Scientists have found "some evidence" of a link between the kind of radiation that comes from cell phones and cancer, but there are two big exceptions.
The first is that it was only found in rats and the second was they were exposed to much higher levels of radiation than humans get from their cell phones.
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The two decade study of 3000 rodents found that 2 to 3 percent of them developed brain cancer, but only the males.
It also found that 5 to 7 percent of the male rats developed malignant schwannomas, a kind of tumour, in the heart.
The tumours are similar to acoustic neuromas, a benign kind of tumour that can occur in people's brains.
"We believe that the link between radio-frequency radiation and tumours in male rats is real," John Bucher, a senior scientist at the National Toxicology Program, said in a statement.
Dr Bucher said the results cannot be compared to the human experience due to the differing levels of radiation than what people usually encounter.
Rats were exposed to radiation nine hours a day for two years, far longer than heavy users of cell phones.
The study looked only at 2G and 3G technology, not 4G or 5G which is less able to penetrate the body.