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.
Male animals, but not females, had a higher rate of malignant schwannomas — arising in nerves in the heart — compared with animals not exposed to cell phone radiofrequency radiation. The increased risk occurred only in male animals exposed to the highest levels of radiofrequency radiation, which exceeded the exposure levels associated with typical cell phone usage.
“The typical cell phone call has radiofrequency radiation emissions that are very, very, very much lower than what we studied,” John Bucher, PhD, senior scientist at the National Toxicology Program (NTP), said during a media teleconference to summarize the study results. “We studied the maximum that one could achieve during a call in a poorer-connection situation. We studied it over 9 hours a day for over 2 years. This is a situation, obviously, that people are not going to be encountering in utilizing cell phones. It’s a situation that allows us to find a potential biological event if one is going to occur.
“I think the message is that typical cell phone use is not going to be directly related to the kind of exposure we used in these studies.”
The complete results, extracted from studies involving about 3,000 laboratory animals, led NTP scientists to conclude the increased rate of malignant schwannomas in male rats was caused by exposure to the radiofrequency radiation. Male rats exposed to the highest levels of radiofrequency radiation had about a 6% incidence of malignant schwannomas versus none in the control groups.
Bucher and colleagues suggested that the evidence for malignant schwannomas met the risk classification standard of “some evidence of carcinogenic activity.” That descriptor ranked just below the highest standard, “clear evidence of carcinogenic activity.” The schwannoma evidence is “the strongest cancer finding in our study,” said Bucher.
The study produced some evidence of an increased incidence of brain tumors in male rats, but the data left the NTP scientists with a “lower level of certainty” that exposure to cell phone radiofrequency radiation caused the tumors. Bucher said the findings related to brain tumors rose only to the level of “equivocal evidence of carcinogenic activity.”
When the NTP released an interim report in 2016, scientists said tumors observed up to that point were “likely related” to cell phone radiofrequency radiation exposures but they had more confidence in the association for the malignant schwannomas and the brain tumors. At the time, they noted the overall tumor incidence was low, even though increased in the exposed versus control groups.
The FDA has yet to finish its review of the NTP data, but a top official said the initial impression is that cell phone radiofrequency radiation does not pose a cancer threat to humans.
“Based on our ongoing evaluation of this issue and taking into account all available scientific evidence we have received, we have not found sufficient evidence that there are adverse health effects in humans caused by exposures at or under the current radiofrequency energy exposure limits,” Jeffrey Shuren, MD, JD, director of the FDA Center for Devices and Radiological Health, said in a statement.
“Even with frequent daily use by the vast majority of adults, we have not seen an increase in events like brain tumors. Based on this current information, we believe the current safety limits for cell phones are acceptable for protecting the public health.”
Bucher briefly reviewed the study design. Animals were assigned to control groups or to groups exposed to various levels of radiofrequency radiation. Radiation exposure began in utero and continued for 2 years. Bucher noted that a 2-year-old rat is the age equivalent of a 70-year-old human.
Animals were exposed to radiofrequency radiation associated with the two most common types of cell phone networks: global system mobile communications (GSM) and code division multiple access (CDMA).
Bucher said NTP scientists will continue to evaluate data from the study, including studies of the effects of cell phone radiofrequency radiation on different types of tissue and DNA. Separate reports of studies involving rats and mice are available on the NTP website.