The development of biomedical approaches and concepts in radiofrequency radiation protection

The approaches and concepts used in the development of radiofrequency radiation (RFR) protection guidelines evolved over the past quarter of a century. The values of exposure limits (EL) proposed by various groups are converging. Early guides specified ELs in incident power density. Recent ELs are based on considerations of the relationship between bioeffects and the magnitude of the whole body average specific absorption rate (WBA-SAR) and current densities induced in the body. Both these quantities may be considered as dosimetric ones. Thresholds for untoward health effects expressed in terms of these quantities were suggested, and may be considered as basic ELs. It is possible to derive a frequency-dependent relationship between incident RFR fields and WBA-SAR and/or induced current densities in the body. ELs specified for the purpose of determining compliance in terms of electric and magnetic field strengths or equivalent plane-wave power density existing at a point where a person could be present, but measured in the absence of the exposed subject, may be considered as derived working limits. The rationales offered for the recommended ELs indicate that the principal consideration in establishing limits for frequencies of 10 MHz and higher is the prevention of thermal injury, thermal being defined as relatable to heating, i.e. an increase in temperature. At lower frequencies, below 100 kHz or 30 kHz, direct effects on membranes of nerve and muscle cells may be the limiting factor. An additional consideration is the hazard of shock and burns from contact with ungrounded large metal objects that are charged by RFR fields. Recent advances in RFR dosimetry led to concerns that exposure to presently accepted derived ELs may result in large local SARs and induced current densities in certain parts of the body. The present review concludes that further refinements to the basis for RFR should be introduced. Threshold for health hazards should be investigated taking into account both direct and thermal bioeffects of RFR. The dose-thermal effects relationships should be quantified using the concepts of SAR, SA and thermal dosage. Several unresolved questions, such as the biological basis for SAR time-averaging, and the limitation of pulse peak power, are briefly discussed.