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Nonionizing Electromagnetic Fields and Cancer: A Review

Nonionizing Electromagnetic Fields and Cancer: A Review

The paper by Salvatore et al discusses a very broad, complex subject, which, in some aspects, is quite controversial. The review touches on many different topics but not always in enough detail to provide clarity. Pertaining to the debated link between 50 to 60 Hz power line field exposure and cancer, the authors characterize their article as "... a review of the basic science that points to this possible association [with cancer]." However, the "basic science" that "points to" occupies much of their presentation, while some major types of evidence and reasoning that "points away" is not mentioned.

We agree that the possible human health hazards posed by environmental electric and magnetic fields is an important issue, principally because it concerns so many people. Thus, it is essential to understand what basic science has found to date. An examination of the scientific literature shows that while there are indeed reports of biological effects associated with "weak" 50-60 Hz fields, there are also [1] reports of initial findings not being confirmed by other, independent research groups, and [2] strong theoretical grounds for skepticism. With this in mind, in the space available, we very briefly review some of the evidence on the "points away" side of the intellectual ledger.

Electric and magnetic fields can, indeed, cause biologic effects and induce changes in biochemical metabolic pathways. The response of voltage-gated membrane proteins (channels, ionic pumps) has a solid experimental [1] and theoretical foundation [2]. Indeed, nature has favored electrical signaling as the basis for important interactions and control in many normal biologic processes. The basic question is: what, if anything, happens to these electric signals when they are exposed to external fields from power lines, appliances, and other sources?

From Air to Tissue

In order to address this question, we must recognize the weakness of the coupling between electric fields in air and biological tissue. A reasonable upper limit for an electric field near a powerline is around 10,000 V/m. The coupling through air is so weak that 20 yards away from a power line, the intratissue electric field directly induced is 1 million times as small, ie, approximately E = 0.1! mV/cm. For a 100 micron diameter white blood cell precursor in the bone marrow, this implies a change in the membrane potential of approximately 0.1 µV (0.0001! mV). In contrast, the inescapable thermal fluctuations in the transmembrane voltage have a root mean square value of about 25 µV (2510!V)[3-4]. Furthermore, the magnitude of the transmembrane voltage change normally associated with a change in the conductance of a voltage-gated channel is typically approximately 1 mV.

To make a long story short, for air fields less than 10 kV/m (extracellular electric fields in the tissue less than approximately E = 10 mV/cm) it is difficult to imagine that the external field can compete with inescapable stochastic fluctuations in biochemical processes [3-5]. Thus, any extra-cellular electric field less than 10! mV/cm induced by 50-60 Hz magnetic fields can be considered "weak", and any biological effect attributed to "weak" fields deserves intense scrutiny above and beyond the normal diligence given any experimental result.

Magnetic fields are not attenuated so strongly. However, for comparison, the 50-60 Hz magnetic field needed to induce 10! mV/cm within the human body is greater than about B = 0.01 T (100 G), while the field associated with a typical power line is only 0.5 G. Thus, any reported effects of "weak" fields cannot be easily associated with typical voltage-gated proteins that nature has so heavily favored during evolution [6].

Also missing from the Salvatore et al review are the "points away" discussions based on other fundamental physical arguments [3-6]. In the case of magnetically sensitive radical pair reactions, the article fails to acknowledge that slow variations due simply to walking in the earths geomagnetic field cause changes larger than those due to powerline magnetic fields [5]. For example, a human approaching a large steel structure, such as a barge, experiences a magnetic field change of about 7 microtesla (0.07 G), about 15% of the earth's field. Thus, reports of biologic effects involving the radical pair mechanism at magnetic fields smaller than about 10 microtesla (0.1 G) are difficult to accept as support for potential hazards. These and other published arguments are highly relevant to scientific progress towards understanding this complex issue, but are not mentioned at all by the authors.

Theory has an important role in the current debate. A particularly promising approach is reverse engineering-ie, taking the design components appropriate for biological organisms and attempting to construct the most sensitive device for field sensing possible. This approach will identify not only fundamental limits but also biological structures likely to be affected by weak electromagnetic fields, if they do, in fact, exist. Such well-known biological examples as sharks and skates, which can sense direct-current (DC) electric fields in water of 10 !V/cm in water, and birds and honeybees, which use differences in the earths magnetic fields for navigation, serve as positive controls.

Some Misleading Points

In some aspects, such as the comparison of 50-60 Hz fields on the scale of photon energy, the presentation of Salvatore et al is grossly misleading. The photon energy is a measure of energy stored in the field oscillation which is important only when the period of oscillation is short in comparison to the time required for field transmissions. The period for a 50-60 Hz sine wave is approximately 20 milliseconds, over which time the field propagates 4,000 miles. Thus, for distances less than about approximately 10,000 miles, quantum mechanical considerations of photon energy are not important in the field energy transmission. For the purposes of the debate on whether electromagnetic fields pose a health hazard, it is much more appropriate to consider the electromagnetic field as a thermodynamic parameter, such as temperature or pressure, rather than in terms of photons. The term "radiation," even when qualified by the adjective "nonionizing," is scientifically inaccurate when applied to this discussion, and is particularly inappropriate because of the public's perception of "radiation."

Another erroneous concept is the discussion of the energy of the field. Literally, the energy in the field is enough to energize a city. The question is how strong is the coupling through air to specific biochemical structures that can alter biochemical processes. In order to foster a rational, scientifically balanced discussion of these issues, it is important to assiduously avoid these types of errors.

A Point of Disagreement

Regarding field-induced carcinogenesis, we do not agree with Salvatore et al that nonpropagating fields are incapable of causing molecular damage. Electroconformational denaturation of proteins with DC fields have been documented [7]. However, such strong tissue fields are very unlikely without direct mechanical contact with the powerline. Thus, we agree that ambient fields are more likely candidates for tumor promoters. Powerline 50-60 Hz electric fields are excluded from the cytoplasm of all but the largest cells (ie, skeletal muscle and nerve cells which are larger than their electrical space constant). The site of strongest interaction is the cell membrane [3,8]. Thus, the plasma membrane is the most probable site of interaction for commercial power-frequency electric fields, and electric field interaction with plasma membrane processes involved in growth control is an essential consideration.

Finally, there have been important experimental studies which have sought to reproduce some of the widely discussed "gene expression" studies [9,10], but no effect of "weak" magnetic fields was found [11,12]. The combination of some experimental studies that show no effects and theoretical grounds for doubting the involvement of major classes of biophysical mechanisms strongly suggests that scientific understanding of this controversial topic is incomplete. Thus, we agree with the general conclusion of Salvatore et al that "... funding of... research is essential to answer the questions of possible carcinogenic or other health effects of nonthermal, nonionizing electromagnetic fields."

References

1. Hille, Bertil: Ionic Channels of Excitable Membranes Chapter 1, Sinauer Associates Inc, Sunderland, 1984.

2. Astumian RD, Chock PB, Tsong TY, et al: Effects of oscillations and energy-driven fluctuation on the dynamic of enzyme catalysis and free-energy transduction. Phys Review 39:12:6416-6435, 1989.

3. Weaver JC, RD Astumian: The response of cells to very weak electric fields: the thermal noise limit. Science 247:459-462, 1990.

4. Adair RK: Constraints on biological effects of weak extremely-low-frequency electromagnetic fields. Phys Review 43:1039-1048, 1991.

5. Bennett Jr, WR: Health and low-frequency electromagnetic fields. Yale University Press, 1994.

6. Astumian RD, Weaver JC, Adair, RK: Retification and signal averaging of weak electric fields by biological cells. Proc Natl Acad Sci 92:3743-3940, 1995.

7. Chen W, Lee RC: Altered ion channel conductance and ionic selectivity induced by large imposed membrane potential pulse. J Biophys 67: 603-612, 1994.

8. Gaylor DC, Prakah-Asante K, Lee RC: Significance of cell size and tissue structure in electrical trauma. J Theor Biol 133:223-237, 1988.

9. Goodman R, Bassett CAL, Henderson A: Pulsing electromagnetic fields induce cellular transcription. Science 220:1283-1285, 1983.

10. Goodman R, Henderson AS: Sine waves enhance cellular transcription. Bioelectromagnetics 7:23-29, 1986.

11. Saffer JD, Thurston SJ: Cancer risk and electromagnetic fields. Nature 375:22, 1995.

12. Lacy-Hulbert A, Wilkins RC, Hesketh TR, et al: Cancer risk and electromagnetic fields. Nature 375:23, 1995.

 
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