McIntosh, R. L. and Anderson, V. SAR versus VAR, and the size and shape that provide the most appropriate RF exposure metric in the range of 0.5-6 GHz. Bioelectromagnetics. 2011 Jan 12.
Basic restrictions for protection against localized heating induced in biological tissues by radiofrequency electromagnetic fields (RF EMFs) are defined in terms of the specific absorption rate (SAR). SAR is mass averaged and expressed as W per kg of tissue mass. The International Commission on Non-ionizing Radiation Protection set limits for localized exposure as SAR averaged over 10 g tissue in the frequency range 100 kHz – 10 GHz. There is a debate about the most appropriate averaging mass for SAR as it may depend on frequency.
The objective was to determine the most appropriate averaging mass and averaging shape for SAR, to compare SAR with an alternative metric in terms of predicting tissue temperature rise.
In their analyses, the authors considered 1, 3, 5, 7 and 10 g as averaging masses, cube and sphere as averaging shapes for SAR. An alternative metric, volumetric energy absorption rate (VAR), which is averaged over a volume rather than over a mass and expressed as W per m3, was suggested. Averages over volumes of 1, 3, 5, 7 and 10 cm3 in the shape of a cube or a sphere were considered. The most appropriate metric type was determined by the degree of correlation with tissue temperature rise (DT) induced by near- and far-field RF exposures for frequencies 0.5, 1, 3 and 6 GHz.
For most frequencies, averaging sizes and shapes (40 comparisons in total), the correlation coefficients (R2) for VAR was equal or greater than that for SAR, though not significantly in most cases. The differences between the two averaging shapes were less discernible. For lower frequencies (0.5-3 GHz) and medium to larger sizes (3-10 g or cm3), comparisons between the two shapes seemed to be in favor of the cube. The optimal averaging size was 10 g for SAR and 10 cm3 for VAR except for at 6 GHz. At this frequency, none of the averaging volumes/masses performed well.
Interpretation and Conclusion
The results indicate that VAR is a superior metric to the SAR in terms of correlation with tissue temperature rise and also in terms of computational ease. The optimal averaging size is 10 g for SAR and 10 cm3 for VAR except for at 6 GHz. The authors suggest that at this frequency incident power flux density can be more appropriate exposure metric type. Since averaging over a cube or a shape is equivalent, the shape that is easier to assess should be specified for safety standards and guidelines.