Introduction to RF Radiation Safety / 9 / 9.3 / 9.3.2

9.3.2 Basic Restrictions

Basic restrictions are the fundamental exposure restrictions in the ICNIRP framework. They are expressed in physical quantities closely related to the substantiated adverse health effects.

Depending on frequency, exposure duration, and the part of the body exposed, the basic restriction quantities include induced electric-field strength, whole-body average specific absorption rate (SAR), local SAR, absorbed power density, and, for brief exposures, specific energy absorption or absorbed energy density.

Most basic-restriction quantities describe fields or energy absorption within the body and cannot be measured directly during a routine site survey. Compliance may therefore require RF dosimetry using calculation, computational modeling, or specialized measurement.

9.3.3 Reference Levels

Reference levels provide a practical means of demonstrating compliance without first calculating quantities inside the body. They are derived from the basic restrictions using conservative exposure conditions.

Reference-level quantities include incident electric-field strength, incident magnetic-field strength, incident power density, incident energy density for brief exposures, and limb current in relevant circumstances. Plane-wave-equivalent quantities may also be used where the guideline permits them. Absorbed power density is not a reference-level quantity; it is a basic-restriction quantity within the body.

Exposure at or below the applicable reference level is taken to comply with the corresponding basic restriction. Exceeding a reference level does not necessarily mean that the basic restriction has been exceeded or that harm will occur. It means that a more detailed assessment is required.

The applicable reference quantity and assessment rule depend on frequency and exposure geometry. In the far field, electric field, magnetic field, or incident power density may provide an adequate compliance measure. In the radiating or reactive near field, electric and magnetic fields may need to be assessed separately, and in some reactive near-field situations compliance must be demonstrated against the basic restrictions.