9.1.3 Early Power-Density Limits

Early RF exposure limits were expressed primarily in terms of externally measurable field quantities, particularly power density. This was a practical approach because available instruments could measure the electromagnetic environment more readily than they could determine the amount of energy absorbed inside the body.

A power-density limit around 10 mW/cm² became influential in several early occupational protection systems. Although it represented an important advance, a single external limit could not fully account for differences in frequency, body size, orientation, exposure duration, polarization, or distance from the source.

Researchers increasingly recognized that the same external field strength could produce different patterns and amounts of absorption. The human body may absorb RF energy more efficiently at some frequencies than at others, while fields close to an antenna may have characteristics quite different from those in the far field. Consequently, external power density alone could not provide a complete description of the interaction between an RF field and the body.

This did not make external measurements obsolete. Modern standards continue to use external field quantities as practical reference levels. However, their relationship to the internal quantities associated with established adverse effects must be understood, particularly in complex near-field or highly localized exposure conditions.