Whittow WG, Panagamuwa CJ, Edwards RM, Vardaxoglou JC. (2008). On the effects of straight metallic jewellery on the specific absorption rates resulting from face-illuminating radio communication devices at popular cellular frequencies. Phys Med Biol 53(5):1167-1182.
Background and Objective
This paper investigates the effects of RF exposures to the human head from electronic personal data assistants (PDA) and how this interacts with straight metallic jewellery. The study is an extension of previous research that suggested jewellery can redistribute, shield, or enhance RF energy. Examples include studs in the nose, the eyebrow, and the ear; as well as long and straight items, like metallic spectacles and hairclips. Results are presented both for simulation work and for direct measurement validation.
To replicate personal electronic data assistants, which are used in front of but away from the face, the study used a finite-difference time-domain (FDTD) model with a continuous wave dipole source positioned in front of the head so as to illuminate the face. The simulations were run for 1800 MHz and 900 MHz exposures. In general, closed form analysis is not available for such complex systems as phone-body interactions and therefore simulation, measurement, and published controls have been used. In both measurement and simulation, the authors modeled various shapes for the head but did not include a model of the hand. A study using 20 adult volunteers was also undertaken to measure the typical distance between the eye and screen of a PDA device when a user was reading text. The worst-case scenario had the PDA at a distance of 100 mm. Three types of head model were used, representing different shapes and types of tissue: a half-wave dipole 100 mm in front of a 200 mm homogeneous cube; a 70 mm dipole 98 mm in front of a 200 mm cube, including a 2 mm fiberglass shell; and a half-wave dipole 100 mm in front of the eyebrows of a realistically shaped head.
Both the size of the pin and its distance from the model were important. The results showed that small pins (<0.2 wavelengths long) will have negligible effect on the 1 gram specific absorption rate (SAR), regardless of the position. The effect becomes substantial when the length is greater than 0.3 wavelengths. The maximum effect was seen with a pin 0.42 wavelengths long. When the pin was either touching the head, or 2 mm from the head, all jewellery sizes had little effect. The largest effect of the pin was also seen when the distance from the model to the pin was approximately 12 mm. The combination of a pin 0.42 wavelengths long and at a distance of 12 mm from the surface of the model had the largest effect, increasing the 1 g SAR by a factor of 16 (from 0.38 W/kg without the pin to 6.12 W/kg). The power absorbed in the whole head model with the pin was 0.18W, an increase of 30% (1.3 times) compared to without the pin. The pin also caused higher SAR values to extend deeper into the head, which could have implications for children who have smaller heads. Results also showed areas where the SAR is reduced when the pin is added. Measurement results reaffirmed that the SAR is increased near the pin and decreased away from the pin, implying that the pin has a focusing effect. The authors caution that the results may overestimate the expected SAR values.
Discussion and Conclusion
From these results, it can be inferred that the effects would be negligible with small straight jewellery touching the skin, such as with piercings. The results also indicate that metallic jewellery insulated from the head by air or a plastic coating may have a greater effect. Such objects could include metallic rim spectacles, microphones, and nose and ear jewellery that may hang away from the head.