High exposure to the type of radiofrequency radiation emitted by mobile phones has been linked to tumours in the hearts of male rats, but not female rats or any mice, according to a draft of US government studies.
The study from the US National Toxicology Program (NTP) exposed rats to radiation ranging from 1.5 to 6 watts per kilogram in 10 minute increments, equalling about 9 hours a day of radiation.
The study's authors were quick to point out the levels of radiation the rats experienced was higher than the amount cellphones users are exposed to and focused on the rodent's entire body.
Tumours called malignant schwannomas were found in the hearts of male rats exposed to radiation beyond normal levels emitted by phones.
Both female and male rats were found to have increased damage to heart tissue or cardiomyopathy.
There was a statistically significant increase in the number of rats with tumours on other organs including the brain, liver and pancreas, although it was unclear if this was related to the radiation.
New born rats were found to have lower birth rates when their mothers were exposed to the radiation during pregnancy and lactation but they eventually reached normal size.
The NTP cautioned against connecting the study's findings with how a human body reacted to cellphone radiation, although did say their findings were similar to some studies of humans.
"These findings should not be directly extrapolated to human cellphone usage," John Bucher, NTP senior scientist, said in a statement on Friday.
"We note, however, that the tumours we saw in these studies are similar to tumours previously reported in some studies of frequent cellphone users."