Male rats exposed to very high levels of the kind of radiation emitted by cellphones developed tumors in the tissues around their hearts, according to a draft report by United States government researchers on the potential health risks of the devices.
Female rats and mice exposed in the same way did not develop tumors, according to the preliminary report from the US National Toxicology Program (NTP), a part of the National Institute of Environmental Health Sciences.
The findings add to years of research meant to help settle the debate over whether cellphone radiation is harmful.
However, NTP scientists and the US Food and Drug Administration (FDA) were quick to say the findings could not be extrapolated to humans and that current safety limits on cellphone radiation are protective.
The 10-year, $25-million studies - to date the most comprehensive assessments of health effects of exposure to radiofrequency radiation on rats and mice- do raise new questions about exposure to the ubiquitous devices.
John Bucher, a senior scientist with NTP, said the tumors seen in the studies are "similar to tumors previously reported in some studies of frequent cellphone users."
Dr. Otis Brawley, chief medical officer of the American Cancer Society, noted that the studies were negative for common tumors.
“These draft reports are bound to create a lot of concern, but in fact they won’t change what I tell people: the evidence for an association between cellphones and cancer is weak, and so far, we have not seen a higher cancer risk in people," he said in a statement on Twitter.
Brawley said if cellphone users are concerned about this data in animals, they should wear an earpiece.
Unlike ionizing radiation such as that from gamma rays, radon and X-rays, which can break chemical bonds in the body and are known to cause cancer, radiofrequency devices such as cellphones and microwaves emit radiofrequency energy, a form of non-ionizing radiation.
The concern with this type of radiation is that it produces energy in the form of heat, and frequent exposure against the skin could alter brain cell activity, as some studies have suggested.
In the NTP study, rats and mice were exposed to higher levels of radiation for longer periods of time than what people experience with even the highest level of cellphone use, and their entire bodies were exposed all at once, according to the draft report.
Cellphones typically emit lower levels of radiation than maximum levels allowed, the draft report said.
NTP, a part of the National Institutes of Health, will hold an external expert review of its complete findings from these rodent studies from March 26 to 28.
Dr. Jeffrey Shuren, head of the FDA's radiological health division, said there is not enough evidence to say cellphone use poses health risks to people.
"Even with frequent daily use by the vast majority of adults, we have not seen an increase in events like brain tumors," he said in a statement. "We believe the current safety limits for cellphones are acceptable for protecting the public health."
Nevertheless, the findings are potentially a concern for device makers, especially the world's three biggest smartphone sellers, Apple Inc, Korea's Samsung Electronics Co Ltd and China's Huawei Technologies.
The CTIA, the trade association representing AT&T Inc, Verizon Communications Inc, Apple Inc, Sprint Corp, DISH Network Corp, and others, said on Friday that previous studies have shown cellphone RF energy emissions have no known heath risks.
"We understand that the NTP draft reports for its mice and rat studies will be put out for comment and peer review so that their significance can be assessed,” the group said.
Samsung and Apple did not immediately respond to requests for comment.