A recent study from Sweeden shows link between brain tumours and rural phone use where output power of phone and base station are highest.
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A difference in the power output level from mobile phones between urban and rural areas has recently been shown. This is caused by adaptive power control (APC) in the cellular telephone and is regulated by the distance between base stations. Thus, in areas with a long distance between base stations, usually rural areas, the output power level is higher than in more densely populated areas–that is, urban areas, with a shorter distance between base stations. APC is used for the Global System for the Mobile Communication (GSM) network. When the Nordic Mobile Telephone System (NMT) started in Sweden in 1981, the highest power was used all the time by the mobile phones. With increasing numbers of users of the NMT, phones in this system were regulated by APC, but only if they were very close to the base station and then only in one step. In previous epidemiological studies we found an association between use of cellular telephones and brain tumours.
(pdf of published study)