Interview

Abstract

Professor Ken Foster is a professor of Bioengineering at the University of Pennsylvania, USA. In this interview he discusses his research and new Electronics Letters paper, ‘Thermal Response of Tissue to RF Exposure from Canonical Dipoles at Frequencies for Future Mobile Communication Systems’, page 360 Professor Ken Foster I have had a longstanding interest in medical uses of radiofrequency and microwave energy. I have also been involved since the early 1970s in the issue of possible health and safety issues with non-ionising radiation. The Mobile Manufacturers Forum (now Mobile and Wireless Forum) established a research project with the objective to enhance the understanding of thermal and dosimetric aspects above 6 GHz. The collaboration between myself and my colleague, Davide Colombi based at Ericsson Research in Sweden, was established thanks to this program. Davide and I performed numerical simulations to determine the amount of absorbed power and subsequent temperature increase in tissue that would result from antennas near the body operating at the maximum levels consistent with current microwave exposure guidelines (IEEE, ICNIRP). We were interested in the effects of the “transition” in these guidelines at 6 or 10 GHz. Below the transition, the guidelines are in terms of the absorbed power (the Specific Absorption Rate or SAR) and above it they limit the incident power density on tissue. We found that the different limits have very different implications for the amount of power that can be transmitted by antennas near the body, and the level of protection that they provide against thermal hazards changes greatly as you cross the transition frequency. The current microwave exposure limits are designed principally to protect against thermal hazards such as burns and thermal pain from excessive exposure to microwave energy. The guidelines need to be protective, but also they should be consistent in level of protection for different exposure frequencies. Excessively restrictive limits can compromise the effectiveness of new communications technology with no discernible benefit in health and safety. ICNIRP and IEEE exposure limits are based on a comprehensive review of the literature on HF fields and have been designed with large safety margins to prevent excessive heating of tissues. The dosimetric issues have been extensively studied for presently used cellular communications bands, but much more work is needed at higher frequencies. As we point out in our Letter, the limits offer very inconsistent levels of protection against thermal hazards above and below the “transition” frequency. Other technical features, such as the area of skin over which exposure is to be averaged and averaging times, also need revision. The ICNIRP limits are inconsistent with its own limits for infrared energy at the frequency (300 GHz) where the two bands meet. The design of the guidelines in the mm wave range (30-300 GHz) has not previously been a big issue since the potential for human exposure to mm waves has been quite limited, particularly from antennas operated close to the body. That will change when 5G communications technologies are rolled out in coming years, which will bring electronics devices including communications handsets operating above 6-10 GHz to the consumer market. There will be a lot more numerical modelling studies related to antennas operating in these frequency ranges and used close to the body. In past years a tremendous amount of work has been invested in developing methods for assessing compliance of ordinary cell phones with national and international exposure limits, and this work will soon be extended to the higher frequencies used for 5G communications. It is a difficult problem due to the short wavelengths and short penetration depths in tissue. More experimental studies will be needed as well, including studies that evaluate reported “non-thermal” effects of mm wave energy.