The electric power industry distributes electricity at 60 Hz (60 complete on-off cycles

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1 Low frequency fields The electric power industry distributes electricity at 60 Hz (60 complete on-off cycles per second) in North America. Much of the rest of the world uses 50 Hz. This is so low a frequency that it is easily possible to see the flicker in electric lights, and so the term extremely low frequency (ELF) is used to describe it. Extremely low frequency electric fields Electric fields are measures of the electric force per charge and have units of newtons per coulomb, or, alternatively, volts per meter. They indicate the influence of the presence of electric charges or charge distributions on other charges. If the electric field E is known at some point in space, then the force on a charge Q at that location is F = QE. Sharks are known to respond to small electric fields, so human sensitivity is possible. The former Soviet Union has set exposure limits for its industrial workers, and people have been known to experience painful skin sensations when they are in regions where the electric field is greater than 15 kv/m. (166) Researchers in the United States found cellular effects at an equivalent external field of 250 kv/m. (168) Physicist R. Adair, a prominent expert, has argued that for constant or ELF 60-Hz electric fields of the electric system less than about 300 V/m (smaller than that found in houses) there are no possible effects. (169) Two arguments support Adair s conclusion: 1. A field of 300 V/m is somewhat larger than the naturally-occurring mean ground-level electric field, to which human beings have always been exposed. 2. The fields everywhere in the cells (of magnitude ~0.020 V/m) are thousands of times smaller than thermal fluctuating fields. According to Bennett, (169) for example, the field

2 Energy, Ch. 8, extension 6 Low frequency fields 2 caused inside the head of a person standing barefoot on the wet tracks of an electric railroad, a terrible idea given the external field of 12 kv/m, would be only 80 µv/m. Virtually all physicists would agree with the view expressed by Adair: (169) Any effects on the cell level of fields in the body generated by weak external ELF fields will be masked by thermal noise effects and, hence, such fields cannot be expected to have any significant effect on the biological activities of the cells. Extremely low frequency microtesla magnetic fields Magnetic fields also indicate the effect of one magnet (or arrangement of circulating charges creating the field) on another magnet or a moving charge. The magnetic force on a segment of wire of length L carrying a current i in a region of magnetic field B is F = ilb sin θ, where θ is the angle between the current direction and the magnetic field direction. In the United States and Europe, concern has focused more on the possible dangers of the magnetic field than the electric field. Several studies have suggested evidence that leukemia and brain cancer in children are associated with home electricity, but these studies have not been replicated and one author was found to have fabricated the data he used. (170) As with electric fields, the energy associated with magnetic fields from electric power is so small that it cannot directly affect the cell by ionization (it is 10 billion times too small to do that). People also wonder whether magnetic fields from power distribution, which are only a few microtesla, could possibly have any effects on cells. Research has shown that honeybees and bacteria have cells containing magnetite, a form of iron (Fe 3 O 4 ) and can consequently be affected by extremely small magnetic fields. (171,172) Humans are known to have magnetite in certain brain cells. (169,171) Also, use of electric blankets and other household appliances has been correlated with

3 Energy, Ch. 8, extension 6 Low frequency fields 3 heightened risk of cancers, (173) and if the electric fields cannot cause problems, the magnetic field might. In contrast to external electric fields, the external magnetic fields can easily penetrate all parts of the cell; if they fluctuate, the magnetic fields can induce electric fields there that could have biological effects. Calculations by Adair and others (169) show that for cells not containing magnetite, the induced electric fields are negligible compared to thermal fluctuations. Earth s magnetic field is very small, 50 µt, so static fields smaller than this must be biologically harmless. Adair (169) also shows that the field caused by walking through Earth s natural magnetic field is comparable to that of a 4 µt 60-Hz alternating field. We may infer that since humans have walked for all their history, an artificial AC field of 4 µt is also benign. TABLE E Measured Magnetic Fields 5 cm from Various Sources Appliance Magnetic Field (µt) Electric blanket 3.4 Fan 3.8 Toaster 6.2 Clock radio 18 TV 21 Hair dryer 30 Desk light 43 Microwave 140 Source: B. Taylor, A Simple Experiment to Survey Low-Frequency Magnetic Fields in the Student s Environment, AAPT Announcer 23(2), 93 (1993). Thus the allowed physics of any deleterious effect that could occur would have to stem from static fields greater than 50 µt or oscillating fields greater than 4 µt affecting hematite-bearing cells. Such human exposures are rarely exceeded by the electricity

4 Energy, Ch. 8, extension 6 Low frequency fields 4 distribution system. Table E06.1 lists measurements made by college students of magnetic fields of ordinary electric appliances. Any risks are probably exceedingly small. The U.S. has had a national electricity delivery system for about three-quarters of a century, and any effects should have shown up as a plague (so far unobserved) of various cancers correlated to exposures. No one has even convincingly shown that electrical industry workers (who have much greater than average exposure) have a higher cancer rate than the general public. (170) A report from the Office of Technology Assessment (OTA) (174) characterized the mechanisms of possible harm as those that could arise from modulation of ion flows, interference with DNA synthesis or RNA transcription, interaction with the response of cells to various other agents, and interaction with biokinetics of cancer cells. One might also argue that if fields could affect hormone production (melatonin, for example), they might cause medical effects. Many studies have been done and all have failed to see the effects of such stress on health (see, for example, Brain et al. (175) ). Small studies have sometimes seemed to show effects, but these vanish when larger cohorts are studied. Other research suggests that the mechanism for small-field effects is that the systems are not in thermal equilibrium. (176) Animal studies of risks of leukemia are overwhelmingly negative according to the report on a workshop on childhood leukemia (low fields were a suspected leukemia risk factor). (177) The National Academy panel on possible effects of electromagnetic fields said No evidence links contemporary measurements of magnetic-field levels to childhood leukemia. (178) The Long Island Breast Cancer Study Project reported no evidence of a link between breast cancer and low frequency fields. (179) A Swedish study found no effect of low-frequency fields on heart

5 Energy, Ch. 8, extension 6 Low frequency fields 5 disease (except perhaps at highest exposure levels) (180) and a study of over 35,000 Southern California Edison utility workers showed no effect whatsoever. (181) All these subjects remain active areas of research despite the negative results because of pressure from activist groups. (182) Despite the lack of evidence of deleterious effects, it is probably prudent to turn off electric blankets before getting into bed (see Reference 173). It also cannot hurt to keep a reasonable distance (10 cm, say) from small appliances except for short times. Magnetic fields decrease very rapidly with increasing distance from the source and if there were any effect, we can presume that it would occur with greater strength the longer the exposure. Can microwave ovens be dangerous? Many people believe that microwave ovens constitute a health hazard and that they are a source of ionizing radiation (radioactive decay products). In my own group s research, we have probed the beliefs of students at Ohio State University and found that some believe microwave ovens give off gamma rays or x rays. (183) In fact, about half of the students surveyed agreed or strongly agreed with the statement that microwaves themselves were hazardous to health, as may be seen in Fig. E Students who volunteered to be interviewed rated microwave ovens as a hazard second only to an x-ray machine. One student said dubiously, You re eating what it does to your food. Students have apparently been confused by teachers, as one interview subject said I learned in school that they [microwave ovens] give off radioactivity...

6 Energy, Ch. 8, extension 6 Low frequency fields 6 photons that hit the body are hazardous to a person's health. 60% 50% 40% 30% 20% Microwave X ray Gamma ray 10% 0% strongly disagree, disagree neutral agree, strongly agree don't know, no answer Fig. E Gamma rays, x rays, and microwaves are seen as hazardous to health by roughly equal proportions of students. Sample sizes are N γ ray = 201, N x ray = 103, and N microwave = 304. Microwave radiation is nonionizing, while both x rays and γ rays are ionizing radiation. X rays (wavelengths of nanometers or tenths of nanometers) have energies typically in the kev range (see the energy converter software elsewhere on this CD). Gamma rays occupy the region of the spectrum with wavelengths less than around m, or 1 pm. Gamma ray energies therefore are typically of order MeV or above. Clearly, both x rays and γ rays have sufficient energy to ionize tissues through which they pass. The blackbody radiation intensity from a microwave oven at a temperature around 300 K is negligible in the x ray and γ ray regions of the spectrum. Microwave ovens are cavities for which the microwave frequency (2.45 GHz) is not tuned to the cavity size, and thus

7 Energy, Ch. 8, extension 6 Low frequency fields 7 the ratio of x ray and γ ray emission is probably even lower than one would predict on the basis of the blackbody spectrum. And, of course, γ rays are higher in energy than x rays, and so are even less likely to be emitted than x rays. Introductory student thinking may represent a widespread misconception among the populace about the sort of radiation emitted from microwave ovens. Further, because of shielding, very little radiation of any kind at all is emitted by microwave ovens in normal operation. Fig. E Ionization energies of atoms. Fig. E The electromagnetic spectrum. The dashed line shows the characteristics of the microwave oven in wavelength (λ = m) and energy (E = x 10-5 ev).

8 Energy, Ch. 8, extension 6 Low frequency fields 8 As a crudest estimate of the danger to health from microwave ovens, suppose that we would have to ionize atoms to have any effect on health. The smallest ionization energies are around 5 ev (Fig. E08.6.2) ranging up to around 25 ev. To get a photon energy of 5 ev, the wavelength would need to be around 250 nm, in the ultraviolet region of the spectrum (Fig. E06.5.3). The bond strengths of ionic and covalent molecular bonds are typically 4 to 10 ev, roughly the same scale as the atomic ionization energies, while microwaves have energies of only around 10 µev. The photon energies needed to break such ionic and covalent bonds is similar to the energy needed to ionize atoms, again corresponding to ultraviolet light of wavelengths around 120 to 350 nm, compared to microwave wavelengths of centimeters (Fig. E08.6.3). Of course, to refine our thinking, we know that there are biological effects in animals at lower energies (tanning in sunlight, for example). The relevant energy scale for these effects is that of the hydrogen bonds used in proteins, RNA, and DNA, which are around 0.5 to 1 ev. This corresponds to wavelengths of the order of magnitude of micrometers. The frequency used in magnetrons in microwave ovens is 2450 MHz, corresponding to a wavelength of about 12 cm. The relevant energy for microwaves of 12 cm wavelength is E = hc λ ~ 1240 ev nm/108 nm ~ 10-5 ev (Fig. E08.6.3) or 10 µev, far below that required to cause such deleterious effects to hydrogen bonds. Any bond energies reputed to be affected are far greater than the scale of normal thermal fluctuations, k B T ~ ev = 25 mev. It is clear from Fig. E that microwave energies never exceed a few mev, far below the energies characteristic of thermal fluctuations.

9 Energy, Ch. 8, extension 6 Low frequency fields 9 Since both x rays and γ rays are ionizing, they can do great damage to cells in living organisms, disrupting cell function. Ionizing radiation is dangerous because it ionizes atoms and molecules in its path. Microwave ovens emit no x rays or γ rays and thus cannot cause ionization, cannot break molecular bonds typical of the molecules found in living things, cannot even attain energies typical of thermal fluctuations. Nonionizing radiation can be dangerous if the field intensities are great enough. To the contrary, the intensity of the fields near microwave ovens are quite small, with B of order 10 mt and E of order 1 kv/m, similar to those for a laptop computer as seen in Table E Microwave ovens, which produce low-intensity, nonionizing radiation, therefore constitute little risk in normal use.