STATIC ELECTRIC AND MAGNETIC FIELDS

 

Martino Grandolfo

 

 

Both our natural and our artificial environments generate electric and magnetic forces of various magnitudes-in the outdoors, in offices, in households and in industrial workplaces. This raises two important questions: (1) do these exposures pose any adverse human health effects, and (2) what limits can be set in an attempt to define “safe” limits of such exposures?

 

This discussion focuses on static electric and magnetic fields. Studies are described on workers in various industries, and also on animals, which fail to demonstrate any clear-cut adverse biological effects at the levels of exposure to electric and magnetic fields usually encountered. Nevertheless, attempts are made to discuss the efforts of a number of international organizations to set guidelines to protect workers and others from any possible dangerous level of exposure.

 

Definition of Terms

When a voltage or electric current is applied to an object such as an electrical conductor, the conductor becomes charged and forces start to act on other charges in the vicinity. Two types of forces may be distinguished: those arising from stationary electric charges, known as the electrostatic force, and those appearing only when charges are moving (as in an electric current in a conductor), known as the magnetic force. To describe the existence and spatial distribution of these forces, physicists and mathematicians have created the concept of field. One thus speaks of a field of force, or simply, electric and magnetic fields.

 

The term static describes a situation where all charges are fixed in space, or move as a steady flow. As a result, both charges and current densities are constant in time. In the case of fixed charges, we have an electric field whose strength at any point in space depends on the value and geometry of all the charges. In the case of steady current in a circuit, we have both an electric and a magnetic field constant in time (static fields), since the charge density at any point of the circuit does not vary.

 

Electricity and magnetism are distinct phenomena as long as charges and current are static; any interconnection between electric and magnetic fields disappears in this static situation and thus they can be treated separately (unlike the situation in time-varying fields). Static electric and magnetic fields are clearly characterized by steady, time-independent strengths and correspond to the zero-frequency limit of the extremely low frequency (ELF) band.

 

Static Electric Fields

 

Natural and occupational exposure

Static electric fields are produced by electrically charged bodies where an electric charge is induced on the surface of an object within a static electric field. As a consequence, the electric field at the surface of an object, particularly where the radius is small, such as at a point, can be larger than the unperturbed electric field (that is, the field without the object present). The field inside the object may be very small or zero. Electric fields are experienced as a force by electrically charged objects; for example, a force will be exerted on body hair, which may be perceived by the individual.

 

On the average, the surface charge of the earth is negative while the upper atmosphere carries a positive charge. The resulting static electric field near the earth’s surface has a strength of about 130 V/m. This field decreases with height, and its value is about 100 V/m at 100 m elevation, 45 V/m at 1 km, and less than 1 V/m at 20 km. Actual values vary widely, depending upon the local temperature and humidity profile and the presence of ionized contaminants. Beneath thunderclouds, for example, and even as thunderclouds are approaching, large field variations occur at ground level, because normally the lower part of a cloud is negatively charged while the upper part contains a positive charge. In addition, there is a space charge between the cloud and ground. As the cloud approaches, the field at ground level may first increase and then reverse, with the ground becoming positively charged. During this process, fields of 100 V/m to 3 kV/m may be observed even in the absence of local lightning; field reversals may take place very rapidly, within 1 min, and high field strengths may persist for the duration of the storm. Ordinary clouds, as well as thunderclouds, contain electric charges and therefore deeply affect the electric field at ground level. Large deviations from the fair-weather field, up to 200%, are also to be expected in the presence of fog, rain and naturally occurring small and large ions. Electric field changes during the daily cycle can even be expected in completely fair weather: fairly regular changes in local ionization, temperature or humidity and the resulting changes in the atmospheric electrical conductivity near the ground, as well as mechanical charge transfer by local air movements, are probably responsible for these diurnal variations.

 

Typical levels of man-made electrostatic fields are in the 1 to 20 kV/m range in offices and households; these fields are frequently generated around high-voltage equipment, such as TV sets and video display units (VDUs), or by friction. Direct current (DC) transmission lines generate both static electric and magnetic fields and are an economical means of power distribution where long distances are involved.

 

Static electric fields are widely used in industries such as chemicals, textile, aviation, paper and rubber, and in transportation.

 

Biological effects

Experimental studies provide little biological evidence to suggest any adverse effect of static electric fields on human health. The few animal studies that have been carried out also appear to have yielded no data supporting adverse effects on genetics, tumour growth, or on the endocrine or cardiovascular systems. (Table 49.9 summarizes these animal studies.)

 

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Table 49.9      Studies on animals exposed to static electric fields

 

Biological end-points

Reported effects

Exposure conditions

Haematology and immunology

Changes in the albumin

and globulin fractions of serum proteins in rats.

Responses not consistent

 

No significant differences in blood cell counts, blood proteins or blood

chemistry in mice

Continuous exposure to fields between 2.8 and 19.7 kV/m

from 22 to 52 days of age

 

 

Exposure to 340 kV/m for 22 h/day for a total of 5,000 h

Nervous system

Induction of significant changes observed in the EEGs of rats.

However, no clear indication of a consistent response

 

No significant changes in the concentrations and utilization rates of

various neurotransmitters in brains of male rats

Exposure to electric field strengths up to 10 kV/m

 

 

Exposure to a 3 kV/m field for up to 66 h

Behaviour

Recent, well-conducted studies suggesting no effect on rodent

behaviour

 

Production of dose-dependent avoidance behaviour in male rats, with

no influence of air ions

Exposure to field strengths up to 12 kV/m

 

 

Exposure to HVD electric fields ranging from 55 to 80 kV/m

Reproduction and development

No significant differences in the total number of offspring nor in the

percentage surviving in mice

Exposure to 340 kV/m for 22 h/day before, during and after

gestation

 

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No in vitro studies have been conducted to evaluate the effect of exposing cells to static electric fields.

 

Theoretical calculations suggest that a static electric field will induce a charge on the surface of exposed people, which may be perceived if discharged to a grounded object. At a sufficiently high voltage, the air will ionize and become capable of conducting an electric current between, for example, a charged object and a grounded person. The breakdown voltage depends on a number of factors, including the shape of the charged object and atmospheric conditions. Typical values of corresponding electric field strengths range between 500 and 1,200 kV/m.

 

Reports from some countries indicate that a number of VDU operators have experienced skin disorders, but the exact relationship of these to VDU work is unclear. Static electric fields at VDU workplaces have been suggested as a possible cause of these skin disorders, and it is possible that the electrostatic charge of the operator may be a relevant factor. However, any relationship between electrostatic fields and skin disorders must still be regarded as hypothetical based on available research evidence.

 

Measurements, prevention, exposure standards

Static electric field strength measurements may be reduced to measurements of voltages or electric charges. Several electrostatic voltmeters are commercially available which permit accurate measurements of electrostatic or other high-impedance sources without physical contact. Some utilize an electrostatic chopper for low drift, and negative feedback for accuracy and probe-to-surface spacing insensitivity. In some cases the electrostatic electrode “looks” at the surface under measurement through a small hole at the base of the probe assembly. The chopped AC signal induced on this electrode is proportional to the differential voltage between the surface under measurement and the probe assembly. Gradient adapters are also used as accessories to electrostatic voltmeters, and permit their use as electrostatic field strength meters; direct readout in volts per metre of separation between the surface under test and the grounded plate of the adapter is possible.

 

There are no good data which can serve as guidelines to set base limits of human exposure to static electric fields. In principle, an exposure limit could be derived from the minimum breakdown voltage for air; however, the field strength experienced by a person within a static electric field will vary according to body orientation and shape, and this must be taken into account in attempting to arrive at an appropriate limit.

 

Threshold limit values (TLVs) have been recommended by the American Conference of Governmental Industrial Hygienists (ACGIH 1995). These TLVs refer to the maximum unprotected workplace static electric field strength, representing conditions under which nearly all workers may be exposed repeatedly without adverse health effects. According to ACGIH, occupational exposures should not exceed a static electric field strength of 25 kV/m. This value should be used as a guide in the control of exposure and, due to individual susceptibility, should not be regarded as a clear line between safe and dangerous levels. (This limit refers to the field strength present in air, away from the surfaces of conductors, where spark discharges and contact currents may pose significant hazards, and is intended for both partial-body and whole-body exposures.) Care should be taken to eliminate ungrounded objects, to ground such objects, or to use insulated gloves when ungrounded objects must be handled. Prudence dictates the use of protective devices (e.g., suits, gloves and insulation) in all fields exceeding 15 kV/m.

 

According to ACGIH, present information on human responses and possible health effects of static electric fields is insufficient to establish a reliable TLV for time-weighted average exposures. It is recommended that, lacking specific information from the manufacturer on electromagnetic interference, the exposure of wearers of pacemakers and other medical electronic devices should be maintained at or below 1 kV/m.

 

In Germany, according to a DIN Standard, occupational exposures should not exceed a static electric field strength of 40 kV/m. For short exposures (up to two hours per day) a higher limit of 60 kV/m is permitted.

 

In 1993, the National Radiological Protection Board (NRPB 1993) provided advice concerning appropriate restrictions on the exposure of people to electromagnetic fields and radiation. This includes both static electric and magnetic fields. In the NRPB document, investigation levels are provided for the purpose of comparing values of measured field quantities in order to determine whether or not compliance with basic restrictions has been achieved. If the field to which a person is exposed exceeds the relevant investigation level, compliance with the basic restrictions must be checked. Factors that might be considered in such an assessment include, for example, the efficiency of the coupling of the person to the field, the spatial distribution of the field across the volume occupied by the person, and the duration of exposure.

 

According to NRPB it is not possible to recommend basic restrictions for avoiding direct effects of human exposure to static electric fields; guidance is given to avoid annoying effects of direct perception of the surface electric charge and indirect effects such as electric shock. For most people, the annoying perception of surface electric charge, acting directly on the body, will not occur during exposure to static electric field strengths less than about 25 kV/m, that is, the same field strength recommended by ACGIH. To avoid spark discharges (indirect effects) causing stress, NRPB recommends that DC contact currents be restricted to less than 2 mA. Electric shock from low impedance sources can be prevented by following established electrical safety procedures relevant to such equipment.

 

Static Magnetic Fields

 

Natural and occupational exposure

The body is relatively transparent to static magnetic fields; such fields will interact directly with magnetically anisotropic materials (exhibiting properties with different values when measured along axes in different directions) and moving charges.

 

The natural magnetic field is the sum of an internal field due to the earth acting as a permanent magnet and an external field generated in the environment from such factors as solar activity or atmospherics. The internal magnetic field of the earth originates from the electric current flowing in the upper layer of the earth’s core. There are significant local differences in the strength of this field, whose average magnitude varies from about 28 A/m at the equator (corresponding to a magnetic flux density of about 35 mT in a non-magnetic material such as air) to about 56 A/m over the geomagnetic poles (corresponding to about 70 mT in air).

 

Artificial fields are stronger than those of natural origin by many orders of magnitude. Artificial sources of static magnetic fields include all devices containing wires carrying direct current, including many appliances and equipment in industry.

 

In direct-current power transmission lines, static magnetic fields are produced by moving charges (an electric current) in a two-wire line. For an overhead line, the magnetic flux density at ground level is about 20 mT for a ± 500 kV line. For an underground transmission line buried at 1.4 m and carrying a maximum current of about 1 kA, the maximum magnetic flux density is less than 10 mT at ground level.

 

Major technologies that involve the use of large static magnetic fields are listed in table 49.10 along with their corresponding exposure levels.

 

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Table 49.10 Major technologies involving the use of large static magnetic fields, and corresponding exposure levels

 

Procedures

Exposure levels

Energy technologies

Thermonuclear fusion reactors

Fringe fields up to 50 mT in areas accessible to personnel.

Below 0.1 mT outside the reactor site

Magnetohydrodynamic systems

Approximately 10 mT at about 50 m;

100 mT only at distances greater than 250 m

Superconducting magnet energy storage systems

Fringe fields up to 50 mT at operator-accessible locations

Superconducting generators and transmission lines

Fringe fields projected to be less than 100 mT

Research facilities

Bubble chambers

During changes of film cassettes, the field is about 0.4-0.5 T at foot level and about 50 mT

at the level of the head

Superconducting spectrometers

About 1 T at operator-accessible locations

Particle accelerators

Personnel are seldom exposed because of exclusion from the high radiation zone. Exceptions

arise only during maintenance

Isotope separation units

Brief exposures to fields up to 50 mT

Usually field levels are less than 1 mT

Industry

Aluminium production

Levels up to 100 mT in operator-accessible locations

Electrolytic processes

Mean and maximum field levels of about 10 and 50 mT, respectively

Production of magnets

2-5 mT at worker’s hands; in the range of 300 to 500 mT at the level of the chest and head

Medicine

Nuclear magnetic resonance imaging and spectroscopy

An unshielded 1-T magnet produces about 0.5 mT at 10 m, and an unshielded 2-T magnet

produces the same exposure at about 13 m

 

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Biological effects

Evidence from experiments with laboratory animals indicates that there are no significant effects on the many developmental, behavioural, and physiological factors evaluated at static magnetic flux densities up to 2 T. Nor have studies on mice demonstrated any harm to the foetus from exposure to magnetic fields up to 1 T.

 

Theoretically, magnetic effects could retard blood flowing in a strong magnetic field and produce a rise in blood pressure. A flow reduction of at most a few per cent could be expected at 5 T, but none was observed in human subjects at 1.5 T, when investigated.

 

Some studies on workers involved in the manufacture of permanent magnets have reported various subjective symptoms and functional disturbances: irritability, fatigue, headache, loss of appetite, bradycardia (slow heart beat), tachycardia (rapid heart beat), decreased blood pressure, altered EEG, itching, burning and numbness. However, lack of any statistical analysis or assessment of the impact of physical or chemical hazards in the working environment significantly reduces the validity of these reports and makes them difficult to evaluate. Although the studies are inconclusive, they do suggest that, if long-term effects do in fact occur, they are very subtle; no cumulative gross effects have been reported.

 

Individuals exposed to a 4T magnetic flux density have been reported as experiencing sensory effects associated with motion in the field, such as vertigo (dizziness), feeling of nausea, a metallic taste, and magnetic sensations when moving the eyes or head. However, two epidemiological surveys of general health data in workers chronically exposed to static magnetic fields failed to reveal any significant health effects. Health data of 320 workers were obtained in plants using large electrolytic cells for chemical separation processes where the average static field level in the work environment was 7.6 mT and the maximum field was 14.6 mT. Slight changes in the white blood cell count, but still within the normal range, were detected in the exposed group compared to the 186 controls. None of the observed transient changes in blood pressure or other blood measurements was considered indicative of a significant adverse effect associated with magnetic field exposure. In another study, the prevalence of disease was evaluated among 792 workers who were occupationally exposed to static magnetic fields. The control group consisted of 792 unexposed workers matched for age, race and socio-economic status. The range of magnetic field exposures varied from 0.5 mT for long durations to 2 T for periods of several hours. No statistically significant change in the prevalence of 19 categories of disease was observed in the exposed group compared with the controls. No difference in the prevalence of disease was found between a subgroup of 198 who had experienced exposures of 0.3 T or higher for periods of one hour or longer when compared with the remainder of the exposed population or the matched controls.

 

A report on workers in the aluminium industry indicated an elevated leukaemia mortality rate. Although this epidemiological study reported an increased cancer risk for persons directly involved in aluminium production where workers are exposed to large static magnetic fields, there is at present no clear evidence to indicate exactly which carcinogenic factors within the work environment are responsible. The process used for aluminium reduction creates coal tar, pitch volatiles, fluoride fumes, sulphur oxides and carbon dioxide, and some of these might be more likely candidates for cancer-causing effects than magnetic field exposure.

 

In a study on French aluminium workers, cancer mortality and mortality from all causes were found not to differ significantly from that observed for the general male population of France (Mur et al. 1987).

 

Another negative finding linking magnetic field exposures to possible cancer outcomes comes from a study of a group of workers at a chloroalkali plant where the 100 kA DC currents used for the electrolytic production of chlorine gave rise to static magnetic flux densities, at worker’s locations, ranging from 4 to 29 mT. The observed versus expected incidence of cancer among these workers over a 25-year period showed no significant differences.

 

Measurements, prevention and exposure standards

During the last thirty years, the measurement of magnetic fields has undergone considerable development. Progress in techniques has made it possible to develop new methods of measurement as well as to improve old ones.

 

The two most popular types of magnetic field probes are a shielded coil and a Hall probe. Most of the commercially available magnetic field meters use one of them. Recently, other semiconductor devices, namely bipolar transistors and FET transistors, have been proposed as magnetic field sensors. They offer some advantages over Hall probes, such as higher sensitivity, greater spatial resolution and broader frequency response.

 

The principle of the nuclear magnetic resonance (NMR) measurement technique is to determine the resonant frequency of the test specimen in the magnetic field to be measured. It is an absolute measurement that can be made with very great accuracy. The measuring range of this method is from about 10 mT to 10 T, with no definite limits. In field measurements using the proton magnetic resonance method, an accuracy of  is easily obtained with simple apparatus and an accuracy of  can be reached with extensive precautions and refined equipment. The inherent shortcoming of the NMR method is its limitation to a field with a low gradient and the lack of information about the field direction.

 

Recently, several personal dosimeters suitable for monitoring exposures to static magnetic fields have also been developed.

 

Protective measures for the industrial and scientific use of magnetic fields can be categorized as engineering design measures, the use of separation distance, and administrative controls. Another general category of hazard-control measures, which include personal protective equipment (e.g., special garments and face masks), does not exist for magnetic fields. However, protective measures against potential hazards from magnetic interference with emergency or medical electronic equipment and for surgical and dental implants are a special area of concern. The mechanical forces imparted to ferromagnetic (iron) implants and loose objects in high-field facilities require that precautions be taken to guard against health and safety hazards.

 

Techniques to minimize undue exposure to high-intensity magnetic fields around large research and industrial facilities generally fall into four types:

 

1. distance and time

2. magnetic shielding

3. electromagnetic interference (EMI) and compatibility

4. administrative measures.

 

The use of warning signs and special-access areas to limit exposure of personnel near large magnet facilities has been of greatest use for controlling exposure. Administrative controls such as these are generally preferable to magnetic shielding, which can be extremely expensive. Loose ferromagnetic and paramagnetic (any magnetizing substances) objects can be converted into dangerous missiles when subjected to intense magnetic field gradients. Avoidance of this hazard can be achieved only by removing loose metallic objects from the area and from personnel. Such items as scissors, nail files, screwdrivers and scalpels should be banned from the immediate vicinity.

 

The earliest static magnetic field guidelines were developed as an unofficial recommendation in the former Soviet Union. Clinical investigations formed the basis for this standard, which suggested that the static magnetic field strength at the workplace should not exceed 8 kA/m (10 mT).

 

The American Conference of Governmental Industrial Hygienists issued TLVs of static magnetic flux densities that most workers could be exposed to repeatedly, day after day, without adverse health effects. As for electric fields, these values should be used as guides in the control of exposure to static magnetic fields, but they should not be regarded as a sharp line between safe and dangerous levels. According to ACGIH, routine occupational exposures should not exceed 60 mT averaged over the whole body or 600 mT to the extremities on a daily, time-weighted basis. A flux density of 2 T is recommended as a ceiling value. Safety hazards may exist from the mechanical forces exerted by the magnetic field upon ferromagnetic tools and medical implants.

 

In 1994, the International Commission on Non-Ionizing Radiation Protection (ICNIRP 1994) finalized and published guidelines on limits of exposure to static magnetic fields. In these guidelines, a distinction is made between exposure limits for workers and the general public. The limits recommended by the ICNIRP for occupational and general public exposures to static magnetic fields are summarized in table 49.11. When magnetic flux densities exceed 3 mT, precautions should be taken to prevent hazards from flying metallic objects. Analogue watches, credit cards, magnetic tapes and computer disks may be adversely affected by exposure to 1 mT, but this is not seen as a safety concern for people.

 

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Table 49.11 Limits of exposure to static magnetic fields recommended by the International Commission on Non-Ionizing Radiation Protection (ICNIRP)

 

Exposure characteristics

Magnetic flux density

Occupational

Whole working day (time-weighted average)

200 mT

Ceiling value

2 T

Limbs

5 T

General Public

Continuous exposure

40 mT

 

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Occasional access of the public to special facilities where magnetic flux densities exceed 40 mT can be allowed under appropriately controlled conditions, provided that the appropriate occupational exposure limit is not exceeded.

 

ICNIRP exposure limits have been set for a homogeneous field. For inhomogeneous fields (variations within the field), the average magnetic flux density must be measured over an area of 100 .

 

According to a recent NRPB document, the restriction on acute exposure to less than 2 T will avoid acute responses such as vertigo or nausea and adverse health effects resulting from cardiac arrhythmia (irregular heart beat) or impaired mental function. In spite of the relative lack of evidence from studies of exposed populations regarding possible long-term effects of high fields, the Board considers it advisable to restrict long-term, time-weighted exposure over 24 hours to less than 200 mT (one-tenth of that intended to prevent acute responses). These levels are quite similar to those recommended by ICNIRP; ACGIH TLVs are slightly lower.

 

People with cardiac pacemakers and other electrically activated implanted devices, or with ferromagnetic implants, may not be adequately protected by the limits given here. The majority of cardiac pacemakers are unlikely to be affected from exposure to fields below 0.5 mT. People with some ferromagnetic implants or electrically activated devices (other than cardiac pacemakers) may be affected by fields above a few mT.

 

Other sets of guidelines recommending limits of occupational exposure exist: three of these are enforced in high-energy physics laboratories (Stanford Linear Accelerator Center and Lawrence Livermore National Laboratory in California, CERN accelerator laboratory in Geneva), and an interim guideline at the US Department of Energy (DOE).

 

In Germany, according to a DIN Standard, occupational exposures should not exceed a static magnetic field strength of 60 kA/m (about 75 mT). When only the extremities are exposed, this limit is set at 600 kA/m; field strength limits up to 150 kA/m are permitted for short, whole-body exposures (up to 5 min per hour).