On basic restrictions and maximum permissible exposure limits for non-ionizing radioation

The Norwegian Armed Forces follow the guidelines of ICNIRP for occupational exposure to non-ionizing radiation. This is consistent with civil regulations. For the frequencies we are interested in (from 100 kHz to 10 GHz) mainly thermal effects are relevant. Only for the lowest frequencies (up to 10 MHz) excitation of nerve tissue may occur. This report discusses the relationship between the basic restrictions or BR (norw. basisverdier), which are legal limits for electromagnetic fields inside the human body, and maximum permissible exposure (MPE) limits (norw. referanseverdier), which are recommended limits of radiation levels in areas where personnel may be present ensuring that the BR are not exceeded. There are two types of BR, a combined or average value for the whole body of 0.4 W/kg, and local BR for various body parts of 10 W/kg for head and torso and 20 W/kg for the extremities. Since the few specific MPE limits listed in the ICNIRP Standard are only valid for exposure to the far field and in relation to the BR for the whole body, there are clear limitations on their use. The IEEE Standard, which rests on the same BR, goes one step further by also providing local MPE limits related to the local BR - but still with the limitation that exposure should not happen in the near field of a transmitter. Also NATO’s NSA STANAG follows mostly the IEEE MPE limits. When using ICNIRPs whole-body MPE limits for fields with varying strength across the body (still excluding near field exposure) the correct procedure is to use an averaged value, provided that local BR are not exceeded. In light of this, we discuss earlier measurements of a portable jamming system as an example. For this case, it can be concluded that: a. the radiation level is acceptable compared to the whole-body MPE limits of the ICNIRP Standard if one uses an averaged value and exposure is considered in the far field of the transmitter, b. the local MPE limits of the newer IEEE Standard suggest that local BR are not exceeded at any single point if exposure is considered in the far field of the transmitter, c. measurements and simulation of thermal effects in the human body of communication radios with similar power indicate that local BR of the ICNIRP Standard will not be exceeded for exposure in the near field of the transmitter. This can, however, be affected by the frequency band and the physical design of the jamming system. The discussion and results in this report may be used as background material for the assessment of improved safety procedures for approval of equipment in the Norwegian Armed Forces, where one also takes into account other thermal loads.