INTERNATIONAL COMMISSION ON NON‐IONIZING RADIATION PROTECTIONICNIRP GUIDELINESFOR LIMITING EXPOSURE TO ELECTRIC FIELDS INDUCED BY
MOVEMENT OF THE HUMAN BODY IN A STATIC MAGNETIC FIELD
AND BY TIME‐VARYING MAGNETIC FIELDS BELOW 1 HZ

PUBLISHED IN: HEALTH PHYSICS 106(3):418‐425; 2014

ICNIRP PUBLICATION – 2014

ICNIRP Guidelines

GUIDELINES FOR LIMITING EXPOSURE TO ELECTRIC
FIELDS INDUCED BY MOVEMENT OF THE HUMAN BODY
IN A STATIC MAGNETIC FIELD AND BY TIME-VARYINGMAGNETIC FIELDS BELOW 1 HZInternational Commission on Non-Ionizing Radiation Protection*
INTRODUCTION
IN THIS document, guidelines are established for the protection of workers moving in static magnetic ﬁelds or being
exposed to magnetic ﬁelds with frequencies below 1 Hz.
This includes, but is not limited to workers engaged in activities related to magnetic resonance imaging (MRI). The
general principles for the development of ICNIRP guidelines are published elsewhere (ICNIRP 2002).
SCOPE
The main objective of this publication is to provide
guidelines for protection of workers against establishedadverse direct health effects arising from exposure to static
magnetic ﬁelds and time-varying magnetic ﬁelds below
1 Hz and to avoid sensory effects which may be annoying
and impair working ability. A two-tier approach is suggested, with a relaxation of the restrictions in conditions
where the workers are made aware of the biological consequences of exposure and are trained to control their own
behavior (ICNIRP 2009a; Jokela and Saunders 2011). The
guidelines are not expected to be relevant for the generalpublic because all exposures to intense magnetic ﬁelds
below 1 Hz are currently found at workplaces.
The guidelines do not apply to the exposure of patients
undergoing medical diagnosis or treatment. Detailed considerations of protection of patients undergoing MRI examinations are given in separate ICNIRP statements(ICNIRP 2009b, 2004). It is also recognized that, for research purposes, there might be a wish to investigate the*ICNIRP c/o BfSVG. Ziegelberger, Ingolstaedter Landstr. 1,
85764 Oberschleissheim, Germany.
For correspondence contact G. Ziegelberger at the above address,
or email at info@icnirp.org.
(Manuscript accepted 24 May 2013)
0017-9078/14/0
Copyright * 2014 Health Physics Society
DOI: 10.1097/HP.0b013e31829e5580
418

effects of static magnetic ﬁelds exceeding the basic restrictions presented by these guidelines (ICNIRP 2009a);such experimental exposures, however, are a matter for theappropriate ethics committees (institutional review boards).
Compliance with the present guidelines may notnecessarily preclude interference with, or effects on, medical devices such as metallic prostheses, cardiac pacemakers,
implanted deﬁbrillators and cochlear implants. ICNIRP
recognizes that practical policies need to be implemented
to prevent inadvertent harmful exposure of persons withimplanted electronic medical devices and implants containing ferromagnetic material and from dangers of objects
unintentionally moving because of attraction by the magnetic force. Advice on avoiding these problems is notwithin the scope of the present document but is available
elsewhere (IEC 2010; Shellock 2012).
These guidelines will be periodically revised andupdated as advances are made in the scientiﬁc knowledge
concerning any aspect relevant for limiting exposure ofstatic and time-varying magnetic ﬁelds below 1 Hz.
PHYSICAL ASPECTS
The basic physical law associated with the inductionof electric ﬁelds by a magnetic ﬁeld is Faraday’s law, whichindicates that the induced electric ﬁeld is directly related tothe change of the magnetic ﬂux through the body or part ofit (e.g., the head). This can be presented asdðB  dSÞ;dt
S=Ei  dl ¼  Xlð1Þwhere Ei is the local induced electric ﬁeld vector, dl is thedifferential length vector along a closed pathway, l, within
an individual exposed to the magnetic ﬂux density B, anddS is the differential area vector directed normal to the differential area. The integrated area, S, is enclosed by the integration pathway. Ei is roughly perpendicular to B. The
magnetic ﬂux may change (1) due to the variation of theﬁeld as a function of time, (2) due to the movement of a bodywww.health-physics.com

Guidelines for limiting exposure to electric ﬁelds c ICNIRP

in a space that results in a relative change to the magnitude
or direction of the magnetic ﬁeld or (3) both cases combined.
The right-hand term of eqn (1) shows the time rate of themagnetic ﬂux in terms of the surface integral of the time
rate of magnetic ﬂux density over the body area of interest.
It is important to note that another fundamental sourceof the induced electric ﬁeld is given by the electromotive
electric ﬁeld EvB = v  B where v is the velocity of apoint in the tissue relative to the ﬁeld. This ﬁeld is associated with the magnetic force causing dielectric polarization, i.e., separating positive and negative charges inthe tissue (Sanchez et al. 2012, 2009; Redzic 2004;
Bringuier 2003). The dielectric polarization increases
until the charges accumulated in tissue boundaries reachequilibrium, where their electric ﬁeld partly counteracts
the EvB ﬁeld (Redzic 2004). For some rotational movements
the magnetic force manifested by the EvB ﬁeld also generates a space (bulk) charge inside a conducting body. The
space and boundary charges may move during the motion.
The currents associated with these movements are added
to the currents generated by the rotational currents determined by Faraday’s law, but in most cases of movementsof biological bodies it can be assumed that rotational
currents and electric ﬁelds dominate over the dielectric
polarization phenomena.
The time constant for achieving the equilibrium of
the polarization is given by T = D/R, where D is the permittivity and R the conductivity of tissue (Redzic 2004).
For human tissues the time constant may be in the order ofmilliseconds, which is relatively small compared to thetime scale of human movements, which are in the range
above 100 milliseconds. Therefore, the time constants ofhuman tissue can be assumed to be short enough to enable
the use of Faraday’s law for the computation of the motioninduced electric ﬁeld relevant to sensory effects below 1 Hzsuch as vertigo (Liu et al. 2003). It remains to be determined whether the assumption of charge equilibrium isvalid for short acceleration or deceleration phases during
the onset and ending of a head movement, in which caserelatively short transient electric ﬁelds may arise in the frequency range relevant to sensory effects above 1 Hz (magnetophosphenes). Additionally, there is considerable lack ofdata of dielectric properties of human tissues below 10 Hz,
which makes the precise calculation of the motion-induced
electric ﬁeld difﬁcult (Gabriel et al. 1996a, b, c, 2009).
The electric ﬁeld induced in the head can be approximated by a linear function of the time derivative of
the average magnetic ﬂux density dB/dt in that region:Ei ¼ C

dB
;
dt

ð2Þ

where Ei is perpendicular to the magnetic ﬁeld B given
as an absolute value, t is time, and C is a conversion

419

factor that depends on the location within the body, thesize of the body, the shape of the body, electrical properties of the tissue as well as on the direction and distribution of the magnetic ﬁeld . This conversion factor applies
to a body rotating in a static magnetic ﬁeld, moving in aﬁeld gradient, and staying stationary in a time-varying
magnetic ﬁeld. The conversion factor can be determinedby computational simulation based on a realistic heterogeneous numerical model of the human body or body region of interest. By using two different human models
placed in a static magnetic ﬁeld, Ilvonen and Laakso(2009) have computed the conversion factor in the vestibular system located in the inner ear. In the case of ahead nodding or shaking in a uniform magnetic ﬁeld directed from left to right (shaking) and from top to down
(nodding), the maximum conversion factor for different
movements varied from 0.066Y0.132 Vmj1 per Tsj1.
The mean of these (maximum) conversion factors was
0.095 Vmj1 per Tsj1. This is close to 0.105 Vmj1
per Tsj1 computed by Dimbylow (2005) for a maximum
conversion factor in the brain at 50 Hz (33 Vmj1 per T).
These data imply that a reasonable estimate for C might
be 0.1 Vmj1 per Tsj1. For a detailed discussion of theconversion factors used for low-frequency guidelines, see
ICNIRP (2010).
The change of the magnetic ﬂux density ($B) is arelevant exposure parameter for limiting movements in astatic magnetic ﬁeld, as will be discussed later. The relationof $B with the induced electric ﬁeld is given byt2

E i;ave ¼

Xt1 Ei ðtÞ  dt
t2  t1¼

C  $B;t2  t1

ð3Þ

where Ei(t) is the instantaneous induced electric ﬁeld, DB
is the magnetic ﬂux density changed during the movement, C is the same conversion factor as in eqn (2) andEi, ave is the electric ﬁeld corresponding to DB. The movement starts at time t1 and the maximum DB is reached at
t2. For example, if $B would be 2 T during 1 s, the average induced electric ﬁeld in the periphery of the brain
would be approximately 0.2 Vmj1 when using 0.1 Vmj1
per Tsj1 for C.
BIOLOGICAL EFFECTS
When the static magnetic ﬁeld exceeds a thresholdof approximately 2 T, the movement-induced electricﬁeld in the head may be high enough to evoke vertigo andother sensory perceptions such as nausea, visual sensations(magnetophosphenes) and a metallic taste in the mouth(WHO 2006; AGNIR 2008; ICNIRP 2009a; Heilmaier
et al. 2011). There is also the possibility of acute neurocognitive effects, with subtle changes in attention, concentrationand visuospatial orientation (van Nierop et al. 2012). All

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Health Physics

these effects are not considered to be hazardous per se,
but they can be disturbing and may impair working ability.
For normal movements, the threshold for peripheral nerve
stimulation is unlikely to be reached with exposures below
8 T, although it is possible that the basic restrictions forperipheral nerve stimulation (ICNIRP 2010) may slightly
be exceeded by very fast movements.
In addition to these movement-induced effects, static
magnetic ﬁelds may cause direct effects arising from (1)induction of electrical ‘ﬂow’ potentials across blood vessels due to the movement of electrolytes in the blood, (2)forces on paramagnetic and diamagnetic components oftissues, (3) changes in chemical reactions due to alteredspin chemistry and (4) deﬂection of ionic currents due tomagnetic (Lorentz) force. These direct interaction mechanisms are not considered to have a signiﬁcant healtheffect when the magnetic ﬂux density is below 7 T (WHO2006; ICNIRP 2009a), above 7 T there is too little research
for any ﬁrm conclusions.
Magnetophosphenes
The most established effect of induced electric ﬁelds
below the threshold for nerve or muscle stimulation is theinduction of magnetophosphenes, the perception of faintﬂickering visual sensations. Magnetophosphenes are evokedby the internal electric ﬁelds induced in the retina (and brain
tissue) by a time-varying magnetic ﬁeld. On the basis ofhuman experiments, the threshold for the induction ofretinal magnetophosphenes has been estimated to lie between about 50 and 100 mVmj1 (Root-Mean-Square) at
20 Hz, rising at higher and lower frequencies (Saunders
and Jefferys 2007; Lövsund et al. 1980) although there isconsiderable uncertainty attached to these values. Available studies indicate that the threshold increases as 1/f atleast down to 5 Hz and probably to lower frequencies(Adrian 1977; Lövsund et al. 1980). The threshold at 1 Hz
would be at least 10 times higher than the minimum
threshold at 20 Hz.
In the case of exposure to a static magnetic ﬁeld,
magnetophosphenes are most likely associated with thetransient electric ﬁeld peaks. As noted in Physical Aspects, these transient peaks arise due to sudden changes
in the velocity of the head. The spectral components ofa short transient extend into the frequency range of themagnetophosphenes.
The increase in the threshold of magnetophosphene
induction below 10 Hz is the reason why the basic restriction for the induced electric ﬁeld can be allowed to increase
as a function of 1/f from 10 Hz down to 1 Hz (ICNIRP 2010).
In the absence of experimental data, this relation is extrapolated to frequencies below 1 Hz until the basic restriction
based on magnetophosphenes reaches the basic restriction
for peripheral nerve stimulation at a frequency of 0.66 Hz.

March 2014, Volume 106, Number 3

Peripheral nerve stimulation
The responsiveness of electrically excitable nerve andmuscle tissue to electric stimuli, including those induced
by exposure to low-frequency electric and magnetic ﬁelds,
has been well established for many years (e.g., Reilly 2002;
Saunders and Jefferys 2007; ICNIRP 2010). Myelinatednerve ﬁbers of the human peripheral nervous system havethe lowest threshold for electrical nerve stimulation. The
minimum threshold value of around 6 Vmj1 (peak)(Reilly 1998; 2002; Reilly and Diamant 2011) has been
estimated based on theoretical calculation using a nerve
model. However, peripheral nerve stimulation induced
during volunteer exposure to the switched gradient magnetic ﬁelds of magnetic resonance (MR) systems suggested that the threshold for perception may be as low as
about 2 Vmj1 (Nyenhuis et al. 2001), based on calculations using a homogeneous human simulation model. A
more accurate calculation of the electric ﬁelds induced in
the tissues of a heterogeneous human model based on datafrom the above MR study has been carried out by So et al.
(2004). These authors estimated the minimum thresholdfor peripheral nerve stimulation to lie between 3.8 and
5.8 Vmj1, based on the assumption that stimulation takes
place in the skin or subcutaneous fat. With stronger stimuli,
discomfort and then pain ensue. Below 10 Hz the threshold rises due to the accommodation of a nerve to a slowly
depolarizing stimulus.
VertigoMovement of the head within a static magnetic ﬁeld
above 2 T frequently gives rise to sensations of vertigo andnausea (Glover et al. 2007). These sensations are predominantly due to the induced electric ﬁeld which affects
the neural output of the vestibular system that is involvedin maintaining balance. Volunteer studies have shown thatvertigo can also be evoked by applying the electric ﬁeld bymeans of galvanic AC or DC currents of the order of 1 mA
fed to the electrodes attached behind the ears in the vicinity of the vestibular system (Fitzpatrick and Day 2004).
Movement-induced vertigo seems not only to be determined by the dB/dt, but also by the time integral of dB/dt,
i.e., DB, the change of magnetic ﬂux density during the
movement, as reported by Glover et al. (2007). They examined the threshold of vertigo sensations in volunteers inside a 7 T MR scanner. The volunteers were positioned at
the iso-center of the magnetic ﬁeld where they nodded andshook their heads. The movements were cyclically repeated
to enhance the sensation of vertigo. All of the subjects
reported mild or severe vertigo sensations and some evenexperienced nausea with rapid movements. The datapoints
in Fig. 1 show the threshold of vertigo in terms of DB andduration of the movement. The peak dB/dt values recorded
during the experiment ranged from 1.5 to 6 Tsj1, the duration of each shake or nod ranged from 0.5 to 6 s, and the

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Guidelines for limiting exposure to electric ﬁelds c ICNIRP

change in magnetic ﬂux density DB varied from 2 to 6 T.
The dB/dt values recorded during nodding were higher thanthose recorded during shaking. This is in agreement with asimple circulating current model which indicates that foraxial shaking (rotation axis parallel to the magnetic ﬁeld) the
induced electric ﬁeld is a minimum, while for nodding(rotation axis perpendicular to the magnetic ﬁeld) a maximum electric ﬁeld is found near the inner ear where thecirculating currents intersect (Jokela and Saunders 2011).
Overall, these results indicated that the threshold of vertigo correlated somewhat better with DB than with thepeak dB/dt and that the most effective frequency range was
below 1 Hz.
It is a common experience from working with clinical
MR imaging that vertigo sensations disappear when movement is slowed down. This indicates that there is a ﬁnite
time during which the sensation of vertigo develops. In
the experiment of Glover et al. (2007) vertigo sensations
were reported by most volunteers when the duration of asingle movement was less than 4 s even though there was
one vertigo observation for longer duration of movement(Fig. 1). As a conservative approach ICNIRP decided to
set the basic restriction so that the basic restriction curve
remains below that single observation. There remains, however, a clear need to obtain more data on vertigo thresholds,
particularly for relatively slow movements.
In addition to the effects of electric ﬁelds induced by amovement, a direct interaction of the magnetic ﬁeld withthe vestibular system cannot be excluded. An altered senseof balance has been observed in volunteers standing stationary in proximity to a 7 T MR scanner (Glover et al.

Fig. 1. The vertigo threshold in terms of magnetic ﬂux densitychange, $B, plotted as a function of duration of a head shake or nod
inside a 7 T MR scanner (Glover et al. 2007). The two line segments
show the basic restriction for magnetic ﬂux change during any 3 s
period during the movement (see Recommendations).

421

2007). This effect was ascribed to a difference in the diamagnetic susceptibility between the linear-movement sensors of the vestibular system and the surrounding endolymph
ﬂuid. A recent study (Roberts et al. 2011) suggests that theLorentz force resulting from interaction between the magnetic ﬁeld and naturally occurring ionic currents in theendolymph ﬂuid might explain the direct effect.
Therefore, since the sensory effects appear to dependon the product of time and dB/dt and given the possibilityof direct magnetic ﬁeld effects on the body, it is importantto restrict both the static magnetic ﬂux density (B) and the
maximum change of the magnetic ﬂux density (DB) experienced by the body during movement.
RECOMMENDATIONS
The objective of this guideline is to prevent peripheral
nerve stimulation and to minimize the possibility of transient sensory effects as a consequence of electric ﬁelds induced in the human body by movements in static magnetic
ﬁelds within occupational settings. The basic restrictions
and reference levels shown in Table 1 have been determined to achieve this objective. The basic restrictions havebeen deﬁned for the change in external magnetic ﬂux
density and for the induced internal electric ﬁeld. ICNIRP
recommends limiting exposure to below both sets of restrictions. Since internal electric ﬁelds cannot be readily
determined, reference levels have been derived to assess
compliance with these basic restrictions. Since the motioninduced electric ﬁeld is a non-sinusoidal ﬁeld, where thespectrum extends above 1 Hz up to 25 Hz, it is necessary
also to apply the basic restrictions and reference levels in
the ICNIRP (2010) guidelines. The restrictions for the
exposure to static magnetic ﬁeld have been speciﬁed in
ICNIRP (2009a).
A distinction is made between controlled and uncontrolled exposures. Basic restrictions for controlled exposure
are intended to be used in work environments where accessis restricted to workers who have been trained to understand the biological effects that may result from exposure,and where the workers are able to control their movements
in order to prevent annoying and disturbing sensory effects. Restrictions for uncontrolled exposure apply to allother occupational situations.
Basic restrictions for DB
In order to prevent transient sensory effects such asvertigo and nausea arising from motion-induced electricﬁeld below a few Hz, ICNIRP recommends that the changeof the magnetic ﬂux density DB should not exceed 2 T
during any 3-s period. Note that the maximum value forthe measured DB may not always occur at the end of the 3-s
period because the direction of dB/dt may change during theperiod. The basic restriction for DB has been plotted in Fig. 1

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March 2014, Volume 106, Number 3

Table 1. Exposure restrictions for controlling movement in a static magnetic ﬁeld and exposure to a time-varying magnetic
ﬁeld below 1 Hz. Above 1 Hz the basic restrictions and the reference levels are presented in the ICNIRP (2010) guidelines.
For uncontrolled exposure the reference levels for a magnetic ﬂux density may be converted to dB/dt by using (eqn 5).
Basic restrictions
Frequency f (Hz)Critical effectExposureconditionb
00Y1
0Y0.66
0.66Y1c$B (T)a

Bpeak to peak (T)Reference levels

Internal electric ﬁeld strength[Vmj1 (peak)]dB/dt [Tsj1 (peak))]

Vertigo due to Vertigo due to PNS effects due
Phosphenes due
PNS effects due
Phosphenes due
movement
time-varying
to movement into movement into movement into movement inin static
B ﬁeldstatic B ﬁeld
static B ﬁeld
static B ﬁeld
static B ﬁeld
B ﬁeldand due toand due toand due toand due totime-varying
time-varying
time-varying
time-varying
B ﬁeldB ﬁeldB ﬁeldB ﬁeldUncontrolled

Uncontrolled

Controlled

Uncontrolled

Controlled

Uncontrolled

1.1
1.1

1.1
0.7/f

2.7
2.7

2.7
1.8/f

2
2

The maximum change of magnetic ﬂux density $B is determined over any 3 s period.
For controlled exposure conditions, a $B of 2 T may be exceeded.
c f in Hzabwhere the constant DB restriction changes to a constantdB/dt restriction at 3-s duration of the movement.
For speciﬁc work applications, exposure to static magnetic ﬁelds up to 8 T can be justiﬁed if the environment
is controlled and appropriate work practices are implemented to control movement-induced sensory effects(ICNIRP 2009a). The probability of vertigo and nausea
will be low if it is possible to move so slowly that the maximum DB does not exceed 2 T during any 3-s period.
In the case of a stationary body in a time-varying
magnetic ﬁeld, the peak-to-peak value of the magnetic ﬂux
density is equivalent to DB and consequently should be
limited to 2 T.
In this context, vertigo and nausea may be annoying
and disturbing, but they are not considered to indicate aserious long-term health effect. Therefore, no additional
reduction factor has been applied to their threshold.
Basic restrictions for induced electric ﬁeld
In order to prevent stimulation of peripheral nervesin controlled exposure, ICNIRP recommends that the induced electric ﬁeld should not exceed the basic restrictionof 1.1 Vmj1 (peak) over the frequency range of motioninduced ﬁeld. This restriction was obtained by converting
the basic restriction of 0.8 Vmj1 (Root-Mean-Square) tothe peak value that applies to all tissues in the frequency
range below 3 kHz (ICNIRP 2010).
Because the stimulation of peripheral nerves is regarded as an adverse health effect, a reduction factor of
5 has been applied to the threshold to account for biological uncertainties.
In order to avoid the induction of magnetophosphenes, the strength of the induced electric ﬁeld should notexceed the basic restrictions for occupational exposure deﬁned by ICNIRP (2010) for time-varying magnetic ﬁelds,with an extension to frequencies below 1 Hz. The linear
increase of the basic restriction for magnetophosphenes asa function of 1/f ceases at 0.66 Hz where it reaches the levelof 1.1 Vmj1 (peak), which is the basic restriction for
peripheral nerve stimulation (Fig. 2). Basic restrictions formagnetophosphenes apply only to uncontrolled exposures,
since workers in controlled exposure situations are considered to be able to avoid this effect by limiting theirmotion speed. Basic restrictions for peripheral nerve
stimulation apply to both conditions.
Like vertigo and nausea, magnetophosphenes may beannoying and disturbing, but they are not considered toFig. 2. Basic restrictions for the induced electric ﬁeld for uncontrolled and controlled exposure conditions. The basic restrictions foruncontrolled exposures are based on protection against magnetophosphenes and peripheral nerve stimulation. The basic restrictions
for controlled exposures are based on protection against peripheral
nerve stimulation only. Above 1 Hz, the basic restrictions are equal
to the occupational basic restrictions in ICNIRP (2010).

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Guidelines for limiting exposure to electric ﬁelds c ICNIRP

cause serious long-term health effects. Therefore, no additional reduction factor has been applied to their thresholds.
Because the waveform of the motion-induced electricﬁeld is a non-sinusoidal transient, the restriction of theinduced electric ﬁeld should be based on the weighted
peak approach:Aij~ ELcosð2pfi t þ ui þ Vi Þje1;

ð4Þ

iwhere t is time and ELi is the exposure restriction (peakvalue) at the ith harmonic frequency f i, where Ai, ui, Vi,
are the amplitude of the ﬁeld, the phase angle of the ﬁeld
and the phase angle of the ﬁlter at f i . More explanations
on the weighted peak method may be found in ICNIRP(2003, 2010).
Reference levels
A practical way for determining compliance with thebasic restrictions for the induced internal electric ﬁeld is toensure that the magnetic ﬂux density does not exceed thereference levels derived conservatively from the basic restrictions. The recommended reference levels in Table 1
join with the ICNIRP (2010) reference levels for magnetic
ﬂux density at 1 Hz when the magnetic ﬂux density isconverted to the peak (amplitude) dB/dt by
pﬃﬃﬃ
dB0¼ 2pf 2BRM S ;ð5Þ
dt
where B0 is the peak value of the sinusoidal magnetic ﬂux
density and BRMS is the Root-Mean-Square value (Fig. 3).
Note that the reference levels are approximately directly proportional to the basic restrictions except for a small difference in corner frequencies. As in the case of compliancewith the basic restrictions for the induced electric ﬁeld,

Fig. 3. Reference levels for dB/dt for uncontrolled and controlled
exposure conditions. Above 1 Hz the reference levels are equal to theoccupational reference levels for magnetic ﬂux density (ICNIRP
2010) converted to the peak dB/dt by using (eqn 5).

423

compliance with the reference levels for dB/dt should be determined by the weighted peak approach.
In order to avoid electrical stimulation of peripheral
nerves, the reference level for peak dB/dt has been set to
2.7 Tsj1 for controlled exposure conditions. Note that toaccount for uncertainties arising from the conversion of thebasic restriction to the reference level a reduction factor of
approximately 3 is included in this reference level (ICNIRP
2010). There is no need for spectral weighting because thereference level limiting the stimulation of peripheral nerves
is constant over a large frequency range (Fig. 3).
AcknowledgmentsVDuring the preparation of this document, ICNIRP wascomposed of the following members: R. Matthes, Chairman (since 2012),
Germany; M. Feychting, Vice-Chairperson (since 2012), Sweden; R. Croft,
Australia; A. Green, Australia; K. Jokela, Finland; J.C. Lin, United States ofAmerica; C. Marino, Italy; A.P. Peralta, The Philippines; R. Saunders, United
Kingdom (until 2010); K. Schulmeister, Austria (until 2012); P. Söderberg,
Sweden; B.E. Stuck, United States of America; A.J. Swerdlow, United
Kingdom (until 2012); Z. Sienkiewicz, United Kingdom; E. van Rongen,
Netherlands; P. Vecchia, Chairman (until 2012), Italy; B. Veyret, France (until2012), S. Watanabe; Japan.
The Commission Task Group, which prepared the draft guidelines,
was lead by K. Jokela, Finland. ICNIRP also gratefully acknowledges theuseful comments received from all the reviewers for their contribution received via the open online consultation.
Finally, the support received by ICNIRP during that period from theInternational Radiation Protection Association, the European Commission(Health and Safety at Work), and the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety is gratefully acknowledged.
All ICNIRP members are requested to ﬁll in and update a declaration of
personal interests. Those documents are available online at www.icnirp.org/cv.htm.

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Jokela K, Saunders RD. Physiologic and dosimetric considerations for limiting electric ﬁelds induced in the body by
movements in a static magnetic ﬁeld. Health Phys 100:
641Y653; 2011.
Liu F, Zhao H, Crozier S. Calculation of electric ﬁelds inducedby body and head motion in high-ﬁeld MRI. J Magn Reson
161:99Y107; 2003.
Lövsund P, Öberg PA, Nilsson SEG. Magneto- and electrophosphenes: A comparative study. Med Biol Eng Comput 18:
758Y764; 1980.
Nyenhuis JA, Bourland JD, Kildishev AV, Schaefer DJ. Healtheffects and safety of intense gradient ﬁelds. In: Shellock FG,
ed. Magnetic resonance procedures: Health effects and safety.
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Redzic DV. Conductors moving in magnetic ﬁelds: Approach toequilibrium. Eur J Phys 25:623Y632; 2004.
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Zee DS. MRI magnetic ﬁeld stimulates rotational sensorsof the brain. Curr Biol 21:1635Y1640; 2011. doi:10.1016/
j.cub.2011.08.029.

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Sanchez CC, Glover P, Power H, Bowtell R. Calculation of theelectric ﬁeld resulting from human body rotation in a magnetic ﬁeld. Phys Med Biol 57:4739Y4752; 2012.
Sanchez CC, Bowtell R, Power H, Glover P, Marin L, Becker A,
Jones A. Forward electric ﬁeld calculation using BEM fortime varying magnetic ﬁeld gradients and motion in strong
static ﬁelds. Eng Anal Bound Elem 33:1074Y1088; 2009.
Saunders RD, Jefferys JG. A neurobiological basis for ELF
guidelines. Health Phys 92:596Y603; 2007.
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implants, and devices: 2012 Edition. Los Angeles, CA: Biomedical Research Publishing Group; 2012.
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van Nierop LE, Slottje P, van Zandvoort MJE, de Vocht F,
Kromhout H. Effects of magnetic stray ﬁelds from a 7 Tesla
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crossover study. Occup Environ Med 69:759Y766; 2012.
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World Health Organization. Static fields. Environmental HealthCriteria 232. Geneva: WHO; 2006.

GLOSSARY
Basic restrictions

Limitations on the quantities that closely match known
biophysical interaction mechanisms with tissue that may
lead to adverse health effects.
Central nervous system (CNS)

The portion of the vertebrate nervous system
consisting of the brain and spinal cord, but not including
the peripheral nerves.
Conductivity (R)A property of materials that determines the magnitudeof the electric current density when an electric ﬁeld isapplied to the material, expressed in units of Siemens permeter (Smj1); the inverse of resistivity.
Electric ﬁeld strength (E)Force exerted by an electric ﬁeld on an electric point
charge, divided by the electric charge. Electric ﬁeld
strength is expressed in Newton per Coulomb or Volt permeter (NCj1 = Vmj1).
Electro-stimulation

Stimulation of excitable tissue in the body by an applied
electrical stimulus.
Electromotive electric ﬁeld (EvB)

Electric ﬁeld induced by a movement of a conducting
body in a magnetic ﬁeld.
Frequency

The number of cycles completed by electromagnetic
waves in 1 s; usually expressed in Hertz (Hz).

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Guidelines for limiting exposure to electric ﬁelds c ICNIRP

Hertz (Hz)The unit for expressing frequency ( f ). One Hertz
equals one cycle per second. 1 kHz = 1,000 Hz, 1 MHz =1,000 kHz, 1 GHz = 1,000 MHz.
Induction
The creation of an electric ﬁeld and current in a
conducting or dielectric body caused by an external timevarying magnetic ﬁeld or by movement of a body in amagnetic ﬁeld.
Magnetic ﬂux density (B)A vector quantity that determines the force on amoving charge or charges (electric current) in a magnetic
ﬁeld. Magnetic ﬂux density is expressed in Tesla (T).
Magnetophosphenes
The sensation of ﬂashes of light caused by electricﬁelds and currents that are induced in the retina by a timevarying magnetic ﬁeld.

425

Permittivity (e)A constant deﬁning the inﬂuence of an isotropic medium on the forces of attraction or repulsion between charged
bodies, and expressed in farad per meter (F mj1).
Reference levels

The Root-Mean-Square and peak electric and magnetic ﬁeld strengths or ﬂux densities and contact currents towhich a person may be exposed without an adverse effectand with acceptable safety factors. Reference levels may
be used in practical situations for determining compliancewith the basic restrictions.
Vestibular system

An organ consisting of motion receptors sensitive tolinear and rotational accelerations of the human body. It is
the sensory organ that provides perception of movement
and sense of balance. The vestibular system is located in
the inner ear.

Nerve
A bundle of nerve ﬁbers.

Static ﬁeld

Nerve ﬁberLong protrusion of a single neuron.

ThresholdAn electric or magnetic ﬁeld that does not vary with time.

NeuronA cell in the nervous system usually consisting of acell body and a number of protrusion: a long one, the axon,
and a number of shorter ones, forming the dendritic tree.
Occupational exposure
Exposure to electromagnetic ﬁelds experienced by individuals as a result of performing their regular or assigned
job activities.
Peripheral nervous system (PNS)
The portion of the vertebrate nervous system consisting of the neuronal tissue found outside the central
nervous system.

The minimum level of a stimulus that will produce
a response or speciﬁed effect.
Time derivative of magnetic ﬂux density (dB/dt)

Change of the magnetic ﬂux density divided by the
duration of the change.
VertigoA type of dizziness, where there is a false feelingof motion.
Waveform

The variation of an amplitude of ﬁeld vector with time.

www.health-physics.com¡¡

Correspondence
COMMENT ON ICNIRP GUIDELINES FOR
LIMITING EXPOSURE TO ELECTRIC
FIELDS INDUCED BY MOVEMENT OF
THE HUMAN BODY IN A STATICMAGNETIC FIELD AND BY
TIME-VARYING MAGNETIC
FIELDS BELOW 1 HZDear Editors:
WE ARE dismayed by the way in which ICNIRP has apparently used our 2007 paper (Glover et al. 2007) to draw up
its recent Guidelines on low frequency magnetic field exposure (Ziegelberger 2014). It appears that these Guidelines
are going to be incorporated into the EMF Directive andmade legally binding within the 28 member countries of
the European Union without further scrutiny [Directive
2013/35/EU See Article 11(2) (2013)].
The guidelines’ limits aimed at preventing vertigo arebased entirely on our paper, which was effectively the firstin the field and which has not been replicated by anyoneelse. Although we stand by our data, it was a very small
study, and the lines on Figure 1 of the guidelines havebeen drawn about single data points from a single subject
at two frequencies (Ziegelberger 2014). This seems to be aremarkably flimsy basis on which to make internationalguidelines.
However, we are keen to point out that there was amore serious problem with using that data in this way.
At the time we wrote the paper, we proposed that the dominant mechanism was induced electric currents. However,as ICNIRP noted in the new guidelines, this mechanism
has been questioned by Roberts, who proposed a Lorentz
force mechanism (Roberts et al. 2011). Crucially, ICNIRP
apparently failed to realize that this Lorentz force mechanism depends on the amplitude and direction of the field,
whereas the induced current mechanism depends on theabsolute rate of change of field. In our experiment, we
asked subjects to move their head at high field; this would
induce an electric current and would have also produced a
Lorentz force, so our previous data was consistent withboth mechanisms.

ISSN: 0017-9078
Copyright © 2014 Health Physics Society
DOI: 10.1097/HP.0000000000000142

We have now carried out further studies that support
Roberts’ Lorentz force mechanism (Antunes et al. 2012;
Glover et al. 2014; Mian et al. 2013). Furthermore, the
new mechanism explains the previously anomalous observation of apparent vertigo-type effects in small rodents,
which are physically too small to be able to develop sufficient current densities in their heads to cause nerve excitation (Houpt and Houpt 2010).
It now seems likely that the perceptual effects of thechanging Lorentz force are the primary reason why movement causes vertigo. Unfortunately, this does not lead toneat, frequency-dependent limits that can be dovetailed
with other limits. We propose that at the current stage ofknowledge, a practical way to limit the experience of vertigo would be to accept some disconnect in the limits andsimply apply the previous static field limit; i.e., 2 T for uncontrolled exposures and 8 T for controlled exposures.
The authors declare no conflicts of interest.
PENNY GOWLAND AND PAUL GLOVER
School of Physics and Astronomy
University of Nottingham
United Kingdom

REFERENCES
Antunes A, Glover PM, Li Y, Mian OS, Day BL. Magnetic field
effects on the vestibular system: calculation of the pressure
on the cupula due to ionic current-induced Lorentz force.
Phys Medicine Biol 57:4477–4487; 2012.
Directive 2013/35/EU of the European Parliament and of theCouncil. On the minimum health and safety requirements regarding the exposure of workers to the risks arising fromphysical agents (electromagnetic fields). Brussels, Belgium:
European Union; 2013.
Glover PM, Cavin I, Qian W, Bowtell R, Gowland PA. Magneticfield-induced vertigo: a theoretical and experimental investigation. Bioelectromagnetics 28:349–361; 2007.
Glover PM, Li Y, Antunes A, Mian OS, Day BL. A dynamic
model of the eye nystagmus response to high magnetic
fields. Phys Medicine Biol 59:631; 2014.
Houpt TA, Houpt CE. Circular swimming in mice after exposure
to a high magnetic field Physiol Behavior 100:284–290; 2010.
Mian OS, Li Y, Antunes A, Glover PM, Day BL. On the vertigo
due to static magnetic fields. Plos One 8:e78748; 2013.
Roberts DC, Marcelli V, Gillen JS, Carey JP, Della Santina CC,
Zee DS. MRI magnetic field stimulates rotational sensors of
the brain. Curr Biol 21:1635–1640; 2011.
Ziegelberger G. ICNIRP Guidelines: for limiting exposure toelectric fields induced by movement of the human body ina static magnetic field and by time-varying magnetic fields
below 1 Hz. Health Phys 106:418–425; 2014.

www.health-physics.com

261

Copyright © 2014 Health Physics Society. Unauthorized reproduction of this article is prohibited.

262

Health Physics

RESPONSE BY ICNIRP TO THE
COMMENTS OF GOWLAND AND GLOVER
Dear Editors:
GOWLAND AND Glover expressed dismay that their 2007 paper (Glover et al. 2007) had been used by ICNIRP in setting limits on exposure to electric fields induced by
movement of the human body in a static magnetic field
and by time-varying magnetic fields below 1 Hz (ICNIRP
2014). As Gowland and Glover correctly state, their study
was the first in the field, and there is a clear need for moredata. At the time of writing the guidelines, their study was
the only research providing useful information for setting
guidelines. Important new findings from two studies wererecently published (Mian et al. 2013; Glover et al. 2014),
but they do not provide sufficient reason for fundamental
changes in the exposure guidelines.
The main target of the criticism is the protection
against vertigo by limiting the change of the magnetic flux
density in order to limit slowly varying induced electricfields. This was indeed an important aspect, but it was
not the only one taken into account. The guidelines explicitly state that “…. given the possibility of direct magnetic fields on the body, it is important to restrict boththe static magnetic flux density (B) and the maximum
change of the magnetic flux density (Δ B) experienced
by the body during movement” (ICNIRP 2014). Given
the present state of knowledge, to which Gowland andGlover have contributed much, it is best to assume thatboth the direct Lorentz force and electric field effects contribute to motion-induced vertigo. The new findings haveincreased the likelihood that the Lorentz force on ioniccurrents in the vestibular organ explains the vertigo effect,
but other mechanisms and particularly those due to the induced electric field still cannot be neglected. It has been
well known for 150 years that the electric field induced by
the galvanic current in the vestibular system causes vertigo.
The new research findings indicate that the balancesystem in humans seems to react more to the change ofthe magnetic flux density than to the magnetic field itself.
It should be noted that the new ICNIRP guidelines should
be applied together with the guidelines for static magnetic
fields (ICNIRP 2009). Therefore, ICNIRP recommends
restricting both the static magnetic flux density and itschange during movement. This does not contradict theLorentz model. A relevant issue is the relatively short integration time of 3 s, during which the change of the magnetic flux density should not exceed 2 T. In a 7 T field, itwould be possible to move into the field in 10 s in order tocomply with the guidelines, while the study of Glover et al.
(2014) indicates that the adaptation time constant would beapproximately 40 s or even longer. However, it is important

September 2014, Volume 107, Number 3

to remember that the 2014 guidelines provide for controlled
exposures where there is need to minimize sensations ofvertigo without a strict requirement to avoid them.
More studies are needed to clarify this important safetyissue concerning movement in strong magnetic fields.
ICNIRP awaits with interest results of new studies. ICNIRP
will regularly revise its Guidelines and Statements. If there
is an obvious need for adjustments and clarifications of
the existing guidelines, they will be provided.
The author declares no conflicts of interest.
GUNDE ZIEGELBERGERICNIRP
Oberschleissheim, Germany

REFERENCES
Glover PM, Cavin I, Qian W, Bowtell R, Gowland PA. Magneticfield-induced vertigo: a theoretical and experimental investigation. Bioelectromagnetics 28:349–361; 2007.
Glover PM, Li Y, Antunes A, Mian OS, Day BL. A dynamic
model of the eye nystagmus response to high magnetic
fields. Phys in Med and Biol 59:631; 2014.
International Commission on Non-ionizing Radiation Protection.
Guidelines on limits of exposure to statis matnetic fields. HealthPhys. 96(4):504–514; 2009.
Mian OS, Li Y, Antunes A, Glover PM, Day BL. On the vertigo
due to static magnetic fields. Plos One 8:e78748; 2013.

ATOMIC BOMB SURVIVOR CATARACT
SURGERY PREVALENCE DATA ARECONSISTENT WITH NON-ZERO
THRESHOLD DOSE—COMMENT ON
ARTICLE BY NAKASHIMA ET AL. 2013.
Dear Editors:
THIS COMMENT is with reference to the recent article
titled “Radiation Dose Responses, Thresholds, and False
Negative Rates in a Series of Cataract Surgery PrevalenceStudies among Atomic Bomb Survivors” by Nakashima
et al. (2013). In this study estimating the dose thresholdfor cataract surgery in atomic bomb survivors, the authors
concluded that the data for each 2‐y period is compatible
with zero dose threshold. This conclusion is surprisingsince it contradicts the recognition of a threshold doseof 0.5 Gy for cataracts in a recent statement by ICRP
(Stewart et al. 2012). Another reason why the possibilityof zero threshold is not credible is that the eye lens has ahigh concentration of reduced glutathione (Ganea andHarding 2006), which would protect the lens from the oxidative damage that may be caused by a small increase inradiation exposure. Since the authors’ conclusion on the

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