Evaluating Partial-Body Radiofrequency Field Exposures: The Need for Better Near-field Dosimetry

Abstract

The potential for excessive radiofrequency (RF) exposure is generally greatest in the near-field of RF sources. Present-day RF exposure standards are primarily derived from dosimetry associated with uniform exposure of the entire body. This is particularly true for the limits on spatial peak specific absorption rate (SAR). Normally, however, the very intense and localized RF fields to which some personnel are exposed are clearly not uniform over the body and this raises the question of the appropriateness of partial-body relaxation provisions in various exposure standards. SAR measurement data obtained during the course of evaluating the effectiveness of RF protective clothing have demonstrated the existence of localized SARs that can substantially exceed recommended limits (e.g., 8 W/kg in any one gram of tissue), even though the local RF field incident on the body does not exceed maximum permissible exposure (MPE) limits for partial body exposures. These data indicate that body curvature plays a significant role in determining the peak SAR and may be most important for the head. The findings also suggest that the field polarization and source position relative to the body are critical determinants of the maximum, local SAR value. These results point to the need for a better understanding of how localized SAR is related to conditions of exposure and challenge the validity of peak SAR limits that are based on the assumption of uniform exposure.