Vrijheid M, Mann S, Vecchia P, Wiart J, Taki M, Ardoino L, Armstrong BK, Auvinen A, Bédard D, Berg-Beckhoff G, Brown J, Chetrit A, Collatz-Christensen H, Combalot E, Cook A, Deltour I, Feychting M, Giles GG, Hepworth SJ, Hours M, Iavarone I, Johansen C, Krewski D, Kurttio P, Lagorio S, Lönn S, McBride M, Montestruq L, Parslow RC, Sadietzki S, Schüz J, Tynes T, Woodward A, Cardis E.Determinants of mobile phone output power in a multinational study – implications for exposure assessment Occup Environ Med Ahead of print May 21, 2009. doi:10.1136/oem.2008.043380

Studies on the potential health effects of mobile phone use are limited by crude exposure assessment, which relies mainly on participants’ self-reported frequency and duration of phone use, proxy measures for exposure to radiofrequency (RF) fields. Exposure to RF fields is dependent on the output power of the phone. Output power can theoretically be substantially reduced by adaptive power control (APC) technology employed in second-generation GSM phones. The power reduction may, depending on circumstances of phone use, include shielding by buildings, distance to the base station, and number of reconnections with different base stations within one call, such as would occur in a moving vehicle.

To assess mobile phone output power in different GSM networks world-wide and under different circumstances of use to inform RF exposure assessment in epidemiological studies.

Special software-modified phones (SMPs) were used by volunteers associated with the INTERPHONE study in Australia, Canada, Denmark, Finland, France, Germany, Israel, Italy, New Zealand, Norway, Sweden, and the U.K. between 2001 and 2005. The SMPs contained software to internally record the time and duration of each call, and the power level at intervals throughout the call. Volunteers used the SMPs for around one month. Additionally, volunteers answered questions about their frequency of phone use in urban or rural areas, inside buildings, and in a moving vehicle.

Analyses were based on 516 subjects from 12 countries who made or received 63,151 calls with the SMPs. For all study centres combined, the mean output power per call was approximately 50% of the maximum power levels, with significant differences between study centres. On average, the phones emitted on maximum power (PMax) for 39% of call time and at minimum power (PMin) for 2.8%, again with substantial differences between centres.  Mean output power depended somewhat on mobile phone operators but not on time period of SMP use (2000/2001 versus 2004/2005). Output power was higher for short than long calls and was slightly lower for calls made during the day. In all study centres together, calls made by SMP users reporting use mainly in rural areas had somewhat higher mean output power and use of PMax than those reporting use in urban areas, although results were mostly driven by findings in Sweden. Mean mobile phone output power was not influenced by use while in a moving vehicle or inside buildings nor was it influenced by age, gender, or SMP model.

Interpretation and Limitations
GSM networks with APC reduced the output power of mobile phones by 2-fold on average.  This is much smaller than the theoretically possible 1000-fold reduction. Reductions of around 2-4 fold have also been observed in 5 prior studies of limited scope. Overall, frequency of SMP use in rural areas, in moving vehicles, and inside buildings was not related to output power, except in Sweden where the output power was significantly higher for rural compared to urban users. This is consistent with previous studies from Sweden, the US, and Germany. The main predictions of output power were study location, network, and duration of call.  A major limitation of this study is that information on phone use circumstances were only available for individuals overall and not for each single phone call.

Exposure indices in epidemiological studies using questionnaires covering call frequency and duration of phone use could be improved by accounting for average power levels of different telecommunications systems.  There appears to be little value in gathering information on circumstances of phone use apart from use in very sparsely populated regions.

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