Mobile phones could pose a higher health risk to rural dwellers because they emit more intense signals in the countryside, Swedish scientists said on Tuesday.

　　Base stations tend to be further apart in more remote areas so the phones compensate with stronger signals.

　　"We found that the risk of brain tumour was higher for people living in rural areas than in towns," said Professor Lennart Hardell, of University Hospital in Orebro, Sweden.

　　"The stronger the signal, the higher the risk," he told Reuters.

　　Use of mobiles has increased rapidly worldwide but there has been no hard evidence that the technology causes health problems ranging from headaches to brain tumours.

　　Some researchers have suggested that radio frequency fields could interfere with biological systems.

　　Health officials have urged the public to limit mobile phone use or to use hands-free devices.

　　Hardell and his colleagues, who studied 1,429 people with malignant and benign brain tumours and 1,470 healthy controls living in the centre of Sweden, said the health risks may not be evident until someone has used a mobile for 10 years or more.

　　Their research is published in the journal Occupational and Environmental Medicine.

　　The scientists found that rural dwellers who had been using a mobile phone for more than three years were three times more likely to be diagnosed with a brain tumour than city dwellers.

　　The risk quadrupled after more than five years of use.

　　The researchers questioned both groups about how often they used their mobile phones and for how long. They also looked at whether they lived in the countryside or in towns.

　　Their findings were adjusted for other environmental factors that might increase the risk of brain tumours.