High exposure to radiofrequency radiation of the type emitted by cell phones has been linked to tumours in tissues surrounding nerves in the hearts of male rats, but not female rats or any mice, according to a draft of US government studies released on Friday.

However, the levels and duration of exposure to the radiation in the studies were much greater than what people experience with even the highest level of cell phone use, and exposed the rodents' whole bodies, the draft report from the US National Toxicology Program (NTP) noted.

"So, these findings should not be directly extrapolated to human cell phone usage,” John Bucher, NTP senior scientist, said in a statement.

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