Federal Ministry of Labour and Social Affairs
Forschungs penavi Sozialforschung
400-E Electromagnetic fields at workplaces Final Report
ISSN 0174-4992
Impressum: Herausgeber: Bundesministerium furArbeit und Soziales Referat Information. Publikation, Redaktion 53107 Bonn Stand: NOvember 2011 Artikel-Nr.: FB 400-E E-Mail;
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Bericht der EMF-Arbeitsgruppe des Bundesministeriums f¨ ur Arbeit und Soziales
Elektromagnetische Felder am Arbeitsplatz Ein neuer wissenschaftlicher Ansatz f¨ ur die Sicherheit und den Gesundheitsschutz der Besch¨ aftigten
Electromagnetic fields at workplaces A new scientific approach to occupational health and safety
F. B¨ orner H. Br¨ uggemeyer S. Eggert M. Fischer H. Heinrich K. Hentschel H. Neuschulz
Stand: November 2011
Executive summary This report provides an in-depth analysis of the physical and physiological background for an effective protection of the health and safety of workers with respect to occupational exposure to electric, magnetic and electromagnetic fields (EMF), based on current scientific knowledge. Answers are given to the concerns being raised by stakeholders and to shortcomings within Directive 2004/40/EC. Therefore, information provided in this report, especially the figures and tables in section 4.1 and 4.2, can serve as a sound base for a review of the risk-related provisions of Directive 2004/40/EC. A revised concept of exposure limit values for the low frequency electric and magnetic fields is based on the physiologically relevant parameter of the peak electric field strength in the tissue and represents common scientific understanding. Based on this concept a set of exposure limit values has been laid down guaranteeing the health and safety of workers without the need for unnecessary and costly measures or unduly impacting the use of certain technologies or industrial processes. For an easy and also cost-effective assessment of the risks due to the exposure to low frequency electric and magnetic fields and in order to avoid unnecessary complex and time-consuming calculations currently necessary to show the compliance of an exposure situation with the exposure limit values, two sets of easier-to-implement action levels are given. These action levels can be compared directly with measurable electric field strengths or magnetic flux densities. Because all EMF-related biological effects in the low frequency range are linked to peak values of the internal electric field strength in the tissue, all exposure limit values and lower and upper action levels are given as peak values and not as rms-values as in Directive 2004/40/EC. The report also addresses the risks of workers with respect to the movement and the projectile risk in static magnetic fields. For the low frequency range it provides a sound solution on how to deal with pulsed electric and magnetic fields, multi-frequency electric and magnetic fields and contact currents. Contact currents are now classified as exposure limit values because of the biological relevance. For both the static and the low frequency range, effects of localized exposure and time or spatial averaging are considered in the report. So far, no changes have been proposed for frequencies higher than 100 kHz.
Preface Due to the ongoing technological development and scientific research regarding occupational exposure to electric, magnetic and electromagnetic fields, this report presents the current knowledge and understanding of open questions and concerns on a solid and well established scientific foundation. This report provides an in-depth analysis and the most up-to-date information available for the ongoing discussion concerning occupational health and safety with regard to workers exposure to static and low frequency electric and magnetic fields. If necessary, it will be updated when new technologies emerge, new studies and results become available or new questions and concerns are being raised. Apart from the considerations in this document, additional guidance and information may be necessary to assist the employer in risk assessment, thus saving time and money while guaranteeing the safety and health of workers at the same time.
Contents 1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
2
Physiological effects of EMF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
2.1
Direct effects of electric, magnetic and electromagnetic fields . . . . . . . .
2
2.1.1
2
2.1.2
Static electric fields . . . . . . . . . . . . . . . . . . . . .
2
2.1.1.2
Low-frequency electric fields . . . . . . . . . . . . . . . .
3
Magnetic fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
Static magnetic fields . . . . . . . . . . . . . . . . . . . .
4
2.1.2.2
Low-frequency magnetic fields . . . . . . . . . . . . . . .
5
High-frequency electromagnetic fields . . . . . . . . . . . . . . . .
6
Indirect effects of electric and magnetic fields . . . . . . . . . . . . . . . . .
6
2.2.1
Electric fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
2.2.2
Magnetic fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
Body models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
Neurophysiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
3.1
Mechanisms and facts for the creation of action potentials . . . . . . . . . .
8
3.2
Electrical stimulation of excitable tissues
2.2
2.3
4
2.1.1.1
2.1.2.1 2.1.3
3
Electric fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . .
12
3.2.1
Basic facts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12
3.2.2
Long stimuli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
3.2.3
Short stimuli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
3.2.4
CNS tissue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
3.2.5
Uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16
3.2.6
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16
Limiting occupational exposure to static and low frequency electric and magnetic fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
4.1
Exposure limit values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
4.1.1
Static electric fields . . . . . . . . . . . . . . . . . . . . . . . . . .
18
4.1.2
Static magnetic fields . . . . . . . . . . . . . . . . . . . . . . . . .
18
4.1.3
Low frequency electric and magnetic fields . . . . . . . . . . . . .
20
4.1.4
Contact currents . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21
Upper and lower action levels . . . . . . . . . . . . . . . . . . . . . . . . . .
23
4.2.1
25
4.2
Upper action level . . . . . . . . . . . . . . . . . . . . . . . . . . . i
4.2.2
4.2.1.1
Electric fields . . . . . . . . . . . . . . . . . . . . . . . . .
25
4.2.1.2
Magnetic fields . . . . . . . . . . . . . . . . . . . . . . . .
26
Lower action level . . . . . . . . . . . . . . . . . . . . . . . . . . .
28
4.2.2.1
Electric fields . . . . . . . . . . . . . . . . . . . . . . . . .
28
4.2.2.2
Magnetic fields . . . . . . . . . . . . . . . . . . . . . . . .
29
Special exposure situations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
31
5.1
Simultaneous exposure to electric and magnetic fields . . . . . . . . . . . .
31
5.2
Simultaneous exposure to multiple field sources operating with the same frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
31
Simultaneous exposure to multiple frequency fields . . . . . . . . . . . . . .
31
5.3.1
Summation formulae . . . . . . . . . . . . . . . . . . . . . . . . . .
31
5.3.2
Assessment of fields with arbitrary temporal behaviour . . . . . .
32
5.3.3
Harmonic content . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
5.4
Localized exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
5.5
Movement in static magnetic fields . . . . . . . . . . . . . . . . . . . . . . .
34
5.6
Interference with active implanted medical devices (AIMD) . . . . . . . . .
34
5.7
Projectile risk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
35
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
37
Annex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
44
A
Quantities, variables, abbreviations and SI-units . . . . . . . . . . . . . . . . . . .
44
B
Tissue data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
45
5
5.3
ii
1
Introduction
On 29 April 2004, the European Parliament and the Council adopted Directive 2004/40/EC on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (electromagnetic fields). This directive is commonly referred to as the EMF Directive. It establishes minimum health and safety requirements for the protection of workers against the risks arising from exposure to static and time-varying electric, magnetic and electromagnetic fields (EMF). The frequency range extends from 0 Hz to 300 GHz. Directive 2004/40/EC obliges the employers to assess the risks arising from electric, magnetic and electromagnetic fields at the workplace and to take adequate measures to eliminate or to minimize such risks where necessary. The Directive refers to a set of exposure limit values (ELV) listed in table 1 in its Annex. The exposure to electromagnetic fields cannot be measured directly because the physiologically relevant physical quantities, e.g. current density and specific absorption rate, only exist inside the human body. In order to facilitate the application of the directive a set of so called action values (AV) was given to simplify the determination of the level of exposure at a workplace. If these action values are not exceeded, an inherent compliance with the exposure limit values is guaranteed. However, the exceedance of the action values does not automatically lead to an exceedance of the exposure limit values. Where action values are exceeded, employers can make further efforts to assess and, if necessary, prove that the exposure is still below the exposure limit values. Since the adoption of Directive 2004/40/EC scientific knowledge with regard to • the concept of limit values, • risks related to the movement in a static magnetic field, • the projectile risk, • risks related to pulsed electric and magnetic fields, • risks related to multi-frequency electric and magnetic fields, • risks related to contact currents, • risks related to implanted medical devices has significantly improved. This report will address these aspects on a solid and well established scientific and technological basis.
1
2
Physiological effects of EMF
The physiological effects of electrical, magnetic and electromagnetic fields on the human body are dependent on the frequency. The effects of static electric fields are limited to the surface of the human body and can cause motion of body hair and corona discharges. Static magnetic fields exert forces on ferro- and dia-magnetic materials as well as charged moving particles. This may lead to acceleration, torque effects and the induction of electric fields in the tissue. In the low-frequency range up to some 100 kHz the main physiological effect is the electrical stimulation of excitable body tissues like muscles, nerves and sensory organs. In the frequency range between several 100 kHz and some MHz electrical stimulation and tissue heating occurs. The higher the frequency, the more the tissue heating effects increase and the stimulation effects decrease. Tissue heating effects are dominant for frequencies above several MHz. A further distinction is made with regard to the interaction with the human body. If there is a direct interaction between EMF and the human body, e.g. stimulation of muscles, nerves or sensory organs or tissue heating, this type of interaction is called an direct effect. If there is an interaction between EMF and objects outside the human body, e.g. contact currents, projectile risk or the interference with implanted medical devices, this type of interaction is called a indirect effect. There are no confirmed long-term health effects related to the exposure to EMF.
2.1
Direct effects of electric, magnetic and electromagnetic fields
2.1.1
Electric fields
The relationship between the external undisturbed electric field strength E0 and the electric field strength hereby induced in the body tissue Ei is established through the condition that the normal component of the displacement current must remain steady at the surface boundary of the human body [30, 41]. For a simple homogeneous ellipsoid model of the body it is expressed by: E0 · k · ε0 · 2π · f = κ · Ei with
k ε0 f κ
(2.1)
field distortion factor; for human beings k ≈ 13 . . . 18 A2 ·s4 permittivity of free space (vacuum); ε0 = µ01·c2 ≈ 8, 854 · 10−12 kg·m 3 0 frequency of the field (mean) conductivity of the body tissue(s)
In principal eqn. 2.1 remains valid even for more natural and anatomically correct body models. However, the variables k and κ become parametric functions. 2.1.1.1
Static electric fields
As an immediate result of eqn. 2.1 for static fields (f = 0) it follows that the electric field strength inside the tissue Ei is (nearly) zero, regardless of the electric field strength of the external static electric field. The external static electric field breaks down completely at the surface of the human body, the inner body is totally shielded from any effect of the external static electric field. Therefore, no direct physiological effect can occur inside the human body. External static electric fields, exceeding E0 ≈ 30 kV/m, can cause corona discharges at the surface of the human body, e.g. fingers, nose, ears, hairs [41, 63]. Those corona discharges depend on the 2
external field strength, posture, the size and form of the body and climatic factors, e.g. relative humidity. Such discharges can be annoying, startling or even painful. Significant external static electric fields can only occur where high DC-voltage is used (DCpowerlines, including switchyards and inverter stations) or can be produced by triboelectricity, e.g. plastics production and other industrial processes where highly insulating solids or liquids are handled and charge separation can occur. 2.1.1.2
Low-frequency electric fields
External low frequency (LF) electric fields can generate internal electric fields in the tissue. Consulting eqn. 2.1 the relationship between external and internal field strength can be rewritten as: Ei =
k · ε0 · 2π · f · E0 κ
(2.2)
As (k ·ε0 ·2π ·f ) is very small compared to κ in the low frequency range, there still exists a shielding effect from the outside to the inside of the human body. However, it is not a complete shielding like in static electric fields. Therefore there is the potential for adverse effects inside the human body. However, external electric fields used by technological processes or near electric powerlines are generally not strong enough to cause adverse health effects. Especially for power frequency fields with an external electric field strength of some (kV/m) the electric field strength in the body tissue is in the range of some (mV/m). With very high electric field strengths currently in use by technological processes (mean value ≈ 180 kV/m for experimental high-voltage transmission lines with voltages >1500 kV [46]) and limited by the breakdown field strength of air (≈ 3000 kV/m for homogeneous fields [41, 46]), it is not possible to generate electrical fields in the tissue of the human body that can trigger any adverse physiological effects like electrophosphenes (E0 >200 kV/m at 50 Hz) or peripheral nerve stimulation (E0 >4000 kV/m at 50 Hz). Very strong electric fields higher than 30 kV/m can also cause corona discharges on the surface of the human body. However, these electrical field strengths in the tissue can interfere with the proper operation of active medical implants, e.g. pacemakers or cardioverter defibrillators, see section 5.6. 2.1.2
Magnetic fields
Magnetic fields exert physical forces on electric charges, but only when such charges are in motion. There are three physical effects of magnetic fields on biological tissues: • Electrodynamic forces and magnetic induction • Magneto-mechanical effects • Electron spin interaction The main effect of a magnetic field is the Lorentz force F~ on a point charge q moving with velocity ~v as described by: ~ F~ = q ~v × B (2.3) Due to these forces charge transfers develop in the biological tissue. They generate differences in the electrical potential and thus an electrical field strengths in the tissue of the human body. The
3
connections between electrical field strength and the magnetic flux density describes Faraday’s law of induction [30]. Z I ~ · dA ~ ~ · d~l = − d B (2.4) E dt The left-hand side of eqn. 2.4 is a line integral over a closed loop and the right hand side is the time derivative of a surface integral of the normal component of the magnetic flux. The equation only calculates the average electric field over the loop, but it is often the only measure available when the actual local field in a complex system can only be estimated with numerical methods that require very detailed knowledge about the fields on the system boundary and its material properties. If we assume that the bulk conductivity of the material is relatively homogeneous, we can also infer the average induced current by Ohm’s law. The equation will register an average electric field when the integral changes with time. If we consider the loop to be of fixed dimensions, this can happen in several different ways: • the magnetic field itself varies with time. This is the typical situation for many field studies in which a spatially homogenous field is modulated, e.g. with a sine wave; • by motion in a field that has spatial variation. This is, for instance, relevant when transporting a person into or out of a magnetic resonance imaging (MRI) machine that has very strong spatial gradients at the opening to the bore; and • the relative orientation between the loop and the field vector is changed. This happens when we rotate the loop in a static field. The field gradients are decisive and can enhance the induction effect. It makes no difference whether a person is stationary in a field changing over time or whether a person moves in a constant magnetic field. In both cases the effect is the same: Induction of an electrical field in the body tissue. 2.1.2.1
Static magnetic fields
Current data suggests possible pathological effects of static magnetic fields, e.g. induced blood flow potentials around the heart which might interfere with the autonomous heart action, increases in blood flow resistance due to magneto-hydrodynamic effects, can occur only in magnetic flux densities exceeding 10 T [54, 55, 56, 71, 99, 112, 107]. However, it must he noted, that the actual data base is very small for flux densities exceeding 8T. Available studies for flux densities exceeding 8 T are often not replicated. Detailed information of direct effects of static magnetic fields, especially for MRI and magnetic resonance spectroscopy (MRS) applications can be found in [54, 55, 56, 99, 112]. However, all conclusions must be carefully examined to determine if they are really due to the direct physiological effects of static magnetic fields, because they are sometimes mixed up with effects from time-varying magnetic fields or movements in static magnetic fields. Changes in a static magnetic field, e.g. time-variation or movement, induces electric fields in body tissues. The induced field may interact with the human body by several mechanisms. The main mechanisms are sensory or nerve stimulation – see section 3. The occurrence of these effects is dependent on the temporal gradient of the field or the spatial gradient of the field and the movement speed of the subject. Movement in a static magnetic field – see section 5.5 – causes a low frequency internal electric field in the tissue.
4
The strongest static magnetic fields are currently used in MRI with flux densities up to 14 T and MRS with magnetic flux densities up to 25 T. Other sources of strong static magnetic fields are thermonuclear reactors, magnetohydrodynamic systems, particle accelerators, and superconducting generators. Industries where strong magnetic field exposure can occur are those involving electrolytic processes such as chlorine or aluminum production and in the manufacture of permanent magnets and magnetic materials. The typical exposures in these industries are a few mT of the working day with peak exposures up to several tens of mT. 2.1.2.2
Low-frequency magnetic fields
The main physiological effect of low-frequency (LF) magnetic fields is the induction of electrical fields in the human body and the stimulation of excitable body tissues, like sensory organs, nerves and muscles. Because different excitable body tissues have different maximum sensitivities with respect to the frequency, the major points of interaction change with the frequency. Table 2.1 shows for some physiological effects their major point of interaction and their frequency range of maximum sensitivity. Maximum sensitivity
Physiological effect
Point of interaction
Metallic taste
Various receptors in the tongue (shift in ion gradients)
Vertigo, nausea Blood flow induced electric fields in tissue
Inner ear (vestibular system) Nerve, muscle excitation (interference with heart action)
≈ 20 Hz
Magnetophosphenes
Retina
≈ 50 Hz
Tactile and pain sensations Loss of muscle control Interference with autonomous heart action
Peripheral nerves Peripheral nerves, muscles
1 Hz < 0.1 . . . 2 Hz
Table 2.1:
Heart
Frequency range of maximum sensitivity and major point of interaction for some physiological effects
Extremely low frequency sensory effects caused by movements in a strong static magnetic field are experienced with flux densities above 2 - 3 T [26, 36, 56]. The maximum sensitivity is expected at frequencies around 0.1 Hz. Sometimes these effects last longer than the actual field exposure and can be detrimental to work performance and quality. Pathological effects of blood flow induced electric fields in the tissue causing nerve and muscle excitation in the immediate vicinity of these blood vessels or that may interfere with the autonomous heart action are expected for flux densities exceeding 8 - 10 T [99]. However, exposure during typical industrial processes, including e.g. electrolysis, electroplating or welding is well far below these threshold values. Magnetophosphenes give the magnetically evoked appearance of light spots in vision. They have a very sharp response peak (maximum sensitivity) at ≈ 20 Hz. For lower frequencies the sensitivity decreases approximately proportional with f , for higher frequencies the decrease in sensitivity is proportional to nearly f 3 . At the frequency 50 Hz there is the maximum sensitivity for nerve and muscle stimulation. However, the response curve is very flat in the frequency range from 10 Hz to some hundreds of Hz. 5
Only for frequencies higher than 3 - 5 kHz the sensitivity decreases approximately proportional with f . In general, the thresholds for the direct stimulation of muscles are much higher than for nervous structures. However, the exposure at most industrial workplaces is far below both threshold values. All these physiological effects have a clearly defined threshold. Any stimuli below the threshold value will not cause an adverse effect, even when applied for a long time [50] – see also section 3. A comprehensive compilation of direct physiological effects of LF magnetic fields can be found in [51, 54, 55, 57, 63, 64, 68, 70, 82, 89, 90, 91, 108]. Low frequency magnetic fields are ubiquitous at workplaces where electric energy is used. Magnetic fields will be produced e.g. by transmission lines, underground cables, distribution lines, transformers, electric railway systems, household appliances, resistance and induction heating systems, hand-held electric tools and arc, spot and resistance welding equipment. Exposure to low frequency magnetic fields at workplaces in terms of magnetic flux density ranges from some nano- or microtesla, e.g. in office buildings, up to several tens or hundreds of millitesla, e.g. at industrial workplaces. The frequency range covered reaches from fractions of 1 Hz, e.g. movement in static magnetic fields, up to some tens or hundreds of kHz, e.g. induction heating. 2.1.3
High-frequency electromagnetic fields
The direct effect of high frequency (HF) electromagnetic fields is the penetration of HFelectromagnetic fields in the body and the absorption of energy in tissues. The energy absorption causes an increase of temperature in the tissue which could lead to an increase in body temperature. To prevent adverse health effects the increase in tissue and body temperature must be limited. A commonly used value is to limit the temperature increase in the tissue caused by an electromagnetic field to a maximum value of 1 ℃ [51, 106]. The penetration depth into the biological tissue depends on the frequency of the electromagnetic field and the electric properties of the body tissue. The higher the frequency of the electromagnetic field and the electrical permittivity of the tissue, the shorter the penetration depth. For continuous-wave exposures with frequencies exceeding 10 GHz the penetration depth is very short and the total energy is absorbed in the top layers of the skin.
2.2 2.2.1
Indirect effects of electric and magnetic fields Electric fields
Static electric fields can accelerate dust particles towards the worker and therefore enhance the dust deposition on the worker. This can lead to allergic and inflammatory reactions in sensitive personnel. Movement or vibration of body hair can also occur in static and time-varying electric fields, creating a possible annoyance. However, the perception threshold of hair vibration shows a wide individual variation [10, 108]. Contact currents occur, if a worker touches a charged object or touches a grounded object while being charged himself, due to exposure to a electric field or due to triboelectricity. The resultant physiological effect is largely dependent on the size of the contact area, e.g. touch or grasp contact, and on the amount of discharge energy and transferred charge, as well as the amplitude and frequency of the continuously flowing contact current. These effects can be annoying, painful or can have life threatening consequences [18, 58, 59, 60]. In general, two different phases of a contact current event can be distinguished:
6
• a spark discharge, i.e. an initial discharge current impulse • a continuous contact current Depending on the specific exposure scenario only one or both phases of the contact current event might be present. Usually, the initial discharge current with a duration in the sub-millisecond range is only present for exposure situations involving either a static or time-varying electric field. In general, a continuous contact current is linked to time-varying electric or magnetic fields, but can also occur in conjunction with ongoing triboelectric processes. The frequency of the continuous contact current depends on the frequency of the causal time-varying electric field, but can also be a DC current in case of triboelectric processes. Therefore it is necessary to limit both phases of the contact current event. The thresholds for perception and pain are lower for touch contact when compared to grasping contact. For a frequency of 50 Hz the perception thresholds for such touch and grasp currents are in the range of 1 . . . 3.5 mA (rms). For frequencies in the 100 kHz and MHz range, the thresholds are up to 40 . . . 50 mA (rms) [3, 10, 13, 21, 22, 39, 58, 59, 60, 106, 108]. If in a certain workplace environment, e.g. high-voltage switchyards, spark discharges or contact currents cannot be avoided by technical measures, workers should be trained to always make grasp contact or instructed to use special work techniques, e.g. equalization of potentials, or work gear, e.g. insulating or conductive gloves. 2.2.2
Magnetic fields
Indirect effects of static and time-varying magnetic fields are translational and rotational forces on ferromagnetic and conductive objects, interference with AIMDs and the heating of conductive objects. A quantitative solution for the translational and rotational forces on a ferromagnetic object being placed in a static magnetic field can be found in chapter 5.7. For the magnetic field characteristic (spatial magnetic gradient) of an unshielded magnet a minimum magnetic flux density of Bz ≈ 60 mT is needed to overcome the initial frictional force, which in turn makes it possible that a sphere is accelerated in the magnetic field and a so-called projectile risk can occur. This result is in good agreement with the value given in [21]. In general, shielded superconducting magnets have higher spatial gradients at their openings to the bore. This leads to a lower minimum magnetic flux density which could constitute a so-called projectile risk. Current data for shielded systems indicates a minimum magnetic flux density in the central axis of a superconducting cylindrical magnet in the range from 30 . . . 40 mT necessary for a projectile risk to occur. For non-spherical objects not only a translational force can exist, but a torque as well. Needleshaped rotational ellipsoids try to turn their long axis parallel to the direction of the field. The magnitude of the torque is proportional to the square of the static magnetic flux density Bz2 , so the maximum torque is to be expected in the center of the magnet and can be higher than the maximum translational force. Personnel working in areas with high static magnetic fields, e.g. MRI, MRS, electrolysis, electroplating, particle accelerators, superconducting generators, should be informed that these torques can occur and trained to avoid any interference with the proper handling of tools and material. Interference mechanisms for static and time-varying magnetic fields with AIMDs are covered in section 5.6. High time-varying magnetic fields can also heat up conductive objects, e.g. passive medical implants and tools. A detailed risk assessments needs to be carried out for workers with passive medical implants who are exposed to high time-varying magnetic fields or who need to handle conductive objects in these fields. 7
2.3
Body models
Directly measurable external quantities, e.g. electric field strength, magnetic flux density or contact current, are linked to the exposure-limiting body internal quantities, e.g. peak electric field strength in the tissue, by using analytical and numerical body models with different resolution and complexity. All calculations throughout this report are done using simple ellipsoid models [63, 75] – mainly used for validation purposes –, detailed anatomical models based on the Visible Human data set [81] and on CAD models of the Virtual Family [23] with voxel sizes in the range from 1 to 5 mm3 . For calculations inside of the eye and the inner ear custom made high resolution models with spatial resolutions of up to 0.1 mm3 were used.
3 3.1
Neurophysiology Mechanisms and facts for the creation of action potentials
The main physiological effect of electrical fields in the body tissue created by low-frequency electric or magnetic fields is electrical stimulation of excitable body tissues, like sensory organs, nerves and muscles. It is therefore of utmost importance to understand the underlying neurophysiological processes which lead to the generation of action potentials, their thresholds, time behavior and other important parameters, in order to limit the exposure to low frequency electric and magnetic fields, thus protecting the health and safety of workers while being exposed to these physical agents.
Figure 3.1:
Schematic structure of a typical CNS or motor neuron (In part from [111])
Fig. 3.1 shows the simplified structure of a typical neuron. Basic components of a neuron are one or more dendrites, a single soma with the cell nucleus, a single axon and one or more axon terminals. The information is passed in form of an electrical signal, i.e. the action potential, between the dendritic inputs and the axon terminal outputs. Coupling to other neuronal structures usually happens in the form of neurotransmitters, i.e. chemical agents, which are released at the axon terminals and picked up by receptor sites on the postsynaptic dendritic spines. The axon hillock is the anatomical part of a neuron that connects the cell body, i.e. the soma, to the axon. It is described as the location where the summation of inhibitory and excitatory postsynaptic potentials from numerous synaptic inputs on the dendrites or cell body occurs. The axon hillock also has a high concentration of voltage-gated ion channels, which are also common on the surface of the soma and at the nodes of Ranvier, but not on the dendritic spines. Most of the length of the axon is insulated by a myelin sheath, i.e. Schwann cells in the peripheral nervous 8
system and oligodendrocytes in the central nervous system, which wrap themselves around the axonal segment forming a thick fatty layer that prevents ions from entering or leaving the axon. The internodal distance d between two nodes of Ranvier lies in the range of 0.2 . . . 2 mm and is linked to the fiber diameter D by the empirical equation: d ≈ 100 · D
(3.1)
The length of the uninsulated gap G at a node of Ranvier usually is only a few micrometers wide (G ≈ 1 . . . 2 µm) [100]. This myelin insulation increases both the energy efficiency of the propagation process since the ionic currents are confined to the nodes of Ranvier – see fig. 3.1 – and the conduction velocity of an action potential va through so-called saltatory conduction – see table 3.2. Table 3.1 gives some rough estimates for the electric properties of the cell membrane of a nerve fiber at a nodal gap and the cell membrane plus the Myelin sheath between two nodes of Ranvier [84]. Specific leakage resistance [kΩ · cm2 ]
Specific capacitance [µF/cm2 ]
1 100
1 0.01
Cell membrane Myelin sheath Table 3.1:
Electrical properties of cell membrane and Myelin sheath
A classification of peripheral nerve fibers according to Erlanger and Gasser [29] together with some basic fiber properties is given in table 3.2. The autonomic, motor and sensory nervous system use different kinds of peripheral nerve fibers. Fiber class
Diameter D [µm]
Conduction velocity va [m/s]
Myelin sheath
Aα Aβ Aγ Aδ
10 - 20 7 - 15 4-8 3-5
60 - 120 40 - 90 15 - 30 5 - 25
very thick thick normal thin
B
1-3
3 - 15
partial
C
0.3 - 1
0.5 - 2
none
Table 3.2:
Classification and properties of peripheral nerve fibers
Class B and C fibers are found in the autonomic nervous system. Class C fibers can also be found in the sensory nervous system innervating nociceptors for slow pain and warmth receptors. Class Aδ fibers are associated with touch and pressure receptors (free nerve endings) as well as thermoreceptors for cold and nociceptors for slow pain. Aα and Aβ fibers of the sensory nervous system are the primary and secondary connections of proprioreceptors, e.g. muscle spindles, with the CNS. Aβ fibers also innervate all cutaneous mechanoreceptors. The lower motor neurons of the motor nervous system consist of Aα and Aγ fibers which innervate the extrafusal and intrafusal muscle fibers, respectively. The distribution of peripheral nerve fibers in the human body comprises fiber diameters in the range from 0.3 . . . 17 µm with relative maxima in the fiber number at diameters of 0.6 µm for unmyelinated fibers and 2.3 µm, 3.8 µm, 6.3 µm, 8.6 µm and 12.8 µm for myelinated fibers 9
[15, 80, 90, 94]. The distribution of myelinated fiber diameters in the central nervous system has significantly different numbers. In the human pyramidal tract more than 89 % of nerve fibers are in the diameter range from 1 . . . 4 µm, approximately 9 % in the diameter range from 5 . . . 10 µm and less than 2% in the diameter range from 11 . . . 20 µm [74, 90]. Another important component of the neuron is its cell membrane.
Figure 3.2:
Schematic structure of a cell membrane (Adapted from [110])
Fig. 3.2 shows the schematic structure of a cell membrane and its basic components. A key component is the phospholipid bi-layer which prevents molecules and ions from leaving or entering the cell through uncontrolled diffusion. Channel proteins form controlled gateways for substances entering or leaving the cell. For neurons two ionic pathways through the membrane are of special interest: • Active ion pumps create and maintain an ionic concentration gradient between the inside of the neuron, i.e. the cytoplasm, and the outside of the neuron, i.e. the extracellular fluid • Voltage-gated ion channels use this concentration difference to selectively transport ions along their concentration gradients Directly linked to these ionic concentration differences between the cytoplasm and the extracellular fluid, i.e. the inside (index ’i’) and the outside (index ’e’) of the neuron, is the existence of a potential difference UM = Φi − Φe or an electric field EM across the cell membrane. Any transport of ions – and therefore charges – across the membrane by pumps or channels changes the difference of the electric potentials and the electric field across the membrane. The concentration of potassium (K+ ) ions inside the neuron is approximately 20-fold larger than the outside concentration, whereas the concentration of sodium (Na+ ) ions on the outside is roughly 9-fold larger than on the inside of the neuron. Similarly, ionic gradients across the cell membrane of a neuron also exist for calcium (Ca++ ), chloride (Cl− ) and magnesium (Mg++ ) [48]. The equilibrium membrane potential – the resting potential Ur – at which the net flow of all ions across the membrane is zero can be calculated with the Goldman equation [38] and leads to a typical electrical potential difference of Ur ≈ −70 . . . 80 mV across the membrane. Membrane potentials are always measured relative to the exterior of the cell. This membrane potential in turn leads to a strong directional electrical field EM across the membrane. Fig. 3.3 shows the various phases of an idealized action potential passing a single point on the cell membrane of an axon. As soon as a stimulus increases the transmembrane potential to more positive values both the voltage-gated sodium and potassium channels begin to open, leading to an increase of both the inward sodium ionic current, causing further depolarization, and the 10
Figure 3.3:
Phases of an idealized action potential (In part from [109])
outward potassium ionic current, responsible for repolarization/hyperpolarization. If the change in membrane potential is only small and does not exceed the threshold, the higher potassium ionic current is counterbalancing the lower sodium ionic current, thereby returning the electrical potential across the membrane to its resting value. These so-called failed initiations of a action potential describe one part of the fundamental “all-or-none” principle which is a key element to the behavior of excitable structures. In other words, action potentials either occur fully or do not occur at all. That means that larger stimuli do not create higher action potentials than smaller stimuli. Instead, the frequency of the action potentials is used to encode the intensity of a stimulus. However, if the change in membrane potential is large enough to exceed a typical threshold level of about 15 . . . 25 mV above the resting voltage, a positive feedback from the already open sodium channels opens even more sodium channels and in rapid succession leads to a runaway condition, where the electrical potential difference across the membrane nearly reaches the levels of the sodium equilibrium potential UNa ≈ +55 mV. Because some of the slower acting potassium channels are also open at this point in time, the peak membrane potential is lower than the sodium equilibrium potential UNa and reaches typical values of approximately +40 mV. This rising phase of the action potential has a time duration of typical 1 ms. The positive feedback of the rising phase finally slows, comes to a stop and is finally transformed into a negative feedback by a special behavior of the sodium channels. Every sodium channel has a built-in shut-off feature which automatically closes an open channel after a certain amount of time. The probability for a sodium channel to stay open decreases with higher potentials across the membrane. This inactivation of the sodium channels occurs much slower than the transition from a closed to open state and takes some additional time for being reset to a normal closed state of the channel. The inactivation of the sodium channels lowers the membrane’s permeability to sodium, thus driving the membrane potential back down. At the same time, the slower acting potassium channels, which lack an automatic inactivation feature, become fully open causing the membrane potential to drop quickly, thus repolarizing the membrane and creating the falling phase of the action potential. Because the potassium channels act much more slowly than the sodium channels, it takes some time to close them again, resulting in a hyperpolarization of the cell membrane (undershoot). Only when the membrane’s permeability to potassium returns to its usual value, the potential across the membrane assumes the resting value again. A previous action potential leaves many sodium and potassium channels in a refractory state,
11
in which they are unable to open again, regardless of any stimulus being present. This absolute refractory period, where no action potential can be created, is maintained until the membrane potential reaches sufficient negative values or even is hyperpolarized for a certain length of time. In the relative refractory period enough ion channels have recovered that a new action potential can be created, however requiring a stimulus, i.e. an initial depolarization of the cell membrane, much larger than usual. These refractory periods guarantee that the action potential usually travels only in one direction along the axon, but also limits the maximum frequency of generating action potentials. For mammalian nerve fibers the absolute refractory period is in the range of 0.4 . . . 1 ms for class A fibers and ≈ 2 ms for class C fibers, whereas the relative refractory period is in the range of several milliseconds. Under lab conditions the maximum repetition rate for action potentials created by externally applied electric stimuli is ≈ 2000 per second. However, the maximum repetition rate for action potentials in the human body is typically in the range of 10 . . . 100 per second and rarely exceeds a value of 500 action potentials per second [16, 90]. The ions exchanged during an action potential make only a negligible change to the total internal and external ionic concentrations. Even with blocked sodium-potassium-pumps a typical axon can generate up to hundreds of thousands of action potentials before a degeneration in amplitude occurs. Because of the thermal motion it is not possible to predict whether a certain channel will be open or closed at any given time. However, the laws of probability allow to make certain predictions of the average behavior of a channel. Typically, a large number (≈ 102 ) of channels contribute to the generation of an action potential. Sodium channel
Potassium channel
Faster than potassium channel (up to a factor of ten) Time constant: ≈ 10 µs (range: 5 . . . 200 µs)
Slower than sodium channel (probability of being open increases with depolarization)
Automatic inactivation (slow recovery, ≈ 10 ms at -70 mV)
No automatic inactivation
3 distinct states: open, closed, inactivated
2 distinct states: open, closed
9 internal states (1 open / 8 closed) when not inactivated
16 internal states (1 open / 15 closed)
Table 3.3:
Fact sheet for sodium and potassium ionic channels
Some important data for sodium and potassium channels is summarized in table 3.3 and can also be found in [44, 47, 84]. As already shown in table 3.2 and discussed in the previous paragraphs, myelinated class A fibers have higher conduction rates, shorter action potential durations, shorter refractory periods and lower electrical stimulation thresholds when compared to unmyelinated class C fibers [94].
3.2 3.2.1
Electrical stimulation of excitable tissues Basic facts
Because of the lower electrical stimulation thresholds of myelinated class A fibers, due to the longer internodal distance d, these fibers are an excellent choice for studying their behavior with regard to setting safety limits.
12
A quantitative solution for the generation and propagation of action potentials as well as a description of the underlying ionic mechanisms in an unmyelinated nerve fiber, e.g. squid giant axon, was first given by Hodgkin and Huxley [49]. Frankenhaeuser and Huxley reformulated the classical Hodgkin-Huxley equations, in terms of electrodiffusion theory, and computed action potentials specifically for saltatory conduction in myelinated axons [31]. The whole mathematical framework is well beyond the scope of this report but some key equations will be presented, which give a very detailed insight into the whole process of electrical stimulation of excitable tissue and the generation of action potentials in nerve fibers. Additional background information can be found in the literature [24, 31, 47, 49, 84, 90, 94, 103, 104]. For a first approach, an individual nerve fiber of infinite length, the center of which is oriented along the spatial z-axis lying in an unbounded extracellular medium (conductivity κe ) is selected. For subthreshold conditions where the excursion of the transmembrane voltage uM = UM − Ur from its resting value are small, the electric properties of the membrane are those of a passive admittance described as a parallel RC network with constant R and C values. Assuming steady state conditions, i.e. ∂uM /∂t = 0, the relationship between the membrane potential uM and the potential of the external stimulus ϕe normalized to their respective baselines is expressed by the differential equation: uM ∂ϕe ∂ 2 uM − 2 =− 2 (3.2) ∂x2 λ ∂z p with λ = rM /ri , where rM is the membrane resistance per unit length and ri the resistance of the intracellular medium per unit length. With the electric field being the negative spatial derivative of the corresponding function for the electric potential, eqn. 3.2 can be rewritten as uM ∂Ez ∂ 2 uM − 2 = 2 ∂x λ ∂z
(3.3)
The term ∂Ez /∂z on the right hand side of eqn. 3.3 is often called the activation or forcing function in the differential equation. Eqn. 3.3 describes some important facts for changes in the membrane potential (hyperpolarization, depolarization) and in the second case the successful initiation of an action potential: • A gradient along the fiber axis in the electric field of the external stimulus must exist. This finding is proven by experimental results, that the electric stimulation of excitable tissue is facilitated, if the electric field of the stimulus is parallel to elongated cells or fibers. A perpendicular field orientation is rather inefficient and requires a much higher stimulus in order to be successful [9, 65, 79, 84, 86, 87, 88, 90]. • The spatial field gradient does not necessarily have to originate from the external stimulus but can also be created by boundary conditions, e.g. beginning, termination, bends or branches, changes in diameter of the fiber, adjacent tissues and structures with different electrical properties. This fact is especially of interest when studying complex excitable tissue structures, e.g. brain, retina. • Peak depolarization or hyperpolarization are expected at locations where ∂Ez /∂z attains its maximum value. • The overall reaction of the cell membrane depends on the entire course of the function E(t, z, . . . ) not just the location or amplitude of its initial or peak values. It is therefore expected that different forms of stimuli, e.g. rectangular, trapezoid, triangular, sinusoidal, exponential, mono- or biphasic, even those having the same amplitude, will have different effects on the overall behavior of the nerve cell membrane.
13
The following sections apply these basic findings to the mechanism of electrical stimulation of peripheral nerves (PNS) and central nervous system of the head (CNS). A review of the current literature reveals that these factors are often not well controlled and are not sufficiently documented. Especially for experimental data it is very difficult to find relevant parameters in the published documents. 3.2.2
Long stimuli
However, a careful review of the literature and additional numerical simulations at membrane level, including parameter variation studies, reveals a threshold for the electric field strength in the tissue for the onset of peripheral nerve stimulation (PNS) in the range of 6 . . . 7 V/m for stimulation pulses longer than 1 . . . 2 ms [14, 17, 25, 42, 66, 83, 90, 101]. This value is quite conservative because many experiments and calculations use point sources for the stimulation current or voltage, which can cause a high spatial field gradient in the tissue, especially for small distances between the field source and the axon under investigation. Because these high spatial field gradients in the tissue are difficult to obtain by using external electric or magnetic fields for stimulation, even higher threshold levels for peripheral nerve stimulation are to be expected in those cases. Lapicque’s law [72, 73], also known as the modified Weiss equation [105], gives the fundamental relationship between the stimulation strength – historically given as a rectangular stimulation current Is – and the duration of the stimulus T with respect to physiological parameters like the rheobase – also historically given as a current IR – and an empirical time constant τe which is linked to the membrane time constant τM = RM · CM (approximately in the range of 1 ms), defined by membrane resistance RM and membrane capacity CM , and the spatial distribution of the stimulus current or the spatial gradient of the electrical field strength in the tissue: Is =
Figure 3.4:
IR 1 − e−T /τe
(3.4)
Graphical representation of Lapicque’s law given by eqn. 3.4
As shown in fig. 3.4 and according to eqn. 3.4 Is , the minimum stimulation strength (or current) with duration T , is required to reach the stimulation threshold. For long stimuli (T → ∞) the value for Is is identical to the rheobase value IR , which defines the stimulation threshold. The time T = τc where the minimum stimulation strength required is twice the rheobase value Is = 2 · IR was named chronaxie by Lapicque. It must be noted that the rheobase value is dependent on physiological parameters and individual exposure conditions. Three fundamental statements can be derived from eqn. 3.4: 1. Stimuli must exceed a threshold, i.e. a minimum stimulation current or minimum electric field strength in the tissue, in order to create an action potential 14
2. Stimuli below the threshold, i.e. the rheobase value, cannot create an action potential even if they are of very long duration 3. Stimuli with shorter durations must be of higher intensity in order to be effective, i.e. create an action potential Some documents [3, 51, 57] present a U-shaped stimulation threshold or exposure limit value curve, which allows for higher values of the current density or electric field strength in the tissue due to accommodation of the nerve fiber for frequencies below 10 Hz. However, this is not endorsed by Lapicque’s law or eqn. 3.4 and is based on a misinterpretation of physiological data as explained below. If a stimulus is constant at a sub-threshold value or increases only slowly with time, e.g. sinusoidal waveform at a low frequency starting at a zero amplitude value, the sodium channels can open gradually which leads to a small rise in membrane voltage and also to an increase in stimulation threshold. This creates a chase condition between the stimulus and the stimulation threshold which can only be overcome with a higher amplitude of the stimulus or a faster rate of change. The behavior of a nerve to adapt to a constant or slowly varying stimulus is called accommodation. However, this behavior is only present if there is a slowly changing stimulus, e.g. sinusoidal or triangular waveforms beginning at a zero value. It is not encountered with long rectangular, exponential or even trapezoid waveforms with steep rising and falling slopes. It is also absent if the sinusoidal waveform starts at its peak value. Therefore, the usage of those higher values must be restricted to certain waveforms and should not be given as a general option without stating the limitations. 3.2.3
Short stimuli
When it comes to short stimulus durations (T → 0), the stimulus charge or the integral of electrical field strength in the tissue ET over the stimulus duration T becomes the new threshold value: ET · T ≥ cs for rectangular stimuli, where cs is a constant threshold value. It must also be noted that the threshold value cs is nearly invariable to dET /dt and therefore does not largely depend on the form of the stimulus, i.e. rectangular, trapezoid, triangular, sinusoidal and exponential stimuli nearly give the same results. From analytical calculations and numerical parameter variation studies and for stimulus durations of less than 10 µs (T ≤ 10µs) a value cs > 2 · 10−3 Vs/m can be obtained, which translates to an electric field strength in the tissue in excess of 200 V/m for a 10 µs stimulus. 3.2.4
CNS tissue
Experimental data in the literature gives lower threshold rheobase values when it comes to the stimulation of CNS tissue of the head, e.g. electro- and magnetophosphenes [6, 76, 77, 90, 91, 95]. It must be noted that this data are not highly reliable because of incomplete dosimetric documentation and often gives only average values for current densities or electric field strength in the tissue or does not take spatial gradients of the electric field in the tissue into account. However, from smaller fiber diameters and shorter fiber lengths higher threshold values would have been expected. As already pointed out in section 3.2.1 and shown in eqn. 3.3, high spatial field gradients resulting from boundary conditions, neighboring tissue structures with different electrical properties and a possible influence from highly specialized receptors, e.g. the rods of the retina of the eye and their neural interface, can make up for a seemingly lower total stimulation threshold value. As a preliminary result and a rough estimate, simulations indicate a total factor in the order of 20 . . . 40 when comparing the threshold levels in the frequency range of maximum sensitivity – see table 2.1 – with those for peripheral nerve stimulation.
15
Applying this factor to the threshold value for the electric field strength in the tissue for the onset of peripheral nerve stimulation for long stimuli – see section 3.2.2 – in the range of 6 . . . 7 V/m gives a threshold value for CNS tissue in the range of 0.15 . . . 0.35 V/m for the same type of stimuli. Similar results are expected when it comes to vertigo and nausea, but high-resolution numerical models, needed to link any exposure to extremely low frequency magnetic fields to those effects, are sparse or not existent. Other CNS tissue, e.g. spinal cord, can be disregarded in this context, because due to the electric properties of the surrounding tissues an ’electric shielding effect’ occurs which results in generally higher threshold values for electrical stimulation [12, 43]. 3.2.5
Uncertainties
A reduction factor fr =
√
10 is introduced in order to address uncertainties
• in modeling, e.g. body models [23, 81] • physiological data, e.g. tissue data [33, 34, 35] • due to individual health status and possible pathological conditions 3.2.6
Summary
Summarizing the findings of this section on neurophysiology, mechanisms and electrical stimulation, some important facts for threshold-level stimuli have to be noted: • The relevant physiological parameter to describe the electrical stimulation of excitable body tissues, like sensory organs, nerves and muscles is the peak electric field strength in the tissue together with its spatial and temporal derivatives. • The location where an electric field ET in the tissue, caused by an external (low frequency) electric or magnetic field, depolarizes or hyperpolarizes the cell membrane of an axon is dependent on its gradients in space and time. This important fact means that exposures to different sources of electric and magnetic fields and to different frequencies in general have different points of interaction with the cell membrane and are therefore independent of each other. In other words, there is hardly any additivity of the different spectral components under practical exposure conditions. • In the case of stimuli with repetition frequencies of less than 300 . . . 800 Hz, every peak value of the electric field strength in the tissue can create an instantaneous action potential. For these stimuli, a threshold for the electric field strength in the tissue for the onset of peripheral nerve stimulation in the range of 6 . . . 7 V/m applies. Due to high spatial field gradients resulting from boundary conditions and a possible influence from highly specialized receptors, the thresholds for CNS tissue stimulation appear to be lower than those for peripheral nerve stimulation by a factor in the order of 20 . . . 40. • For stimuli with repetition frequencies exceeding several kHz many stimuli, e.g. periods of a sinusoidal waveform, are necessary in order to create an action potential. This behavior is caused by a slow drift in the membrane potential due to subsequent stimulation and is attributed to the different time behavior of the sodium and potassium channels, leading to a so-called delayed action potential. Published data [44, 90] shows that for stimuli with a repetition frequency of 5 kHz a delayed action potential is evoked after 5 . . . 10 stimuli or periods, whereas for a repetition frequency of 50 kHz approximately 50 . . . 100 stimuli or periods are necessary to create a delayed action potential.
16
• The probability of creating an action potential is very small for frequencies exceeding ≈100 kHz and requires a high electric field strength in the tissue (>200 V/m). Especially for continuous-wave signals these field strengths in the tissue can lead to significant tissue heating effects, which must be controlled. • The generation of action potentials is instantaneous or nearly instantaneous. However, with time frames of less than 1 . . . 2 ms, no time averaging can be justified. This also means that root-mean-square (RMS) values, which by definition are an average, are a poor metric and should be avoided. The usage of peak values for measurement and calculation purposes is highly recommended. However, for single-frequency, continuous-wave sinusoidal waveforms, √ the peak values can be derived from RMS values by multiplication with a factor of 2. • The geometric dimensions of the main areas of field interaction and the neurological structures involved in the generation of an action potential are very small and therefore do not allow for any spatial averaging. However, from a practical point of view, e.g. for measuring and calculation purposes, some spatial averaging is inevitable, but has to be controlled carefully. A detailed analysis of this issue has to take into account several parameters, e.g. location (in the tissue or outside the body) and source (dimension, distance), and is beyond the scope of this document. All these important facts need to be taken into account when limiting the exposure to low frequency electric and magnetic fields for the protection of the health and safety of workers.
17
4
Limiting occupational exposure to static and low frequency electric and magnetic fields
The major goal of Directive 2004/40/EC is the protection of the health and safety of workers. This means, that any physiological effects caused by an exposure to EMF must be limited in such a way, that they do not pose a potential threat to the health and safety of workers. Any • interference with autonomous heart action • loss of muscle control • significant pain • severe form of vertigo and nausea • whole-body heat stress and excessive localized tissue heating qualifies as a potential threat to the health and safety of workers and the risk of such an occurrence should therefore be controlled. Other effects, like phosphenes, may or may not pose a potential safety threat, depending on the working environment and the duty of the worker. The same is true for effects like metallic taste and minor tactile sensations at threshold level. The proposed exposure limit values and action values presented in the next subsections of this document are based on this valuation.
4.1
Exposure limit values
As described in section 3 the relevant metric to quantify physiological effects based on electrical stimulation of excitable body tissue is the electric field strength in the tissue together with its spatial and temporal derivatives. Because these effects are threshold-based, the peak value of the electric field strength in the tissue is the relevant parameter which needs to be limited. If this peak field strength in the tissue remains below the identified stimulation threshold at all times, no stimulation will occur [42, 43, 44, 63, 89, 90]. 4.1.1
Static electric fields
The exposure limit values for static electric fields are indicated in table 4.1. As stated in section 2.1.1.1, the external static electric field cannot penetrate the body surface. Therefore the exposure limit value is solely based on indirect effects of the static electric field and is indicated as direct measurable external field quantity. 4.1.2
Static magnetic fields
The exposure limit values for static magnetic fields are indicated in table 4.2. Sections 2.1.2.1, 2.2.2 and 5.7 give the rationale for setting these exposure limit values stated as directly measurable external field quantities. It has to be noted that these exposure limit values only apply if the worker is stationary with respect to the static magnetic fields. For all time-varying exposures, including movements in static magnetic fields, the exposure limit values as indicated in section 4.1.3 do also apply. Additional information can be found in section 5. 18
External electric field strength [kV/m]
(a,b)
30 Note:
(a)
Value refers to the spatial maximum
(b)
If there is a risk that the worker touches any grounded or ungrounded object, additional restrictions due to contact currents – see section 4.1.4 – may apply
Table 4.1:
Exposure limit value for static electric fields
Maximum magnetic flux density Exposure of head and trunk (a,b,c,d) Exposure of limbs [T] [T] 2 Note:
Table 4.2:
(a,b,c,e)
8
(a)
Value refers to the spatial maximum
(b)
Personnel with active medical implants, e.g. pacemakers, cardioverter defibrillators, should not be exposed to static magnetic fields with flux densities higher than 0.5 mT at the location of the implant. For additional information see section 5.6
(c)
Magnetic flux densities in excess of 30 mT are allowed if any projectile risk or any risk from translational or rotational forces on metallic objects or implants can be excluded
(d)
For controlled environments where access is limited to specially instructed and trained workers, where special work practices and measures are in force and where a detailed risk analysis shows that any risks to the health and safety of the workers or any negative impact on their duties or the safety of others with regard to vertigo, nausea and phosphenes can be excluded, magnetic flux densities up to 8 T are allowed
(e)
For controlled environments magnetic flux densities in excess of 8 T are acceptable for a limbs only exposure
Exposure limit values for static magnetic fields
19
4.1.3
Low frequency electric and magnetic fields 100
Peak electric field strength in the tissue [V/m]
Exposure of the trunk / Controlled environment Whole body exposure / Exposure of the head 10
1
0.1
0.01
