Details of numerical calculations of induced currents and fields in the body

In this page we give chapter and verse on the results of numerical calculations.  It is therefore necessarily quite detailed.

The calculations all come from Dimbylow 2005 (the results for the adult male were originally published in earlier papers Dimbylow 1998 and Dimbylow 2000).  The paper presents calculations for several different organs.  We have extracted the result for the "limiting case" organ, ie the organ where a given external field induced the highest current or internal field.

Dosimetry relevant to central nervous system

In many exposure limits, the focus is on the central nervous system, so we start by giving the dosimetry relevant to the central nervous system.

Older exposure limits - e.g. NRPB 1993 and ICNIRP 1998 - specify limits on the induced current density, newer limits - e.g. ICNIRP 2010 - the induced field, so we give both.

ICNIRP 1998 includes the spinal cord in the definition of the central nervous system, but other definitions don't, so we give both alternatives. more on parts of the body the limits apply to

Induction by external magnetic field

External magnetic field - induced current density

(averaged over 1 cm2)

  • adult male: 5.50 mA/m2 per mT = 1.82 mT per 10 mA/m2 = 0.364 mT per 2 mA/m2
  • adult female: 4.89 mA/m2 per mT

both in the retina, for a lateral field.  From Dimbylow 2005 table 3.

The limiting case is therefore the male.  Because the limiting case is in the retina anyway, it makes no difference whether the spinal cord is included or not.

External magnetic field - induced electric field

(99th percentile voxel)

including spinal cord:

  • adult male: 48.6 mV/m per mT in spinal cord = 2.06 mT per 100 mV/m = 0.412 mT per 20 mV/m
  • adult female: 31.4 mV/m per mT in brain

excluding spinal cord:

  • adult male: 33.0 mV/m per mT in brain = 3.03 mT per 100 mV/m = 0.606 mT per 20 mV/m
  • adult female: 31.4 mV/m per mT in brain

all for a lateral field.  From Dimbylow 2005 table 6.

The limiting case is therefore the male whether or not the spinal cord is included.

Induction by external electric field

External electric field - induced current density

(averaged over 1 cm2)

  • adult male: 0.211 mA/m2 per kV/m
  • adult female: 0.217 mA/m2 per kV/m = 46.1 kV/m per 10 mA/m2 = 9.22 kV/m per 2 mA/m2

both in retina (value in heart is higher but is not part of central nervous system) for grounded person.  From Dimbylow 2005 table 4.

The limiting case is therefore the female. Because the limiting case is in the retina anyway, it makes no difference whether the spinal cord is included or not.

External electric field - induced electric field

(99th percentile voxel)

including spinal cord:

  • adult male: 3.42 mV/m per kV/m in spinal cord = 29.2 kV/m per 100 mV/m = 5.85 kV/m per 20 mV/m
  • adult female: 2.92 mV/m per kV/m in spinal cord

excluding spinal cord:

  • adult male: 1.65 mV/m per kV/m in brain
  • adult female: 2.02 mV/m per kV/m in brain = 49.5 kV/m per 100 mV/m = 9.90 kV/m per 20 mV/m

all for grounded person.  From Dimbylow 2005 table 7.

The limiting case is therefore the male if the spinal cord is included and the female if it is not.

Dosimetry for ICNIRP 2010 exposure guidelines

 ICNIRP (the International Commission on Non-Ionizing Radiation Protection) issued new exposure guidelines for 1 Hz - 100 kHz in 2010.  The EU Directive on occupational exposure is based on the same values.  On this page we discuss the dosimetry needed to apply these guidelines in practice.  

The ICNIRP basic restrictions are for induced fields in the tissues.  They give separate limits for the brain ("central nervous system" or CNS) and the whole body ("peripheral nervous system" or PNS).  For the CNS, the dosimetry is simple in principle, we take the most sensitive organ in the brain, which is usually the retina.  (Note that ICNIRP 1998 included the spinal cord in the CNS whereas ICNIRP 2010 excludes it.)  For the PNS, dosimetry is less well developed.  ICNIRP suggest using the values for the skin and we follow this here.  But there are questions about how well the skin is modeled and the effect of clothing, headgear etc.

The normal source of dosimetric calculations are those performed by Dimbylow at HPA on two models: a standard adult male ("Norman") and a standard adult female ("Naomi").  The normal practice is to take the more sensitive of these two and the most sensitive direction.  ICNIRP gives figures themselves too, which in most cases are the same as the Dimbylow figures.  At one place in the document (p824) they give ranges, then at a later point (p827), for magnetic fields but not electric fields, they give the actual value they use.  

The following tables give the figures for 50 Hz.

The figures for occupational exposure

 

magnetic field

electric field

 

CNS

(brain)

PNS

(whole body)

CNS

(brain)

PNS

(whole body)

basic restriction

100 mV/m

800 mV/m

100 mV/m

800 mV/m

dosimetric conversion factors
ICNIRP p824

23-33 mV/m / mT

20-60 mV/m / mT

1.7-2.6 mV/m / kV/m

12-33 mV/m / kV/m

ICNIRP p827

33 mV/m / mT

60 mV/m / mT

not stated

Dimbylow

33 mV/m / mT

(male, brain, lateral field)

(Dimbylow 2005 table 6. Female is 31.4.)

36 mV/m / mT

(female, skin, lateral field)

(Dimbylow 2005 table 5)

2.02 mV/m / kV/m

(female, brain, grounded)

(Dimbylow 2005 table 7.  Male is 1.65.)

33.1 mV/m / kV/m

(female, skin, grounded)

(Dimbylow 2005 table 5)

value used here

33 mV/m / mT

60 mV/m / mT

2.02 mV/m / kV/m

33.1 mV/m / kV/m

resulting field

3.03 mT

13.3 mT

49.5 kV/m

24.2 kV/m

limiting value of field

(for whole-body exposure)

3.03 mT

(CNS)

24.2 kV/m

(PNS)

The figures for public exposure

 

magnetic field

electric field

 

CNS

(brain)

PNS

(whole body)

CNS

(brain)

PNS

(whole body)

basic restriction

20 mV/m

400 mV/m

20 mV/m

400 mV/m

dosimetric conversion factors
ICNIRP p824

23-33 mV/m / mT

20-60 mV/m / mT

1.7-2.6 mV/m / kV/m

12-33 mV/m / kV/m

ICNIRP p827

33 mV/m / mT

60 mV/m / mT

not stated

Dimbylow

33 mV/m / mT

(male, brain, lateral field)

36 mV/m / mT

2.02 mV/m / kV/m

(female, brain, grounded)

33.1 mV/m / kV/m

value used here

33 mV/m / mT

60 mV/m / mT

2.02 mV/m / kV/m

33.1 mV/m / kV/m

resulting field

606 µT

6.67 mT

9.9 kV/m

12.1 kV/m

limiting value of field

(for whole body exposure)

606 µT

(CNS)

9.9 kV/m

(CNS)