Probabilistic approach to ranking sources of uncertainty in ELF magnetic field exposure limits

There is a need to improve the biological data, dosimetry, and risk assessment methodology used for setting guidelines for occupational exposures to extreme-low-frequency magnetic fields. This paper illustrates how a probabilistic approach can be used to determine priorities for future research based upon the analysis of biological and dosimetric variables that affect stimulation of the heart by magnetically-induced currents. A model was constructed to predict a level of whole-body magnetic-field exposure below which no cardiac stimulation is expected (Bncs). For each iteration of the model, a value was selected from cardiac stimulation threshold, shape factor, and conductivity distributions by Latin Hypercube sampling, and a value for Bncs was computed. The very wide range of simulated Bncs magnetic-field values obtained indicates that there is considerable uncertainty about what constitutes a "safe" level of exposure. The results show that the major occupational-exposure guidelines are very conservative with respect to risks of cardiac stimulation. The minimum Bncs value computed (0.01 T) by either a circular-loop model or an ellipsoid dosimetric model is ten times the highest recommended workday exposure value in a guideline. The lowest 5% Bncs value calculated for a circular-loop model is about 50-times the ICNIRP occupational exposure limit; the lowest 5% Bncs value calculated for an ellipsoid model is more than 100 times the ACGIH occupational exposure limit. A new finding is that the method specified by several guidelines for determining guideline compliance when exposures occur at multiple frequencies (additive weighting of harmonics) leads to substantially lower Bncs estimates relative to a probably more valid rms-weighting method. Probabilistic sensitivity analyses indicate that lack of knowledge of the threshold for cardiac stimulation is the greatest source of uncertainty as to what is a "safe" level of exposure to extreme-low-frequency magnetic fields. Additional research to address this source of uncertainty may be expected to have the greatest potential impact to reduce overall uncertainty about safe magnetic field exposures.