Extremely low frequency (ELF) and very low frequency (VLF) electric and magnetic fields encompass the frequency range above static (> 0 Hz) fields up to 30 kHz. For this paper ELF is defined as being in the frequency range > 0 to 300 Hz and VLF in the range > 300 Hz to 30 kHz. In the frequency range > 0 to 30 kHz, the wavelengths vary from ∞(infinity) to 10 km and so the electric and magnetic fields act essentially independently of each other and must be treated separately. The electric field strength (E) is measured in volts per metre (V/m), the magnetic field strength (H) is measured in amperes per metre (A/m) and the magnetic flux density (B) in tesla (T).

Considerable debate about possible adverse health effects has been expressed by workers using equipment that operates in this frequency range. By far the most common frequency is 50/60 Hz, used for the generation, distribution and use of electric power. Concerns that exposure to 50/60 Hz magnetic fields may be associated with an increased cancer incidence have been fuelled by media reports, distribution of misinformation and ongoing scientific debate (Repacholi 1990; NRC 1996).

The purpose of this article is to provide an overview of the following topic areas:

• sources, occupations and applications
• dosimetry and measurement
• interaction mechanisms and biological effects
• human studies and effects on health
• protective measures
• occupational exposure standards.

Summary descriptions are provided to inform workers of the types and strengths of fields from major sources of ELF and VLF, biological effects, possible health consequences and current exposure limits. An outline of safety precautions and protective measures is also given. While many workers use visual display units (VDUs), only brief details are given in this article since they are covered in greater detail elsewhere in the Encyclopaedia.

Much of the material contained here can be found in greater detail in a number of recent reviews (WHO 1984, 1987, 1989, 1993; IRPA 1990; ILO 1993; NRPB 1992, 1993; IEEE 1991; Greene 1992; NRC 1996).

Sources of Occupational Exposure

Levels of occupational exposure vary considerably and are strongly dependent upon the particular application. Table 1 gives a summary of typical applications of frequencies in the range > 0 to 30 kHz.

Table 1. Applications of equipment operating in the range > 0 to 30 kHz

Power generation and distribution

The principal artificial sources of 50/60 Hz electric and magnetic fields are those involved in power generation and distribution, and any equipment using electric current. Most such equipment operates at the power frequencies of 50 Hz in most countries and 60 Hz in North America. Some electric train systems operate at 16.67 Hz.

High voltage (HV) transmission lines and substations have associated with them the strongest electric fields to which workers may be routinely exposed. Conductor height, geometrical configuration, lateral distance from the line, and the voltage of the transmission line are by far the most significant factors in considering the maximum electric field strength at ground level. At lateral distances of about twice the line height, the electric field strength decreases with distance in an approximately linear fashion (Zaffanella and Deno 1978). Inside buildings near HV transmission lines, the electric field strengths are typically lower than the unperturbed field by a factor of about 100,000, depending on the configuration of the building and the structural materials.

Magnetic field strengths from overhead transmission lines are usually relatively low compared to industrial applications involving high currents. Electrical utility employees working in substations or on the maintenance of live transmission lines form a special group exposed to larger fields (of 5 mT and higher in some cases). In the absence of ferromagnetic materials, the magnetic field lines form concentric circles around the conductor. Apart from the geometry of the power conductor, the maximum magnetic flux density is determined only by the magnitude of the current. The magnetic field beneath HV transmission lines is directed mainly transverse to the line axis. The maximum flux density at ground level may be under the centre line or under the outer conductors, depending on the phase relationship between the conductors. The maximum magnetic flux density at ground level for a typical double circuit 500 kV overhead transmission lines system is approximately 35 μT per kiloampere of current transmitted (Bernhardt and Matthes 1992). Typical values for the magnetic flux density up to 0.05 mT occur in workplaces near overhead lines, in substations and in power stations operating at frequencies of 16 2/3, 50, or 60 Hz (Krause 1986).

Industrial processes

Occupational exposure to magnetic fields comes predominantly from working near industrial equipment using high currents. Such devices include those used in welding, electroslag refining, heating (furnaces, induction heaters) and stirring.

Surveys on induction heaters used in industry, performed in Canada (Stuchly and Lecuyer 1985), in Poland (Aniolczyk 1981), in Australia (Repacholi, unpublished data) and in Sweden (Lövsund, Oberg and Nilsson 1982), show magnetic flux densities at operator locations ranging from 0.7 μT to 6 mT, depending on the frequency used and the distance from the machine. In their study of magnetic fields from industrial electro-steel and welding equipment, Lövsund, Oberg and Nilsson (1982) found that spot-welding machines (50 Hz, 15 to 106 kA) and ladle furnaces (50 Hz, 13 to 15 kA) produced fields up to 10 mT at distances up to 1 m. In Australia, an induction heating plant operating in the range 50 Hz to 10 kHz was found to give maximum fields of up to 2.5 mT (50 Hz induction furnaces) at positions where operators could stand. In addition maximum fields around induction heaters operating at other frequencies were 130 μT at 1.8 kHz, 25 μT at 2.8 kHz and in excess of 130 μT at 9.8 kHz.

Since the dimensions of coils producing the magnetic fields are often small there is seldom high exposure to the whole body, but rather local exposure mainly to the hands. Magnetic flux density to the hands of the operator may reach 25 mT (Lövsund and Mild 1978; Stuchly and Lecuyer 1985). In most cases the flux density is less than 1 mT. The electric field strength near the induction heater is usually low.

Workers in the electrochemical industry may be exposed to high electric and magnetic field strengths because of electrical furnaces or other devices using high currents. For instance, near induction furnaces and industrial electrolytic cells magnetic flux densities can be measured as high as 50 mT.

Visual display units

The use of visual display units (VDUs) or video display terminals (VDTs) as they are also called, grows at an ever increasing rate. VDT operators have expressed concerns about possible effects from emissions of low-level radiations. Magnetic fields (frequency 15 to 125 kHz) as high as 0.69 A/m (0.9 μT) have been measured under worst-case conditions close to the surface of the screen (Bureau of Radiological Health 1981). This result has been confirmed by many surveys (Roy et al. 1984; Repacholi 1985 IRPA 1988). Comprehensive reviews of measurements and surveys of VDTs by national agencies and individual experts concluded that there are no radiation emissions from VDTs that would have any consequences for health (Repacholi 1985; IRPA 1988; ILO 1993a). There is no need to perform routine radiation measurements since, even under worst-case or failure mode conditions, the emission levels are well below the limits of any international or national standards (IRPA 1988).

A comprehensive review of emissions, summary of the applicable scientific literature, standards and guidelines has been provided in the document (ILO 1993a).

Medical applications

Patients suffering from bone fractures that do not heal well or unite have been treated with pulsed magnetic fields (Bassett, Mitchell and Gaston 1982; Mitbreit and Manyachin 1984). Studies are also being conducted on the use of pulsed magnetic fields to enhance wound healing and tissue regeneration.

Various devices generating magnetic field pulses are used for bone growth stimulation. A typical example is the device that generates an average magnetic flux density of about 0.3 mT, a peak strength of about 2.5 mT, and induces peak electric field strengths in the bone in the range of 0.075 to 0.175 V/m (Bassett, Pawluk and Pilla 1974). Near the surface of the exposed limb, the device produces a peak magnetic flux density of the order of 1.0 mT causing peak ionic current densities of about 10 to 100 mA/m² (1 to 10 μA/cm²) in tissue.

Measurement

Prior to the commencement of measurements of ELF or VLF fields, it is important to obtain as much information as possible about the characteristics of the source and the exposure situation. This information is required for the estimation of the expected field strengths and the selection of the most appropriate survey instrumentation (Tell 1983).

Information about the source should include:

• frequencies present, including harmonics
• power transmitted
• polarization (orientation of E field)
• modulation characteristics (peak and average values)
• duty cycle, pulse width, and pulse repetition frequency
• antenna characteristics, such as type, gain, beam width and scan rate.

Information about the exposure situation must include:

• distance from the source
• existence of any scattering objects. Scattering by plane surfaces can enhance the E field by a factor of 2. Even greater enhancement may result from curved surfaces, e.g., corner reflectors.

Results of surveys conducted in occupational settings are summarized in table 2.

Table 2. Occupational sources of exposure to magnetic fields

Source: Allen 1991; Bernhardt 1988; Krause 1986; Lövsund, Oberg and Nilsson 1982; Repacholi, unpublished data; Stuchly 1986; Stuchly and Lecuyer 1985, 1989.

Instrumentation

An electric or magnetic field-measuring instrument consists of three basic parts: the probe, the leads and the monitor. To ensure appropriate measurements, the following instrumentation characteristics are required or are desirable:

• The probe must respond only to the E field or the H field and not to both simultaneously.
• The probe must not produce significant perturbation of the field.
• The leads from the probe to the monitor must not disturb the field at the probe significantly, or couple energy from the field.
• The frequency response of the probe must cover the range of frequencies required to be measured.
• If used in the reactive near-field, the dimensions of the probe sensor should preferably be less than a quarter of a wavelength at the highest frequency present.
• The instrument should indicate the root mean square (rms) value of the measured field parameter.
• The response time of the instrument should be known. It is desirable to have a response time of about 1 second or less, so that intermittent fields are easily detected.
• The probe should be responsive to all polarization components of the field. This may be accomplished either by inherent isotropic response, or by physical rotation of the probe through three orthogonal directions.
• Good overload protection, battery operation, portability and rugged construction are other desirable characteristics.
• Instruments provide an indication of one or more of the following parameters: average E field (V/m) or mean square E field (V²/m²); average H field (A/m) or mean square H field (A²/m²).

Surveys

Surveys are usually conducted to determine whether fields existing in the workplace are below limits set by national standards. Thus the person taking the measurements must be fully familiar with these standards.

All occupied and accessible locations should be surveyed. The operator of the equipment under test and the surveyor should be as far away as practicable from the test area. All objects normally present, which may reflect or absorb energy, must be in position. The surveyor should take precautions against radiofrequency (RF) burns and shock, particularly near high-power, low-frequency systems.

Interaction Mechanisms and Biological Effects

Interaction mechanisms

The only established mechanisms by which ELF and VLF fields interact with biological systems are:

• Electric fields which induce a surface charge on an exposed body which results in currents (measured in mA/m²) inside the body, the magnitude of which is related to the surface charge density. Depending on the exposure conditions, size, shape and position of the exposed body in the field, the surface charge density can vary greatly, resulting in a variable and non-uniform distribution of currents inside the body.
• Magnetic fields also act on humans by inducing electric fields and currents inside the body.
• Electric charges induced in a conducting object (e.g., an automobile) exposed to ELF or VLF electric fields may cause current to pass through a person in contact with it.
• Magnetic field coupling to a conductor (for example, a wire fence) causes electric currents (of the same frequency as the exposing field) to pass through the body of a person in contact with it.
• Transient discharges (sparks) can occur when people and metal objects exposed to a strong electric field come into sufficiently close proximity.
• Electric or magnetic fields may interfere with implanted medical devices (e.g., unipolar cardiac pacemakers) and cause malfunction of the device.

The first two interactions listed above are examples of direct coupling between persons and ELF or VLF fields. The last four interactions are examples of indirect coupling mechanisms because they can occur only when the exposed organism is in the vicinity of other bodies. These bodies can include other humans or animals and objects such as automobiles, fences or implanted devices.

While other mechanisms of interaction between biological tissues and ELF or VLF fields have been postulated or there is some evidence to support their existence (WHO 1993; NRPB 1993; NRC 1996), none has been shown to be responsible for any adverse consequence to health.

Health effects

The evidence suggests that most of the established effects of exposure to electric and magnetic fields in the frequency range > 0 to 30 kHz result from acute responses to surface charge and induced current density. People can perceive the effects of the oscillating surface charge induced on their bodies by ELF electric fields (but not by magnetic fields); these effects become annoying if sufficiently intense. A summary of the effects of currents passing through the human body (thresholds for perception, let-go or tetanus) are given in table 3.

Table 3. Effects of currents passing through the human body

Source: Bernhardt 1988a.

Human nerve and muscle cells have been stimulated by the currents induced by exposure to magnetic fields of several mT and 1 to 1.5 kHz; threshold current densities are thought to be above 1 A/m². Flickering visual sensations can be induced in the human eye by exposure to magnetic fields as low as about 5 to 10 mT (at 20 Hz) or electric currents directly applied to the head. Consideration of these responses and of the results of neurophysiological studies suggests that subtle central nervous system functions, such as reasoning or memory, may be affected by current densities above 10 mA/m² (NRPB 1993). Threshold values are likely to remain constant up to about 1 kHz but rise with increasing frequency thereafter.

Several in vitro studies (WHO 1993; NRPB 1993) have reported metabolic changes, such as alterations in enzyme activity and protein metabolism and decreased lymphocyte cytotoxicity, in various cell lines exposed to ELF and VLF electric fields and currents applied directly to the cell culture. Most effects have been reported at current densities between about 10 and 1,000 mA/m², although these responses are less clearly defined (Sienkiewicz, Saunder and Kowalczuk 1991). However, it is worth noting that the endogenous current densities generated by the electrical activity of nerves and muscles are typically as high as 1 mA/m² and may reach up to 10 mA/m² in the heart. These current densities will not adversely affect nerve, muscle and other tissues. Such biological effects will be avoided by restricting the induced current density to less than 10 mA/m² at frequencies up to about 1 kHz.

Several possible areas of biological interaction which have many health implications and about which our knowledge is limited include: possible changes in night-time melatonin levels in the pineal gland and alterations in circadian rhythms induced in animals by exposure to ELF electric or magnetic fields, and possible effects of ELF magnetic fields on the processes of development and carcinogenesis. In addition, there is some evidence of biological responses to very weak electric and magnetic fields: these include the altered mobility of calcium ions in brain tissue, changes in neuronal firing patterns, and altered operand behaviour. Both amplitude and frequency “windows” have been reported which challenge the conventional assumption that the magnitude of a response increases with increasing dose. These effects are not well established and do not provide a basis for establishing restrictions on human exposure, although further investigations are warranted (Sienkievicz, Saunder and Kowalczuk 1991; WHO 1993; NRC 1996).

Table 4 gives the approximate ranges of induced current densities for various biological effects in humans.

Table 4. Approximate current density ranges for various biological effects

Source: Sienkiewicz et al. 1991.

Occupational Exposure Standards

Nearly all standards having limits in the range > 0-30 kHz have, as their rationale, the need to keep induced electric fields and currents to safe levels. Usually the induced current densities are restricted to less than 10 mA/m². Table 5 gives a summary of some current occupational exposure limits.

Table 5. Occupational limits of exposure to electric and magnetic fields in the frequency range > 0 to 30 kHz (note that f is in Hz)

Protective Measures

Occupational exposures that occur near high voltage transmission lines depend on the worker’s location either on the ground or at the conductor during live-line work at high potential. When working under live-line conditions, protective clothing may be used to reduce the electric field strength and current density in the body to values similar to those that would occur for work on the ground. Protective clothing does not weaken the influence of the magnetic field.

The responsibilities for the protection of workers and the general public against the potentially adverse effects of exposure to ELF or VLF electric and magnetic fields should be clearly assigned. It is recommended that the competent authorities consider the following steps:

• development and adoption of exposure limits and the implementation of a compliance programme
• development of technical standards to reduce the susceptibility to electromagnetic interference, for example, for pacemakers
• development of standards defining zones with limited access around sources of strong electric and magnetic fields because of electromagnetic interference (e.g., for pacemakers and other implanted devices). The use of appropriate warning signs should be considered.
• requirement of specific assignment of a person responsible for the safety of workers and the public at each site with high exposure potentials
• development of standardized measurement procedures and survey techniques
• requirements for the education of workers on the effects of exposure to ELF or VLF electric and magnetic fields and the measures and rules which are designed to protect them
• drafting of guidelines or codes of practice for worker safety in ELF or VLF electric and magnetic fields. ILO (1993a) provides excellent guidance for such a code.

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