Initial conversion of the FAQ into html was done by Bob Mueller and Dennis Taylor of the General Clinical Research Center at the Medical College of Wisconsin.
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.
However, a workshop held by the World Health Organization (WHO) in 2004 [131] concluded that:
"...scientific research can provide some measure of confidence that short-term, acute exposures up to about 1-2 T [1000-2000 milliT] should be safe... However, it is not possible to determine whether there are any long-term health consequences even from exposure in the milliT range because, to date, there are no well-conducted epidemiological studies with sufficient power to be able to come to any conclusion on this, and there are no good long-term animal studies."
"While there are huge benefits to be gained from use of static magnetic fields, particularly in medicine, possible adverse health effects from exposure to them must be properly evaluated so that the true risks and benefits can be assessed. This said, WHO does not want to imply that all use of these fields should be unnecessarily restricted until appropriate research has been conducted and safety assured. An analysis of the interaction mechanisms suggests that short-term health effects can be predicted and so avoided. Further, there is no currently understood mechanism that would appear to lead to any long-term adverse health consequence."
No. The nature of the interaction of biological material with an electromagnetic source with 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, radiofrequency (RF) energy, microwaves, magnetic fields from electrical power systems (power-frequency fields), and static magnetic fields are all electromagnetic energy; but they have different frequencies.
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.
We usually talk about the electromagnetic sources as though they 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 the energy per particle (or photons, as these particles are called) is important for determining 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 energy, and microwaves, 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 energy, 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.
Probably not. The magnetic field component appears to be much more 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].
Static magnetic fields are generally measured in Tesla (T), milliTesla (mT), and microTesla (microT, µT) where:
In the US, fields are sometimes still measured in Gauss (G) and milliGauss (mG), where:
In the FAQ, mT (millitesla) will be the preferred term.
Magnetic fields can be measured either as magnetic flux density or as 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:
Residential and environmental exposure to static magnetic fields is dominated by the Earth's natural field, which ranges from 0.03 to 0.07 milliT, depending on location. Static magnetic fields under direct current (DC) transmission lines are about 0.02 milliT. 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 milliT 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 150-4000 milliT [1, 2].
Direct effects on ferromagnetic objects and electronic equipment are the only things that most people would notice below about 1000 milliT. There is really no threshold for effects on ferromagnetic objects; a good compass will twitch at fields as low as 0.01 milliT, but it takes a much larger field (above 1 milliT) 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 milliT.
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 milliT [80].
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 milliT for long periods of time, with maximum exposures up to 60 milliT [2, 3]; but another study reports average fields of only 2-4 milliT [4]. Workers in plants using electrolytic cells are reported to be exposed to fields of 4-10 milliT for long periods of time, with maximum exposures up to 30 milliT [5, 6]. Individuals working with particle accelerators are exposed to fields above 0.5 milliT for long periods of time, with exposures above 300 milliT for many hours, and maximum exposures of up to 2,000 milliT [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 milliT may remain from steel in the structure that has been permanently magnetized. 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 milliT fields from particle accelerators, and Barregard et al [6] found no excess cancer in workers exposed to 10 milliT 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 milliT [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 (see Q9) 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.
In a 2004 review of the epidemiological studies of occupation exposure to static magnetic fields Feychting [133] concluded:
"There are only a few epidemiological studies available [about static magnetic field exposure and long-term health effects], and the majority of these have focused on cancer risks... Overall, few occupational studies have focused specifically on effects of static magnetic field exposure, and exposure assessment have consequently been poor or non-existent. Results from studies that have estimated static magnetic field exposure have not indicated any increased cancer risks, but they are generally based on small numbers of cases and crude exposure assessment. Control of confounding has been limited, and it is likely that the "healthy worker" effect have influenced the results... A problem in epidemiological studies of static magnetic fields is that workers in exposed occupations are also exposed to a wide variety of other potentially harmful agents, including some known carcinogens. In conclusion, the available evidence from epidemiological studies is not sufficient to draw any conclusions about potential health effects of static magnetic field exposure."
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 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 may 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].
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 rather limited [135]. Beniashvili et al [12] found no increase in mammary cancer in mice exposed to a 0.02 milliT field. Mahlum et al [13] found that exposure of mice to a 1000 milliT field did not cause mutations, and other investigators found a similar lack of mutagenesis in fruit flies exposed to 1000-3700 milliT fields [14, 15, 16].
There have been two whole organism reports of possible genotoxicity. In 1995, Koana et al [65] found evidence for increased mutations in fruit flies exposed to a 600 milliT field for 24 hours, but only if they were deficient in the ability to repair DNA damage. 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, 102].
In 2000 Teichman et al [101] reported that 1500 and 7000 milliT static fields did not cause mutations in bacteria (the Ames assay); and in 2002, Scheriber et al [104] reported a similar absence of mutagenic activity after exposure to a 1599 milliT field. However, in 2003, Zhang et al [117] reported that while a 9000 milliT static field did not cause mutations in normal (wild-type) bacteria or in bacteria that were defective in DNA repair, it did cause mutations in bacteria that were deficient in their ability to handle oxidative stress (that is, bacteria that could not detoxify certain types of reactive free radicals).
In 2004, Potenza et al [123] reported that exposure to a 200-250 milliT static field for several hours altered isolated bacterial DNA, but had no effects on the DNA of whole bacteria.
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."
In a 2004 review of the effects of static magnetic fields on animals, Saunders [135] concluded that:
"Various experimental studies carried out over the last 30-40 years have examined the effects of the chronic or acute exposure of laboratory animals to static magnetic fields... No adverse effects of such fields on the growth and development of tumours have been firmly established. Overall, however, far too few animal studies have been carried out to reach any firm conclusions."
In a 2004 review of the effects of static magnetic fields on cells, Miyakoshi [134] concluded that:
"A further area of interest is whether static magnetic fields cause DNA damage, which can be evaluated by determination of the frequency of micronucleus formation... This method has been used to confirm that a static magnetic field alone has no such effect. However, the frequency of micronucleus formation increases significantly when certain treatments (e.g., X-irradiation) are given prior to exposure to a 10 T [10,000 milliT] static magnetic field. It has also been reported that treatment with trace amounts of ferrous ions in the cell culture medium and exposure to a static magnetic field increases DNA damage, which is detected using the comet assay..."
Probably not. In general, static magnetic fields do not appear to have this type of epigenetic activity; but here are a few studies that suggest that intense static magnetic fields might enhance the effects of other genotoxic agents.
Three studies [14, 30, 31] found that 140-3700 milliT static fields did not enhance the mutagenic effects of ionizing radiation. Three other studies have reported some evidence hat strong static magnetic fields can enhance genotoxic injury produced by ionizing radiation:
Three studies [94, 101, 104] have found that 1500-7200 milliT static fields did not enhance the mutagenic effects of chemical carcinogens.
Repair of radiation-damage was reported not be affected by a 140 milliT field [31], but was inhibited at 4000 milliT [33]. Two studies [34, 78] reported that 1300-4700 milliT 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 milliT static fields did not enhance the development of chemically-induced mammary tumors, but a third study [12] reported that a 0.02 milliT static field did enhance the development of chemically-induced mammary tumors.
No. Laboratory studies of the effects of static magnetic fields show that these fields do not have any consistent effects on tumor growth, cell growth, immune system function, or hormonal balance.
Tumor growth: In general, static magnetic fields of 13-1150 milliT appear to have no effect on the growth of either chemically-induced [36] or transplanted [37, 38, 39] tumors. However, there is one report that suggests that a 15 milliT static field increases the growth rate of chemically-induced tumors [35].
Cell growth: In general, static magnetic fields of 45-2000 milliT have no effect on the growth of human [20, 33, 39, 67, 97, 113], animal [25, 31, 39, 42, 72, 74, 100, 102, 113] or yeast [66, 125] cells. However, there are some reports of static fields effects on cell growth:
Immune system effects: In most studies, static magnetic fields of 13-2000 milliT appear to have no effect on the immune system of animals [38, 40, 41, 42], although one study reports that the implantation of small magnets into the brains of rats enhanced their immune response [43]. Two studies of humans [5, 44] have reported that workers in aluminum reduction plants, where exposure to static magnetic fields is common, have minor alterations in the numbers of some types of immune cells; but these minor changes in cell number are of no known clinical significance, and may not even be related to magnetic field exposure. A study of human white blood cells found no effect of a 4275 milliT field on the inflammatory response of normal or leukemic cells [112]. In 2003, Onodera et al [116] reported that exposure of immune system cells to 10,000 milliT field caused the loss of some cell types if the cells has been stimulated to divide, but no effect if the cells had not been stimulated into division.
Hormonal effects: There are some reports that static magnetic fields of the order of the natural earth field (about 0.05 milliT) can affect melatonin production in rats [45, 46, 47], although other studies with stronger (e.g., 2000 milliT fields [68]) have not seen such effects. The one study with human volunteers showed no effects on melatonin production of overnight exposure to a 2-7 milliT static field [99]. While it has been suggested that melatonin might have "cancer-preventive" activity [48, 49], there is no evidence that static magnetic fields affect melatonin levels in humans, or that melatonin has anti-cancer activity in humans.
A wide range of other possible biological effects have been assessed. Recent reports include:
In a 2004 review of the cellular effects of static magnetic fields, Miyakoshi [134] concluded that:
"Studies have shown that a static magnetic field alone does not have a lethal effect on the basic properties of cell growth and survival under normal culture conditions, regardless of the magnetic density. Most but not all studies have also suggested that a static magnetic field has no effect on changes in cell growth rate [or] cell cycle distribution...Many studies have found a strong magnetic field that can induce orientation phenomena in cell culture."
In a 2004 review of the effects of static magnetic fields on animals, Saunders [135] concluded that:
"Various experimental studies carried out over the last 30-40 years have examined the effects of the chronic or acute exposure of laboratory animals to static magnetic fields... few adverse effects were identified... Generally, the acute responses found during exposure to static fields above about 4 T [4000 milliT] are consistent with those found in volunteer studies, namely the induction of flow potentials around the heart and the development of aversive/avoidance behavior resulting from body movement in such fields. No consistently demonstrable effects of exposure to fields of approximately 1T [1000 milliT] and above have been seen on other behavioral or cardiovascular endpoints... Overall, however, far too few animal studies have been carried out to reach any firm conclusions."
In a 2004 review of the physiological effects of exposure of human to high-field [1500-8000 milliT] MRI units, Chakeres and Vocht [132] concluded:
"There were no clinically significant changes in the subjects' physiologic measurements at 8 T [8000 milliT]. There was a slight increase in the systolic blood pressure with increasing magnetic field strength. There did not appear to be any adverse effect on the cognitive performance of the subjects at 8 T [8000 milliT]. A few subjects commented at the time of initial exposure on dizziness, metallic taste in the mouth, or discomfort related to the measurement instruments or the head coil. There were no adverse comments at 3 months. The 1.5 T [1500 milliT] study had two of the four neuro-behavioral domains exhibiting adverse effects (sensory and cognitive-motor). These studies did not demonstrate any clinically relevant adverse effects on neuro-cognitive testing or vital sign changes. One short-term memory, one sensory, and one cognitive-motor test demonstrated adverse effects, but the significance is not clear."
Yes. While the laboratory evidence does not suggest a link between static magnetic fields and cancer, studies have reported that static magnetic fields do have "bioeffects", particularly at field strengths above 2000 milliT [1, 50, 51, 52, 53, 54, 55]. These "bioeffects" have no obvious connection to cancer.
Possibly. A few biological effects have been reported in laboratory systems for fields as low as 8 milliT [130], and some organisms appear to be able to detect changes in the strength and/or orientation of the Earth's static magnetic field (0.03-0.05 milliT) [1, 54]. In addition, the rates of some chemical reactions can be affected by magnetic fields as low as 1 milliT [56, 57, 71, 98, 129].
In 2004, Ye et al [130] reported that they could affect nerve conduction in crayfish with a field as low as 8 milliT.
There are known biological mechanisms through which strong (greater than 2000 milliT) static magnetic fields could cause biological effects [1, 50], but these mechanisms could not account for biological effects of static fields with intensities of less than about 200 milliT [1, 50, 136].
Biological effects could be mediated through effects on free radical reaction rates at field strengths as low as 1 milliT [56, 57, 71, 98, 129]; but there is no current evidence that such effects have any significance for human health [71, 77].
Application of the Hill criteria [Q9] shows that the evidence for a causal association between exposure to static fields and the incidence of cancer is weak to nonexistent.
Yes. There have been a number of such reviews of the epidemiological and laboratory literature. None of these reviews have concluded that static magnetic or electrical fields of the intensity encountered in residential and occupational settings are human health hazards.
In 2002, the International Agency for Research on Cancer(IARC) [109] concluded that:
A 2004 review by the United Kingdom (British) National Radiological Protection Board (NRPB) [121] concluded that for static electric fields:
"The most plausible and coherent set of data from which guidance can be developed concerns perceptual and annoying responses in static electric fields... Any annoying or other stressful effects should be avoided if exposure is below the threshold for cutaneous perception of around 20 kV/m.... There is insufficient evidence from animal and cellular studies to enable the thresholds for long term effects from chronic exposure to static electric fields to be determined."
For static magnetic fields the NRPB [121] concluded that:
"Studies of workers exposed to strong static magnetic fields of up to several tens of milliT do not overall demonstrate increased health risks... Acute effects on the heart or nervous system associated with flow potential induced during movement in the field should not occur if exposures are below 2 T [2000 milliT]. Particular caution should be applied with exposure to fields in excess of about 5-8 T [5000-8000 milliT]. There is insufficient evidence from animal and cellular studies to enable the thresholds for long term effects from chronic exposure to static magnetic fields to be determined."
A workshop held by the World Health Organization (WHO) in 2004 [131] concluded that:
"...scientific research can provide some measure of confidence that short-term, acute exposures up to about 1-2 T [1000-2000 milliT] should be safe. With regard to patient and volunteer exposure, a few studies report the absence of acute physiological responses at 8 T [8000 milliT] in healthy volunteers, but care must be taken moving in and out of such a field. For occupational exposure, present limits are based on avoiding the sensations of vertigo and nausea induced by such movement. However, it is not possible to determine whether there are any long-term health consequences even from exposure in the [milliT] range because, to date, there are no well-conducted epidemiological studies with sufficient power to be able to come to any conclusion on this, and there are no good long-term animal studies."
"While there are huge benefits to be gained from use of static magnetic fields, particularly in medicine, possible adverse health effects from exposure to them must be properly evaluated so that the true risks and benefits can be assessed. This said, WHO does not want to imply that all use of these fields should be unnecessarily restricted until appropriate research has been conducted and safety assured. An analysis of the interaction mechanisms suggests that short-term health effects can be predicted and so avoided. Further, there is no currently understood mechanism that would appear to lead to any long-term adverse health consequence. However, given the widespread and rapidly expanding use of static magnetic fields, priority should be given to key studies that could give sound scientifically based assurance that these assumptions are correct."
Yes. A number of governmental and professional organizations have developed exposure standards, or have modified or reaffirmed their previous standards. For pacemakers and implanted medical device standards also see Q22.
The standards are based on several considerations.
Effects on cardiac pacemakers have been reported for fields as low as 1.7 milliT [73]. The most common effect was triggering of the asynchronous mode; the effect is very model and orientation dependent, and in the models tested normal operation resumed when the pacemaker was removed from the field [73]. Some pacemakers also exhibited significant torque when exposed [73]. For this reason current static field guidelines restrict exposures for wearers of cardiac pacemakers to 0.5 milliT [50, 59]. It would be prudent to apply this restriction to other implanted electronic devices, and to prosthetic devices as well, although not all standards are explicit on this point.
In contrast to the above, a 2000 study [96] found that MR imaging could be safely performed at 500 milliT in patients with cardiac pacemakers.
A 2002 study [110] reports that static fields from MRI units can cause unpredictable effects on the reed switches of some pacemakers at field strengths as low as 500 milliT.
There is no consistent evidence for such effects.
Fertility: Mur et al [82] found no significant effects on the fertility in men exposed to 4-30 milliT static fields in the aluminum industry; and Evans et al [87] found no effect of fertility in female MRI operators. One animal study reported evidence for decreased male fertility at 1500 milliT [83], but two other studies at 500-700 milliT found no such effect [84, 95]. A fourth animal study reported decreased female fertility at 80 milliT, but not at 30 milliT [93].
Miscarriages: Baker et al [85] found that MRIs done at 1500 milliT in the second and third trimester did not increase the miscarriage rate; and Evans et al [87] found no significant effect on miscarriage rates in female MRI operators. Two animal study reported decreased fetal viability at 30 milliT [86, 93] and 80 milliT [93], but other studies at 500-1000 milliT [90, 95] and 6300 milliT [89] found no such effect.
Birth defects: Baker et al [85] found that MRIs done at 1500 milliT in the second and third trimester did not produce birth defects in humans; and Evans et al [87] found no increase in birth defects in children of female MRI operators. One animal study reported adverse effects on fetal development at 1500 milliT [83]; but other studies found no increase in birth defects at 30 [86], 500-1000 milliT [13, 90, 92, 95] , 4700 milliT [128] or 6300 milliT [89]. Three animal MRI studies done at 1500-4700 milliT [88a, 88b, 127] reported increases in birth defects, but heating due to the radiofrequency (RF) radiation used in MRI cannot be ruled out as a factor. Two other animal MRI studies at 1500-4700 milliT found no increases in birth defects [91, 103].
In a 2004 review of the effects of static magnetic fields on animals, Saunders [135] concluded that:
"Various experimental studies carried out over the last 30-40 years have examined the effects of the chronic or acute exposure of laboratory animals to static magnetic fields... No adverse effects of such fields on reproduction and development... have been firmly established. Overall, however, far too few animal studies have been carried out to reach any firm conclusions."
In a 2004 review of the epidemiological studies of occupation exposure to static magnetic fields Feychting [133] concluded:
"There are only a few epidemiological studies available [about static magnetic field exposure and long-term health effects]... There are some reports on reproductive outcomes... Overall, few occupational studies have focused specifically on effects of static magnetic field exposure, and exposure assessment have consequently been poor or non-existent... A few studies have reported results on reproductive outcomes among aluminum workers and MRI operators, but limitations in study designs prevent conclusions... In conclusion, the available evidence from epidemiological studies is not sufficient to draw any conclusions about potential health effects of static magnetic field exposure."
There is no consistent evidence for such effects.
Ozone and nitrogen oxides: It has been suggested that ozone and nitrogen oxides created when high voltage lines (AC or DC) arc might be a health hazard. Both compounds could be produced, but there is no evidence that either are produced at levels high enough to be a health hazard. This is discussed in some additional detail in the Q21B of the Powerline FAQ.
Radon and ionized airborne pollutants: Henshaw and Fews [105] have speculated that the radioactive decay products of radon, and other potentially-carcinogenic airborne particles, might be attracted to strong electric field sources, and that there could be enhanced exposure to such carcinogenic agents near high-voltage power lines. They went on to theorize that this provided a mechanism for an association between AC power lines and childhood leukemia. In 1999, Henshaw and Fews [106] amended their hypothesis to suggest that ions produced by corona from high voltage power lines might attach to aerosol pollutants (for example, motor vehicle exhaust) and increase the probability that these pollutants would be deposited in the lung. They directed their theories to AC sources, but they could also be applied to DC sources. The authors have so far presented no evidence that this increased pollutant exposure actually occurs.
The basic observation of increased deposition of radon daughter aerosols on very strong electric (not magnetic) field sources is plausible. However, there are major theoretical problems with the Henshaw/Fews hypotheses which indicate that the postulated mechanisms are extremely unlikely to produce adverse human health effects under real-world exposure conditions. The problems with the theory are is discussed in considerable detail in the Q32 of the Powerline FAQ.
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