The ICNIRP Exposure Guidelines are published in Health Physics, 1998, vol 74, pp494-522. The basic restrictions (10 mA m-2 for occupational exposure and 2 mA m-2 for general public exposure at 50 Hz) are contained in table 4. The relevant column is headed “current density for head and trunk”.
ICNIRP then published “Response to Questions and Comments on ICNIRP” in Health Physics, 1998, vol 75, pp438-439. Question 10 reads in full:
“10. Question: Is the basic restriction of 10 mA m-2 based only on the threshold for acute effects in the central nervous system, or does it apply to other tissues in the trunk of the body?
Answer: the basic restriction of 10 mA m-2 is intended to protect against acute effects on central nervous system tissues in the head and trunk of the body, with a safety factor of 10. ICNIRP recognizes that this basic restriction may permit higher current densities in body tissues other than the central nervous system under the same exposure conditions.”
Thus ICNIRP intends the basic restriction to apply to the central nervous system only, but both publications need to be read together to establish this, and a reading of the main “Guidelines” document only without the “Responses to Questions” would give the erroneous impression that it applies to the whole head and trunk.
The practical effect of this can be discerned from publications which give numerical calculations of induced currents in different tissue types in the body.
For example, for magnetic fields, consider Dimbylow P J, Phys. Med. Biol. (1998) Vol 43 pp 221-230. Averaged over a 1 cm2 surface as required by ICNIRP, table 2 shows that at 50 Hz, the basic restriction of 10 mA m-2 applied to the head and torso corresponds to a uniform magnetic field of 260 µT , whereas table 3 shows that applied to the central nervous system only it corresponds to 1810 µT. In other words, the erroneous application to the whole head and trunk is, for uniform fields, 7 times more restrictive than the correct application to the central nervous system.
For electric fields, consider Dimbylow P J, Phys. Med. Biol. (2000) Vol 45 pp 1013-1022. Again averaged over a 1 cm2 surface as required by ICNIRP, table 1 shows that at 50 Hz, the basic restriction of 10 mA m-2 applied to the head and torso corresponds to a uniform electric field of 5.8 kV m-1 , whereas table 2 shows that applied to the central nervous system only it corresponds to 47.8 kV m-1 . In other words, the erroneous application to the whole head and trunk is, for uniform fields, 8 times more restrictive than the correct application to the central nervous system.
The EU Recommendation of 1999 dealt with this issue correctly by adding a footnote 2 to the corresponding table 1:
“2. The basic restriction on the current density is intended to protect against acute exposure effects on central nervous system tissues…. However, since the basic restriction refers to adverse effects on the central nervous system, this basic restriction may permit higher current densities in body tissues other than the central nervous system under the same exposure conditions.”
