Concerns have been raised about the possible biological effects of non-onizing radiation since at least the late 1950s with respect to radar, other radio, and microwave sources.More recent concerns have arisen about the potential effects of low-intensity fields, including low frequency fields from the electric power generating, transmission, and distribution system and the devices it energizes, as well as intermediate, radio-frequency (RF), and higher-frequency radiation from devices such as cell phones, broadcast antennas, Wi-Fi, security monitors, and so forth. These are concerns about the direct effects of radiation on humans or other organisms. They are distinct from the electromagnetic compatibility issues that concern interference by the fields from one device with the function of another, though human health can be indirectly affected by electromagnetic interference with t he function of medical devices, including hospital equipment or pacemakers.
Because of the difficulties in establishing the direct bio- logical effects of long-term low-level exposures, the lack of an understood mechanism, and difficulties in obtaining re- producible results, the guidelines for exposure limits have been set based on relatively short- term exposures (minutes) that show clear-cut damage with the addition of a substantial safety factor. The cur- rent guidelines from the U.S. Federal Communications Commission (FCC) for limiting exposures in free space to the general public for the frequency range 100 kHz–100 GHz are given in Table 1. These guidelines are based on American National Standards Institute (ANSI) and IEEE recommendations. For cell phones, the specific absorption rate (SAR) is limited to phenomena are often hours or days. The most favored proposed mechanism for effects from low-level, long- term exposures involves radicals, such as super oxide O2 *, NOx, and H2O2, which is readily converted into the radical OH-, molecules with unpaired electron spins that are highly reactive. These molecules are both signal- ing molecules and molecules that can cause damage to important biological molecules, such as lipids and DNA. Damages, such as aging, cancer, and Alzheimer’s, are associated with radi1.6 W/kg averaged over 1 g of tissue. These limits have been set based on providing a significant safety factor over exposure levels known to cause damage, where the primary damaging mechanism is heating and an increase in temperature. At low frequencies, the limits are based on induced current densities that would excite nerve firing, and the permissible exposures recommended by IEEE C95.6 are shown in Table 2. The International Commission on Nonionizing Radiation Protection (ICNIRP) sets electric field exposure limits at 50 Hz to 5 kV/m and magnetic flux density limits at 100 nT. It also sets guidelines for general public exposures in the frequency range 3 kHz–10 MHz at E = 83 V/m, B = 27 nT and a whole-body SAR = 0.08 W/kg, and 1.6 W/kg over 1 g.
In general, environmental exposures at any frequency do not exceed these guidelines, especially for the general pub- lic. Instances of occupational exposures approaching or exceeding the guidelines are less uncommon . However, the time constants for cell growth cycles and many other growth phenomena are often hours or days. The most favored proposed mechanism for effects from low-level, long- term exposures involves radicals, such as super oxide O2 *, NOx, and H2O2, which is readily converted into the radical OH-, molecules with unpaired electron spins that are highly reactive. These molecules are both signal- ing molecules and molecules that can cause damage to important biological molecules, such as lipids and DNA. Damages, such as aging, cancer, and Alzheimer’s, are associated with radical concentrations that are elevated for extended periods of time . In this article, we present the possible theoretical mechanisms and experimental data that show long-term exposures to relatively weak static, low-frequency, and RF magnetic fields can change radical concentrations. As a con- sequence, a long-term exposure to fields below the guideline levels may affect biological systems and modify cell growth rates, while an organism’s built-in mechanisms may compensate for these changes.
Much of the public concern dates from epidemiological studies that show small, though statistically significant increases in childhood leukemia for children living near power lines and possible increases in brain tumors for heavy use of cell phones. The early study by Wertheimer and Leeper  has shown an increase that was just statistically significant in childhood leukemia for children living near power lines. Of the many additional studies since then, about half show small correlations with proximity to power lines and/or weak magnetic fields, and about half do not . However, the possibility that there may be a cause and effect for a long-term exposure to low levels of low-frequency electromagnetic fields has led to the classification by the International Agency for Research on Cancer (IARC), an agency of the World Health Organization (WHO), as a possible cause of cancer. However, this classification has not been included in the International Commit- tee on Electromagnetic Safety or ICNIRP reference levels because of conflicting results and a lack of physical mechanisms by which weak magnetic fields could be expected to modify biological systems.The IARC has published an extensive review of the research epidemiological & laboratory research used in its determination concerning cancer ; the WHO has previously published a similar mono- graph concerning low-frequency field effects and various diseases, including cancer .
While public concern about the field effects is primarily about adverse health effects, there is also consider- able interest in the potential of using either low- or high- frequency fields beneficially. At present, medical uses of electromagnetic fields involve relatively high intensities. For example, RF fields are used for their heating effect in diathermy and ablation of tissues, and pulsed lower-fre- quency magnetic fields have entered medical practice to encourage healing of recalcitrant bone fractures. A long- term goal of research in this area is to find reliable field effects at lower levels that could be used as noninvasive diagnostic or treatment tools or as research probes of underlying biological processes.
It has long been known that magnetic fields can change chemical reaction rates and radical concentrations. Most of these studies were done with relatively large magnetic fields, 1 mT or greater. Reviews of much of this work have been done by Grissom  and Steiner and Ulrich . These reviews show that both changes in nuclear spin states and changes in the angular momentum for electrons in a molecule occur with variations in the magnetic field and affect chemical reaction rates. Some of the earliest work on the effects of nuclear polarizations states on chem- ical reaction rates of alkyl radicals is described in . This work is followed by numerous papers showing the effects of nuclear polarization and nuclear spin states on chemical reaction rates, including Kaptein , Charlton and Bargon , Den Hollander et al. , and Buchachenko . Wood- ward et al. , among others, find many RF absorption spectra lines in the range 1–160 MHz. Reviews of dynamic spin chemistry by Nagakura et al.  and by Hayashi  present detailed descriptions of the theory for the conversion of singlet to triplet states for radical pairs and the resulting changes in radical concentrations as a function of magnetic field strength, orientation, and the viscosity of the medium.
Radicals perform a wide variety of biological func- tions. Reactive oxygen species (ROS), such as super oxide, O2 ), and nitrogen species, such as NOx, are used both as signaling molecules and to attack bacteria and other pathogens. O2 ) is released by neutrophils to as part of the immune systems response in killing bacte- ria. NO can activate guanylate cyclase, which results in a rise in cyclic guanosine monophosphate in smooth muscle tissue and vasorelaxation. It is also involved in the activation of macrophages . In addition, the ion-radical mechanism for the phosphorylation of a very large number of biological molecules is affected by mag- netic fields, and phosphorylation is an important step in many biological signaling systems and the activation of biological processes .
Our work in this area was triggered by the observation that reducing the Earth’s magnetic field to less than 1 nT inhibited the growth of fibrosarcoma HT1080 cells  and the theoretical and experimental work by Batchelor et al. . Data from one such experiment involving radi- cals are shown in Figure 1, and additional work is summa- rized by Brocklehurst and McLauchlan .
A peak value for the concentration of the radical near the Earth’s magnetic field with a magnetic flux density range below 1 mT is shown in Figure 1. This result, along with the results given in Figure 2 from , shows a large number of resonances in the radical spectra throughout the RF spectrum, provides the theoretical bases by which weak magnetic fields can change radial concentrations.
It is clear from these results that changes in magnetic fields on the order of tens of microtesla can change the concentrations of radicals. We have elaborated on these results to show that one can expect to change radical con- centration when magnetic fields are applied at frequen- cies corresponding to resonances and at level crossings –. Some of these resonances may have narrow line widths corresponding changes in nuclear spin states . In addition, as the static magnetic field (SMF) is varied in intensity and as the angle between the static and ac mag- netic field changes, the recombination rates between the fragments of a radical pair will change . More recent work shows a quantum limit for the detection of weak mag- netic fields by changes in chemical reactions using radicals to be on the order of tens of nanotesla .
The proposed hypothesis, which is based on extensive work by others, e.g., , , , , and, extended by some of our own , is that weak magnetic fields change the rate of recombination for radical pairs that are generated by the metabolic activity in cells, which, in turn, change the concen- tration of radicals such as O2 * and molecules such as H2O2. Most of the time, the signaling properties of these molecules generate antioxidants and other radical scavengers so that damaging health effects are not seen, and, in some cases, positive effects, such as the activation of the immune system, may be observed. However, long-term exposure to elevated magnetic fields can lead to elevated radical concentrations and an association with aging, cancers, and Alzheimer’s. This hypothesis is supported by some theoretical and experimental results. However, because biological systems contain a lot of feedback, feedforward, and repair processes, changes in radical concentrations will often have no observable effects. There is much work that needs to be done to illuminate the conditions in which magnetic fields can lead to either positive health effects or negative health effects, and observable effects may only occur when the exposures are combined with other biological stresses.
Some Theoretical Observations
Radicals are created during many biological reactions,including the metabolic processes in mitochondria. Figure 3
shows a schematic for the formation of a radical pair in either a singlet (S) state, where the spins are aligned with
electron spins with opposite spins, or a triplet (T) state, with the spins parallel.
are not allowed to recombine, and the opportunity for them to diffuse away increases so that they can react with other
molecules. The coupling between the unpaired electrons and the nuclei in each fragment of the radical pair is different
and, typically, can be described by magnetic fields in the range 10 nT–3 mT . For many radicals, this is stronger
than the Earth’s magnetic field flux density of about 50 nT so that the quantum numbers describing the state of each
fragment are determined by the sum F of the electron angular momentum and electron spin J and the nuclear spin
I (see Figure 4).
amounts by the external magnetic fields [see Figure 4(a)]. Changes in the applied magnetic field shift the size of the energy barrier for the recombination and the recombination rate. Nuclear magnetic spectra may have very narrow absorption lines with bandwidths of a few cycles with corresponding lifetimes for excited states of seconds or longer. Magnetic fields at the frequency corresponding to differences in the energy levels can drive molecules between energy levels of different nuclear spin states and change the concentration in these energy levels, which, in turn, can change the recombination lifetimes for radial pairs , as shown in Figures 4(b) and 5. Note that these narrow line widths can lead to saturation effects with magnetic fields in the range 10-8 - 10-9 T . With large molecules that contain many atoms with nuclear spins, the calculations of the recombination rates are very complex as the contributions to the magnetic field seen by the electron that is active is dependent on the nuclear spin of each atom, its distance from the electron, and the shielding by other electrons in different orbits. For examples, see the calculations in , , , , and . For our purposes, we will assume that the sum of these fields is large enough so that coupling can lead to relatively sharp resonances, and the nuclear spin states are important in determining the recombination rates for the radical pairs. Nuclear resonance spectroscopy at radio frequencies shows that nuclear spin states may have lifetimes of seconds or longer and corresponding resonant line widths of a few cycles . We postulate that, in weak magnetic fields, where the magnetic coupling between the active electrons and the nuclei in the radicals is stronger than the perturbing external field, that we will also see shifts in radical concentrations that are frequency and amplitude dependent with relatively narrow line widths , as shown in Figure 5. This figure also gives an explanation for effects seen when the ambient magnetic is shielded , for then level energy differences are below the natural line widths and spontaneous transitions can occur.
The experiments that most clearly show that weak magnetic fields affect biological processes and radical concentrations are those that involve changes in the SMF. The fact that birds, salmon, and other animals can sense small changes in the Earth’s magnetic field and use them for navigation says that biological systems can sense small changes in these fields. Experiments in vitro that show changes in the growth rates of cells are more relevant to potential health effects. The results in reference  have shown a reduction in the growth rate of E. coli by reducing the SMF below 18 nT. It has also been shown that we can reduce the growth rates of HT1080 fibrosarcoma cells by 20–30% by reducing the SMF to less than 1 nT, while normal fibroblast cell are reduced by less than 10%. In addition, we have data that show that changes in magnetic field change the growth rate of cancer cells more than normal cells of the same type. Typically, the interior of a a quiescent normal cell is more negative with respect to the exterior than growing cells or cancer cells of the same type. For example, a normal fibroblast cell might have a membrane potential of -70 mV and a fibrosarcoma -30 to -35 mV . Radicals have
been shown to modify the channel currents of Na+,K+, and Ca++ . Preliminary data on fibrosarcoma cells in our lab show both changes in oxidative stress and membrane potential for changes in magnetic fields from 45 to 100 nT and 200 nT (unpublished results).
45% on day three in proportion to the SMF control group, and at three days, it led to a decrease of 45% in O * 2 - and
an increase in H2O2 of 50%. Note that the calculated SAR is estimated to be approximately 0.12 W/kg. Other results 
have shown that the exposure of HT1080 fibrosarcoma cells to 45 nT SMFs oriented vertical to the plane of growth or to SMFs combined with weak 5- and 10-MHz RF magnetic fields of 10 nTRMS perpendicular to the static field inhibits the growth rate. Cell numbers were reduced up to 30% on day two for the cells exposed to the combination of SMF and
a 10-MHz RF magnetic field compared with the SMF control cells. In addition, cells exposed to 10-MHz magnetic fields
for 8 h increased H2O2 production by 55% . The results demonstrate an overall magnetic-field-induced biological
effect that shows elevated H2O2 levels with accompanying decrease in cellular growth rates. These effects are time
dependent, and different cells can respond in opposite directions.
Both the forgoing results are believed to occur through the interaction of the RF fields with hyperfine transitions
between energy level associate with the generation or absorption of the radicals in the cells. In addition, exposure at 1 mW and an estimated SAR of 0.76 W/kg for 10 h have been shown to reduce the growth rate of E. coli by a more than a factor of two while doing very little to B. subtilis .
We have shown that both a theoretical base and the experimental results exist, demonstrating that weak static, low-frequency, and/or high-frequency magnetic fields can affect the concentration of radicals. There are also results that indicate that weak magnetic fields can change the growth rate of cells. However, there are many experiments where no changes are seen. This, we believe, is due to the many feedback and repair processes in the body. Droge  has shown in Figure 7
how extended elevations of ROS and nitrogen oxide species lead undesired biological effects, such as aging, cancer,
and Alzheimer’s. The question becomes: What does all of this mean for people designing wireless power-transfer systems?
Typical systems have been designed so that the fringing fields meet current safety standards that have been set on relatively short-term exposures. For example, a system for charging car batteries using capacitive coupling at 6.78 MHz has a calculated maximum electric field of 33 V/m at 0.25 m from the charging plates, and the magnetic flux density is expected to be less than a few microtesla. A 6.6-kW system being developed under contract through Oak Ridge National Labs for charging car batteries using two coils separated 160 mm at 22–26 kHz with 85% efficiency has fringing magnetic fields of
less than 6.l25 nT and fringing electric fields less than 87 V/m at 0.8 m. These values are moderately close to the ICNIRP standards of 83 V/m and 27 nT. However, the magnetic flux density is only a little less than 10 nT, which has been shown
to change a smooth muscle cell growth rate over a period of days. As people are not likely to stand next to their car for days, long-term effects are not likely to be important. However, there may well be other situations where designers
may need to be concerned about the possible effects of long-term exposures.
We think that there are now both the theoretical bases and sufficient experimental results for further consideration of the possibility that long-term exposures to magnetic fields can lead to both useful applications in treating diseases and to undesired health effects. It is expected that these effects are frequency, amplitude, and time dependent. They will also be dependent on other biological conditions that can lead to changes in radical concentrations. In short, we have only begun to
scratch the surface, and there is a lot of exciting research to be done before we can understand the ways in which low levels of magnetic fields can be used to control biological systems.
We appreciate the support of Khurram Afridi, Robert Erickson, and Dragan Maksimovic´ for obtaining information on current wireless transfer systems and the University of Colorado and the Milheim Foundation for financial support. In addition, the contributions of the many students and, in particular, Lucas Portelli, Carlos Martino, Cynthiea Bingham, Julian Cyrus, Aly Ashraf, and Tosin Feyintola, who have worked on this topic at the University of Colorado are greatly appreciated.
About the Authors
Frank Barnes (Frank.Barnes@colorado.edu) is a distinguished professor emeritus at the University of Colorado, Boulder. He was elected to the National Academy of Engineering in 2001 and received the Gordon Prize 2004 for innovations in Engineering Education from the National Academy. He is a Fellow of the IEEE and the American Association for the Advancement of Science and has served as vice president, Publication Activities of the IEEE and as the chair of the IEEE
Electron Devices Society. He and his students have built lasers, flash lamps, superconductors, avalanche photo diodes,
and other electron devices as well as working on the effects of electric and magnetic fields on biology. Recently, they have
shown that weak magnetic field can both increase and decrease the growth rate of two kinds of cancer and E.coli. His other work includes energy storage for renewable energy and the integration of wind and solar energy into the grid.
Ben Greenebaum (email@example.com) is emeritus professor of physics at the University of Wisconsin-Parkside. He has been engaged in research on biological effects of electromagnetic fields on biological systems since 1972, primarily collaborating on experiments on cellular and subcellular systems. He was an editor of the peer-reviewed journal Bioelectromagnetics from 1993 to 2006.
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