Researchers claim the higher cancer rates could be caused by phones outside urban areas emitting more intense signals, which might interfere with the function of body cells.

``The stronger the signal, the higher the risk,'' Professor Lennart Hardell, of University Hospital in Orebro, said in a Reuters report.

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

Hardell studied 1,429 people with malignant and benign brain tumors and 1,470 healthy ``controls'' living in the center of Sweden. Both groups were questioned about how often they used their mobile phones and for how long.

Hardell said health risks may not be evident until someone has used a mobile for more than 10 years.

However, researchers found rural dwellers who had been using mobile phones for more than three years were three times more likely to be diagnosed with brain tumors than people living in the city.The risk jumped to four times after more than five years of use.

``We still cannot exclude that there might be other undetected risks in the countryside, but we have tried to adjust the results [for other environmental factors that might increase the risk], as far as we know,'' Hardell said.

But Hong Kong experts are at odds with Hardell's findings. Chinese University biochemistry professor Chan King-ming said such research has so many parameters that is hard to draw conclusions from the data.

``The issue is worthy of concern, but the public need not panic,'' he told The Standard.

As the research method is unclear, it is difficult to say the same conditions exist in Hong Kong, he said.

``There is no hard evidence that radiation from mobile phones can cause health problems. The public should be skeptical of the findings.''

Health official have urged mobile users to use hands-free kits and to resist using them for long periods. Some have advised users, especially pregnant women, not to hang phones on their chests.

Professor Raymond Cheung, of University of Hong Kong's Division of Neurology, says there is no hard evidence to show that mobile phones cause health problems.

He stressed that primary brain tumor is rare, and that the majority of brain-cancer cases are caused by the diffusion of cancer cells from other parts of the body, such as the lungs, intestines and breasts. But he said that, although there is no hard evidence to connect mobile phones with health problems, they do emit electronic waves that increase the temperature of human tissue. Hands-free kits can help decrease exposure to electronic waves, he said.