Standards: Exposure Limits for Brief High Intensity Pulses of Radiofrequency Energy Between 6 and 300 GHz

This Standards paper considers limits of the International Commission on Nonionizing Radiation (ICNIRP) and Institute of Electrical and Electronics Engineers (IEEE C95.1-2019) on “brief exposures” to radiofrequency energy (RF-EMF), that vary significantly within the 6 minute averaging time specified in the limits. This situation arises in two important contexts: exposures from cellular base stations employing Multiple Input Multiple Output (MIMO) antennas with beamforming that sweep beams around the service area, and exposure to pulsed RF-EMF with high peak power but low duty cycle. The ICNIRP and IEEE limits on brief exposures between 6 and 300 GHz are assessed using numerical and analytical solutions to a one-dimensional thermal model of tissue. Analysis is done both for rectangular pulses of RF-EMF (i.e., the step response to a suddenly imposed RF-EMF field), and to impulses of RF-EMF energy. The analysis shows that both sets of guidelines are uneven in the level of protection they offer against excessive transient heating of skin, with the IEEE limits being far more conservative than the ICNIRP limits. The thermal model suggests a simple modification to the pulse fluence limits that provides a more consistent level of protection against excessive temperature increases for pulses of varying duration and frequency. The Standards Paper does not consider possible nonthermal hazards or possible thermoacoustic effects produced by brief but very high amplitude RF-EMF pulses that produce high rates of change of tissue temperature but small temperature increases. The conclusion points to the need for further research and development to establish and refine exposure limits for pulsed RF-EMF.