Mobile phones will not cause cancer or slow the brain, according to a recently published six-year study, but the jury is not out.

The Mobile Telecommunications and Health Research (MTHR) Programme, possibly the largest investigation so far into health risks associated with mobile telephone technology, was set up to resolve uncertainties identified by previous evaluations of the possible health risks associated with the widespread use of mobile phone technology.

The researchers studied mobile phones, mobile phone base stations --including newer 3G stations-- and the TETRA emergency services radio system used in the UK. The extensive research included five epidemiological studies and eight volunteer studies, three of which explored reported hypersensitivity to signals emitted by phones and base stations.

The study involved commissioning Dr Phil Chadwick of Microwave Consultants Limited to design a system producing exposures representative of those in real phones. This model was used in all the volunteer studies so as to eliminate variation in results due to phone design. The resulting device was modified from a commercially available phone and produced in two variants. One simulating a 900 MHz GSM mobile phone and the other a TETRA radio. In both cases the waveform of the emitted fields contained all the significant characteristics of a real signal.

The system had a headset that enabled it to be mounted in one of the standard positions used to assess exposure. Each phone was capable of producing three different exposure conditions: CW (constant RF); modulated (RF that varies in the same way as a mobile phone signal); and sham (ideally no RF, but in practice, an exposure at most 100 times less than in other two conditions). The various output modes were selected using hexadecimal codes so that neither researcher nor subject knew which exposure condition had been selected. The maximum Specific Absorption Rate was 1.3 W/kg averaged over 10 g.

"None of the research so far suggests that biological or adverse health effects are produced by radio frequency exposure from mobile phones. Reassuringly no epidemiological association was found between short-term and long-term mobile phone use (less than ten years) and cancers of the brain or nervous system," states the report.

"Studies on volunteers provided no evidence that brain function is affected by exposure to the signals emitted by mobile phones or by TETRA radios used by the emergency services."

Likewise, studies on electrical hypersensitivity did not support the theory that unpleasant symptoms are experienced as a result of exposure to signals from mobile phones or base stations.

Base station emissions were also measured and exposures confirmed low, although exposure in the immediate vicinity of micro cell installations was found to be higher than those from the larger macro cell installations.

Not surprisingly, the study confirmed that the use of a phone or hand held device while driving does increase risk of accident, but researchers found it caused no greater risk than other in-car distractions. Researchers recommended however, that precautionary advice from vendors of mobile devices was limited and that policy makers need to adopt alternative, more effective methods of communicating the risks associated with operating machinery or driving vehicles while using hand held devices.

The program recognises that some concerns still remain and has proposed an extension of the study to keep working on these. Priorities will include work to assess whether long-term exposure (greater than ten years) increases the risk of developing cancers of the brain and nervous system and the effects of mobile phone exposure specifically in children.

The debate of whether or not mobile phones cause cancer or affect the brain looks set to continue for some time yet. Evidence contradicting the MTHR project's findings was supplied in a study of 300 people that found frequent mobile phone users did demonstrate slowed brain function. These researchers also want to continue their study over a longer period to examine over 17,000 people.