While source type and its distance from the subject are influential parameters, the fundamental exposure metrics are the physically measurable quantities of currents, external electric field and magnetic field strengths, and incident power density (when appropriate), which induce electric and magnetic fields that bring about the interaction of radiofrequency (RF) electromagnetic energy with biological systems. Induced fields are the primary cause for biological effect of RF exposure, regardless of the mechanism. Thus, in order to achieve a quantitative understanding of biological response, the induced electric field and the derived dosimetric quantities of specific absorption rate (SAR) and current density must be quantified and correlated with the observed phenomenon. In the established exposure guidelines, reference levels expressed in terms of physical quantities are introduced for practical exposure assessment purposes since the basic restrictions are often specified as dosimetric quantities that may be impractical to measure. The dosimetric quantity SAR, current density, and their determinations are tissue-type dependent and require a region of specific tissue mass for averaging. Thus, a smaller averaging region is scientifically more relevant and precise. It is emphasized that the sensitivity and resolution of present-day computational resources and experimental measurement techniques can provide accurate SAR values with a spatial resolution on the order of a 1 mm, in dimensions. Furthermore, most exposure guidelines are promulgated on a 4 W kg SAR to prevent any whole-body exposure from raising the body temperature to 1 degrees C above the norm at 37 degrees C. Special guidance may be needed for higher ambient temperature and humidity variations.