While most public concern about electromagnetic (EM) fields and cancer has concentrated on power-frequency, microwave (MW) and radiofrequency (RF) fields, claims have been made that static magnetic fields cause or contribute to cancer.
There is very little theoretical reason to suspect that static fields might cause or contribute to cancer or any other human health problems (Q17), and there is very little laboratory (Q11-Q16, Q23) or epidemiological evidence (Q8-Q10, Q23) for a connection between static fields and human health hazards.
No. The nature of the interaction of an electromagnetic source with biological material depends on the frequency of the source, so that different types of electromagnetic sources must be evaluated separately.
X-rays, ultraviolet (UV) light, visible light, MW/RF, magnetic fields from electrical power systems (power-frequency fields), and static magnetic fields are all sources of electromagnetic energy. These different electromagnetic sources are characterized by their frequency or wavelength.
The frequency of an electromagnetic source is the rate at which the electromagnetic field changes direction and/or amplitude and is usually given in Hertz (Hz) where 1 Hz is one change (cycle) per second. The frequency and wavelength are related, and as the frequency rises the wavelength gets shorter. Power-frequency fields are 50 or 60 Hz and have a wavelength of about 5000 km. By contrast, microwave ovens have a frequency of 2.54 billion Hz and a wavelength of about 10 cm, and X-rays have frequencies of 10^15 Hz and, and wavelengths of much less than 100 nm. Static fields, or direct current (DC) fields do not vary regularly with time, and can be said to have a frequency of 0 Hz and an infinitely long wavelength.
The interaction of biological material with an electromagnetic source depends on the frequency of the source. We usually talk about the electromagnetic spectrum as though it produced waves of energy. This is not strictly correct, because sometimes electromagnetic energy acts like particles rather than waves; this is particularly true at high frequencies. The particle nature of electromagnetic energy is important because it is the energy per particle (or photons, as these particles are called) that determines what biological effects electromagnetic energy will have [62].
At the very high frequencies characteristic of hard UV and X-rays, electromagnetic particles (photons) have sufficient energy to break chemical bonds. This breaking of bonds is termed ionization, and this part of the electromagnetic spectrum is termed ionizing. The well-known biological effects of X-rays are associated with the ionization of molecules. At lower frequencies, such as those characteristic of visible light, RF, and MW, the energy of a photon is very much below those needed to disrupt chemical bonds. This part of the electromagnetic spectrum is termed non-ionizing. Because non-ionizing electromagnetic energy cannot break chemical bonds there is no analogy between the biological effects of ionizing and nonionizing electromagnetic energy [62].
Non-ionizing electromagnetic sources can still produce biological effects. Many of the biological effects of soft UV, visible, and IR frequencies also depend on the photon energy, but they involve electronic excitation rather than ionization, and do not occur at frequencies below that of IR (below 3 x 10^11 Hz). RF and MW sources can cause effects by inducing electric currents in tissues, which cause heating. The efficiency with which an electromagnetic source can induce electric currents, and thus produce heating, depends on the frequency of the source, and the size and orientation of the object being heated. At frequencies below that used for broadcast AM radio (about 10^6 Hz), electromagnetic sources couple poorly with the bodies of humans and animals, and thus are very inefficient at inducing electric currents and causing heating [62].
Thus in terms of potential biological effects the electromagnetic spectrum can be divided into four portions:
No. Static electromagnetic sources do not produce radiation.
In general, electromagnetic sources produce both radiant energy (radiation) and non-radiant energy (fields). Radiated energy exists apart from its source, travels away from the source, and continues to exist even if the source is turned off. Fields are not projected away into space, and cease to exist when the energy source is turned off. For static electromagnetic fields there is no radiative component.
No. Only the magnetic field component appears to be relevant to possible health effects.
Magnetic fields are difficult to shield, and easily penetrate buildings and people. In contrast to magnetic fields, electrical fields have very little ability to penetrate skin or buildings. Because static electric fields do not penetrate the body, it is generally assumed that any biologic effect from routine exposure to static fields must be due to the magnetic component of the field, or to the electric fields and currents that these magnetic fields induce in the body [1,54].
5) What units are used to measure static magnetic fields?
Static magnetic fields are generally measured in Tesla (T), milliTesla (mT), and microTesla (microT, µT) where:
1000 mT = 1 T
1000 (µT) = 1 mT.
In the US, fields are sometimes still measured in Gauss (G) and milliGauss (mG), where:
10,000 G equals 1 T
1 G = 100 microT
1 microT (µT) = 10 mG.
In the FAQ, mT (millitesla) will be the preferred term.
Magnetic fields can be specified in either magnetic flux density or magnetic field strength. In the US and Western Europe field strengths are usually specified in units of magnetic flux density (Tesla or Gauss). In some of the Eastern European literature, however, magnetic fields are specified in Oersteds (Oe), which are units of magnetic field strength. When dealing with exposure of non-ferromagnetic material, such as animals or cells, magnetic flux density and magnetic field strength can be assumed to be equal, so:
1 Oersted = 1 Gauss = 100 microT = 0.1 mT
6) What sort of static magnetic fields are common in residences?
Residential and environmental exposure to static magnetic fields is dominated by the Earth's natural field, which ranges from 0.03 to 0.07 mT, depending on location. Static magnetic fields under direct current (DC) transmission lines are about 0.02 mT. Small artificial sources of static fields (permanent magnets) are common, ranging from the specialized (audio speakers components, battery-operated motors, microwave ovens) to trivial (refrigerator magnets). These small magnets can produce fields of 1-10 mT within a cm or so of their magnetic poles. The highest static magnetic field exposures to the general public are from magnetic resonance imaging (MRI), where the fields range from 150-2000 mT [1,2].
Direct effects on ferromagnetic objects and electronic equipment are the only things that most people would notice below about 1000 mT. There is really no threshold for effects on ferromagnetic objects; a good compass will twitch at fields as low as 0.01 mT, but it takes a much larger field (above 1 mT) to make ferromagnetic objects move in a dangerous way. Electronics can be affected by quite low fields; a high resolution color monitor, for example, can show color distortions at static fields as low as 0.1 mT.
A source of exposure to static fields that blurs the distinction between residential and occupational exposure is electric trains. Static fields in electric trains can be as high as 0.2 mT [80].
7) What sort of static magnetic fields are common in work places?
Persons with occupational exposures to static fields include operators of magnetic resonance imaging (MRI) units, personnel in specialized physics and biomedical facilities (for example, those working with particle accelerators), and workers involved in electrolytic processes such as aluminum production. Some aluminum manufacturing workers are reported to be exposed to fields of 5-15 mT for long periods of time, with maximum exposures up to 60 mT [2,3]; but another study reports average fields of only 2-4 mT [4]. Workers in plants using electrolytic cells are reported to be exposed to fields of 4-10 mT for long periods of time, with maximum exposures up to 30 mT [5,6]. Individuals working with particle accelerators are exposed to fields above 0.5 mT for long periods of time, with exposures above 300 mT for many hours, and maximum exposures of up to 2,000 mT [7].
Another source of exposure to static magnetic fields is the residual fields that can remain after strong static magnets are removed. For example, after a clinical MRI unit is removed from a room, a residual field of as high as 2 mT may remain from steel in the structure that has been permanently magnitized. Such fields are not sufficiently strong to be a concern for human health, but they may be strong enough to interfere with the operation of sensitive electronic equipment. These residual fields can be reduced (although not always eliminated) by professional "degaussing".
There have been relatively few studies of cancer incidence in workers exposed to static magnetic fields. Budinger et al [7] found no excess cancer in workers exposed to 300 mT fields from particle accelerators, and Barregard et al [6] found no excess cancer in workers exposed to 10 mT fields in a chlorine production plant.
There are also studies of aluminum reduction plant workers [8,9,10,61]. In general the studies of aluminum reduction plant workers were not designed to analyzed the effects of static fields, but these workers are exposed to static fields of 5-15 mT [2,3,4]. In the aluminum reduction plant studies, the only excess cancer reported was lymphoreticular tumors, and this was seen in one study [8]. The only aluminum reduction plant study to look specifically at static field exposure and cancer reported no excess of nervous system or hematopoietic cancers [61].
There are certain widely accepted criteria [11,63,64], often called the "Hill criteria" [11], that are weighed when assessing epidemiological and laboratory studies of agents that may cause human cancer. Under these criteria one examines the strength, consistency, and specificity of the association between exposure and the incidence of cancer, the evidence for a dose-response relationship, the laboratory evidence, the biological plausibility of the association, and the coherence of the proposed association with what is known about the agent and about cancer.
These criteria must be applied with caution [11,63,64]:
Application of the Hill criteria shows that the current epidemiological evidence for a connection between static magnetic fields and cancer is weak to non-existent.
Thus the epidemiological evidence for an association between static magnetic fields and cancer is weak and inconsistent, and fails to show a dose-response relationship.
When epidemiological evidence for a causal relationship is weak to non-existent, as in the case of static magnetic fields and cancer, laboratory studies would have to provide very strong evidence for carcinogenicity in order to tip the balance.
Carcinogens, agents that cause cancer, can be either genotoxic or epigenetic (in older terminology these were initiators and promoters). Genotoxic agents (genotoxins) can directly damage the genetic material of cells. Genotoxins often affect many types of cells, and may cause more than one kind of cancer. Genotoxins generally do not have thresholds for their effect; so as the dose of the genotoxin is lowered the risk gets smaller, but it may never go away. Thus evidence for genotoxicity at any field intensity would be relevant to assessing carcinogenic potential [62, 75].
An epigenetic agent is something that increases the probability that a genotoxin will damage the genetic material of cells or that a genotoxin will cause cancer. Promoters are a particular kind of epigenetic agent that increase the cancer risk in animals already exposed to a genotoxic carcinogen. Epigenetic agents (including promoters) may affect only certain types of cancer. Epigenetic agents generally have thresholds for their effect; so as the dose of an epigenetic agent is lowered a level is reached at which there is no risk. Thus evidence for epigenetic activity at field intensities far above those actually encountered in residential and occupation settings would not be clearly relevant to assessing carcinogenic potential [62, 75].
12) Are static magnetic fields genotoxic?
No. A broad range of whole organism and cellular genotoxicity studies of static fields have been carried out. Together these studies offer no consistent evidence that static magnetic fields are genotoxic.
Whole organism genotoxicity studies with static magnetic fields have been somewhat limited. Beniashvili et al [12] found no increase in mammary cancer in mice exposed to a 0.02 mT field. Mahlum et al [13] found that exposure of mice to a 1000 mT field did not cause mutations, and other investigators found a similar lack of mutagenesis in fruit flies exposed to 1000-3700 mT [14,15,16] fields.
There have been two whole organism reports of possible genotoxicity. In 1995 Koana et al [65] found evidence for increased mutations in repair deficient fruit flies exposed to a 600 mT field for 24 hours. No effects was seen in fruit flies that had normal DNA repair capacity. In 2001, Suzuki et al [103] reported that exposure of mice to a 3000 or 4700 milliT static field for 24-72 hours caused chromosome damage in their bone marrow cells [103].
Cellular genotoxicity studies have been more extensive. Published laboratory studies have reported that static magnetic fields do not cause any of the effects that indicate genotoxicity. Static magnetic fields do not cause DNA strand breaks [76], chromosome aberrations [18,19,20,21,22,23,79], sister chromatid exchanges [18,20,22,24], cell transformation [19,25], mutations [26,27,28,94], or micronucleus formation [78].
In 2000 Teichman et al [101] reported that 1500 and 7000 milliT static fields did not cause mutations in bacteria (the Ames assay).
Some studies of static electrical fields have also been conducted. These have been reviewed by McCann et al [29], who concluded that while there were some reports of genotoxicity for static electrical fields, "all reports of positive results have utilized exposure conditions likely to have been accompanied by auxiliary phenomena such as corona, spark discharge, and transient electrical shocks, whereas negative reports have not."
13) Do static magnetic fields enhance the effects of other genotoxic agents?
Probably not. In general, static magnetic fields do not appear to have this type of epigenetic activity. There are a few studies that suggest that static magnetic fields might enhance the effects of other genotoxic agents, but none of these studies has been replicated.
Three studies [14,30,31] have found that 140-3700 mT static fields do not enhance the mutagenic effects of ionizing radiation. A fourth study [32] reported that 1100-1400 mT static fields caused a slight increase in the number of chromosome aberrations produced by exposure to high doses of ionizing radiation, and a fifth study reported that a 4000 mT field slightly increased radiation-induced cell killing [33].
Two studies [94, 101] have found that 1500-7200 mT static fields do not enhance the mutagenic effects of chemical carcinogens.
Repair of radiation-damage was reported not be affected by a 140 mT field [31], but to be inhibited at 4000 mT [33]. Two studies [34,78] reported that 1300-4700 mT static fields did not enhance the mutagenic effects of a known chemical genotoxins, and might even inhibit such activity.
Two studies [35,36] found that 150-800 mT static fields did not enhance the development of chemically-induced mammary tumors, but a third study [12] reported that a 0.02 mT static field did enhance the development of chemically-induced mammary tumors.
14) Do laboratory studies indicate that static magnetic fields have any biological effects that might be relevant to cancer or other human health hazards?
