 m HORIZONTAL

ELLIPSOIDAL

VERTICAL

Β FIELD

APPROXIMATE

Β FIELD

Figure 13. Induced current directions in ellipsoidal proximation to a human. J is current density. (Reproduced with permissionfromreference 32. Copyright 1990 Institute ofElectrical and Electronics Engineers, Inc.)


3.


53

force (F) on a magnetic particle with a dipole moment m i n a magnetic field gradient (VB) is given by F = m-VB The saturation magnetization for these particles, which are 20-30 n m o n a side, leads to magnetic moments that range from 10~ to 5 χ 10" A-m . If these par ticles are arranged into chains in which the moments of each crystal are strongly coupled to each other, then the torque (7) on the resulting magnetic dipole is given by


15

13

2

Τ = |m||£|sin where φ is the angle between the dipole and the applied flux density, B. Also, some evidence suggests (19-26) that these fields at the cyclotron or Larmor fre quencies of important ions can be biologically important, i f these signals are applied in a coherent manner for periods of time on the order of minutes or longer. Thus, the future is likely to show that we will need to characterize both the A C and D C magnetic fields, the angle between them, and the length of expo sure in order to be able to assess the importance of the exposure to magnetic fields for a given biological system.

References 1. Polk, C. In Biological and Clinical Effects ofLow-Frequency Magnetic and Electric Fields; Llaurado, J. G.; Sances, Α., Jr.; Battocletti, J. H., Eds.; Thomas: Springfield, IL, 1974;p21. 2. Wachtel, Η., University of Colorado, private communication, 1992. 3. Barnes, F. S. BioelectromagneticsSuppl.1992, 1, 67-87. 4. Zaffanella, L. E. "Survey of Residential Magnetic Field Sources—Interim Report"; Research Project No. 2942-06; Electric Power Research Institute, Palo Alto, CA, 1992. 5. "Corona and Field Effect"; computer program (public domain software); Bonneville Power Administration, U.S. Department of Energy: Vancouver, WA, 1978. 6. Zaffanella, L. E. "Magnetic Field Management, Overhead Power Lines"; Electric Power Research Institute, EMF Science and Communication Seminar, San Jose, CA, October 15-19, 1991. 7. Kaune, W. T.; Zaffanella, L. E. In Proceedings of the IEEE Transmission and Dis -tribution Conference; The Institute of Electrical and Electronics Engineers, Inc.: Piscataway, NJ, 1991, pp 735-742. 8. Barnes, F. S.; Banerjee, A. K.; Harwick, P. J., unpublished observations. 9. Dietrich, F. M.; Feero, W. E.; Jacobs, W. L. "Safety of High Speed Guided Ground


54

10. 11.


12.

13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.

31.

ELECTROMAGNETC I FIELDS Transportation Systems"; final report to the U.S. Dept. of Transportation Federal Railroad Administration Office of Research and Development. U.S. Government Printing Office: Washington, D. C; DOT/FRA/ORDL-93-07, DOT-VNTSC-TRA -93-13. Mader, D. L.; Barrow, D. Α.; Donnelly, Κ. E.; Scheer, R. R.; Sherar, M . D. J. Bioe -lectromagn. Soc. 1990, 11(4), 283-297. Barnes, F. S.; Savitz, D.; Wachtel, H.; Fuller, J. Bioelectromagnetics 1989, 10, 1321. Buffler, P. A . "Future Epidemiologic Studies of Health Effects of Electric and Mag -netic Fields"; Research Project No. 2964-11; Electric Power Research Institute, Palo Alto, CA, 1992. Nancy Wertheimer, private communication. Kaune, W. T.; Phillips, R. D. Bioelectromagnetics 1980, 1, 117. McLeod, K. J.; Lee, R. C.; Ehrlich, H. P. Science (Washington, D.C.) 1987, 136, 1465-1469. Walleczek, J.; Liburdy, R. P. FEBS Lett. 1990, 271(12), 157-160. Kalmijn, A . D. In Sensory Biology ofAquatic Animals; Atema, J.; Fay, R. R.; Pop -per, A. N.; Tavolga, W. N., Eds.; Springer-Verlag: New York, 1987; Chapter 6. Kirschvink, J.; Kirschvink, A . K.; Diaz-Ricci, J. C.; Kirschvink, S. J. Bioelectro -magnetics Suppl. 1992, 1, 101-115. Liboff, A. R. J. Biol. Phys. 1985, 13, 99. Durney, C. H.; Rushforth, C. K.; Anderson, A . A . Bioelectromagnetics 1988, 9, 315-336. Bawin, S. M . ; Adey, W. R. Proc. Natl. Acad.Sci.U.S.A. 1976, 6, 1999-2003. Blackman, C. F. In Interactions Between Electromagnetic Fields and Cells; Chiabrera, Α.; Nicolini, C.; Schwan, H. P., Eds.; Plenum: New York, 1985. Blackman, C. F.; Benane, S. G.; Rabinowitz, J. R.; House, D. E.; Joines, W. T. Bioelectromagnetics 1985, 6, 327-337. Liboff, A . R. In Interactions Between Electromagnetic Fields and Cells; Chiabrera, Α.; Nicolini, C.; Schwan, H. P., Eds.; Plenum: New York, 1985. McLeod, B. R.; Liboff, A . R. Bioelectromagnetics 1986, 7, 177-189. Durney, C. H.; Rushforth, C. K.; Anderson, A . A . Bioelectromagnetics 1988, 9, Smith, E. In Proceedings of the IEEE Symposium on Electromagnetic Compatibil -ity; Institute of Electrical and Electronics Engineering, Inc.: Piscataway, NJ, 1982. Barnes, F. S. Health Phys. 1989, 56(5), 759-766. Tenforde, T. S.; Kaune, W. T. Health Phys. 1987, 53(6), 585-606. "Biological Effects of Power Frequency Electric and Magnetic Fields"; Background Paper OTA-BP-E-53, U.S. Office of Technological Assessment; U.S. Government Printing Office: Washington, DC, 1989. Koontz, M . D.; Mehegan, L. L.; Dietrich, F. M . ; Nagda, N. L. "Assessment of Chil -dren's Long Term Exposure to Magnetic Fields" (The Geomet Study); Final Report No. TR-101406, Research Project No. 2966-04; Electric Power Research Institute: Palo Alto, CA, 1992.


3.


55


32. Barnes, F. S. IEEE Trans. Magn. 1990, 26, 5. 33. Hauf, R. In Nonionizing Radiation Protection; Suess, M . J., Ed.; WHO Report No. ISBN 92-890-1101-7, European Series No. 10; WHO Regional Office for Europe: Copenhagen, Denmark, 1982; Vol. VIII, pp 175-188. 34. Creighton, J. L; Banks, R. S.; Duening, T. "Sourcebook for Utility Communications on EMF"; Technical Report No. TR-100580; Research Project 2955-07; Electric Power Research Institute: Palo Alto, CA, 1992. 35. Gauger, J. R. "Household Appliance Magnetic Field Survey"; Technical Report No. E06549-3; IIT Research Institute: Chicago, IL, 1984.

RECEIVED for review March 8, 1994. ACCEPTED revised manuscript February 2, 1995.


Electromagnetic Fields - ACS Publications - American Chemical Society

Recommend Documents