London: Male rats exposed to very high levels of the kind of radiation emitted by cellphones developed tumours in the tissues around their hearts, according to a draft report by US government researchers on the potential health risks of the devices.

Female rats and mice exposed in the same way did not develop tumours, according to the preliminary report from the US National Toxicology Program (NTP), a part of the National Institute of Environmental Health Sciences.

Although intriguing, the findings can not be extrapolated to humans, NTP scientists and the US Food and Drug Administration (FDA) said on Friday. They noted that the animal studies were meant to test extreme exposures to cell phone radiation, and that current safety limits on cellphone radiation are protective.

However, the two 10-year, $25 million studies do raise new questions about exposure to the ubiquitous devices. In the studies, about 6 per cent of male rats whose entire bodies were exposed to the highest level of cell phone radiation developed schwannomas - a rare type of tumour - in nerve tissue near their hearts. Reuters