S Kuhn, E Cabot, A Christ, M Capstick, and N Kuster (2009). Assessment of the radio-frequency electromagnetic fields induced in the human body from mobile phones used with hands-free kits.  Phys. Med. Biol. 54:5493–5508

Debate exists on the specific absorption rate (SAR) of wired hands-free kits for mobile phones compared to exposure from the use of the mobile phone at the head. Several studies have shown that an increased SAR is possible under worst-case scenarios.  However, there are still several knowledge gaps concerning exposure from the wire coupling and from the earbud of hands-free kits.  This study addressed several questions, including the extent to which wired and wireless hands-free kits reduce radiofrequency field exposure emitted from mobile phones.

The study involved dosimetric evaluation of phone configurations with and without hands-free kits, including worst-case scenarios, such as having the phone lying on a desk in front of the user.  Evaluation of SAR at the cheek and ear (when earbud used) and in the trunk was considered.  SAR measurements were taken on a phantom model covered with tissue simulating liquid.  Two mobile phones were used:  Nokia 6120c (with an integrated antenna) and the Motorola V1050 (with an external antenna).  Three wired hands-free kits were used, two stereos (Nokia HS-47 and Motorola H120) with an intra-aural earbud and a microphone integrated in the cable and one mono with a microphone boom and an intra-aural earbud (Plantronics MX250). Three wireless hands-free kits using Bluetooth communication were also selected. The phones were evaluated in the GSM900, GSM1800, and UMTS1950 signals. An evaluation was also made of the attenuation of the SAR by absorbing material, and different configurations of the coupling cable were tested. Computer simulation and modeling using the phantom model were also used. In this case, the evaluation of the SAR in the trunk and in the head from a mobile phone with hands-free kit was performed with the mobile phone in typical pants and shirt pocket positions.

Maximum 10g SAR in the head with a wired hands-free kit was five times lower than the ICNRIP limit of 2 W/Kg. In some worst-case situations, the SAR increased somewhat when using the hands-free kits compared to the phone directly at the ear. For SAR in the trunk, the worst-case was without hands-free kits and with the mobile phone’s back side touching the phantom. The SAR decreased by 4–15 dB when a coupling cable was positioned at a distance of 15 mm. Direct contact of the cable to the body resulted in an attenuation as high as 13–26 dB. From the simulation methods, on average, the SAR in the anatomical models was 2.5 dB lower than the SAR measured in the flat phantom, which was explained as being due to layers of fat tissue. Compared to a phone in the pant pocket position, having it in the shirt pocket position resulted in a head SAR that was considerably higher, in this case resulting from less attenuation along the hands-free kit wire.

Discussion and Conclusion
In the worst case scenario, the SAR was always lower than the limits introduced by ICNRIP in 1996.  It was confirmed that, in general, hands-free kits lead to a considerable reduction of the exposure of the head.  Wireless hands-free kits cause a low but constant level of exposure, whereas the exposure from wired hands-free kits depends on several factors, such as output power of the phone. Some hands-free kit configurations were found to increase exposure, but the increase basically was only possible because of modern phones that have been optimized for minimal SAR and optimized radiation efficiency.  In comparison to historical evaluations, exposures were lower. The authors concluded that the SAR at the earbud mainly depends on the output power and frequency of the mobile phone; additional compliance test procedures for wired hands-free kit were not deemed necessary.

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