241
Interaction of Extremely Low–Frequency Electromagnetic Fields
higher current densities in body tissues other than the CNS. The choice of the averaging
algorithm is an important source of uncertainty in the quantification of safety guidelines.
For safety standards and guidelines, there are two different documents published by
ICNIRP and IEEE (ICNIRP 1998a,b; IEEE 2002). ICNIRP provides basic restrictions
on induced current densities based on only the threshold of acute effect in the tissues
of the CNS and in the head and trunk of the body. The basic restrictions are 10 mA/m 2
for occupational exposure and 2 mA/m 2 for general public exposure at 50 Hz (ICNIRP
1998a). The ICNIRP document states that current densities should be averaged over a
cross-section of 1 cm 2 perpendicular to the current direction. Dimbylow (2008) identified uncertainties and problems in the practical application of the basic restrictions on
current densities. For the procedure to average 1 cm 2 of the tissues of the CNS, Bahr,
Bolz, and Hennes (2007) discussed the accuracy of the calculation method, the variability of models and parameters, and the implication for quantifying guidelines. They computed one voxel value of the in situ electric field for a 5-mm resolution model. Although
basic restrictions of existing ICNIRP guidelines are based on the current densities, the
IEEE standard used the in situ electric fields over an averaging distance of 5 mm on an
arbitrary direction as a measure. In the latter case, the electrostimulation is identified as
the relevant biological effect in the frequency range up to 3 kHz.
ICNIRP opened a draft of new guidelines for time-varying electric and magnetic
fields up to 100 kHz, on July 29, 2009 (ICNIRP 2009). This consultation document was
opened until October, 2009 on the ICNIRP Web site, and then the document was withdrawn from the ICNIRP Web site. In the draft guideline, instead of current densities,
ICNIRP proposed the in situ electric fields averaged over a cube (5 × 5 × 5 mm 3 ) as a
measure for basic restrictions. For this reason, the draft guideline mentioned that in the
newly prepared guidelines, the physical quantity used to specify the basic restrictions on
exposure to EMF is the in situ electric field strength E, as it is the electric field that affects
nerve cells and other electrically sensitive cells. Because of the relatively large uncertainties in the conductivity of tissues, induced electric field appears to be a more stable dose
quantity than the induced current densities. It is also mentioned that a biologically reasonable averaging distance might extend from 1 to 7 mm. The basic restrictions of draft
guidelines are 100 mV/m for occupational exposure and 20 mV/m for the general public
exposure, instead of 10 and 2 mA/m 2 , respectively.
In order to provide some insights into the difference between the draft guidelines of
the ICNIRP and IEEE standards, Hirata et al. (2010b) compared the in situ electric fields
averaged over a volume of 5 × 5 × 5 mm 3 and a straight line of 5 mm. They used Japanese
adult male and female models named TARO and HAMAKO for uniform ELF electric
and magnetic field exposures. They used a quasistatic FDTD method for the calculation
of the induced electric fields in these anatomical Japanese models. For magnetic field
exposure, the human was considered to be standing in free space and three orientations
of magnetic fields were assumed: AP, TOP, and LAT with 1 mT at 50 Hz. They compared
the computed results with the results of the SPFD method for exposure to magnetic
field. For electric field exposure, the human was considered to be standing on the perfect
conductor under the vertical electric fields with 1 kV/m at 50 Hz. They developed three
algorithms for calculating the averaged in situ electric fields in nerve tissue. The volumeaveraged electric field in the nerve tissue decreased with the averaging volume. The 99th