Among the female mice, however, the evidence was unclear as to whether the cancers observed in them are associated with RFR. The final reports represent the consensus of NTP and a panel of external scientific experts who reviewed the studies in March after draft reports were issued in February.
Indeed, the only clear link between cellphone radiation and cancer was found among male rats (not female rats or male or female mice), and the researchers stressed that the findings do not apply to humans.
Cell phones emit radio frequency radiation (RFR) that scientists have long anxious could cause cancer. By contrast, people are mostly exposed in specific local tissues close to where they hold the phone.
However, scientists at the agency cautioned that the exposure levels and durations were far greater than what people typically encounter, and the experiment used what is now outdated cell technology. The report also says that the radiation levels were much higher than what people are exposed to when using their cellphones. The lowest exposure level was the same as the maximum exposure allowed for cell phone users, a power level that "rarely occurs with typical cell phone use", according to the NTP statement. The highest exposure level in the studies was four times higher than the maximum power level permitted.
The study pointed out that its focus on 2G and 3G was still relevant as the more modern 4G technology has supplanted them, explaining, "While newer technologies have continued to evolve, it is important to note that these technologies have not completely replaced the older technologies". The studies didn't look into the kinds of RFR used for WiFi or 5G networks.
One of the strengths of the studies was that scientists could control how much radiation the rats and mice were getting, which isn't possible when studying how humans use cell phones, Wyde said.
Interestingly, the researchers found that the male mice that were exposed to cellphone radiation actually lived longer than those that were not exposed.
The animals were housed in chambers specifically designed and built for these studies. The animals were exposed for up to two years, for about nine hours a day (with 10 minutes of exposure and 10-minute breaks in between). RFR levels ranged from 1.5 to 6 watts per kilogram in rats, and 2.5 to 10 watts per kilogram in mice.
The studies did not investigate exposure to the radio frequency radiation used for Wi-Fi or 5G networks.
"We do believe that the tumor responses that we have seen in our studies is real and they are associated with radiofrequency radiation", Bucher said. In addition, the studies will try to identify biomarkers that may indicate early effects of radio-frequency radiation exposure in rodents, such as changes in heart rate or molecular changes that might be predictive of cancer.
While some experts, including those at the Food and Drug Administration, debate the level of concern between "clear evidence" and using terms like "some evidence" of adverse risk, Theodora Scarato - executive director of the Environmental Health Trust - says even a small cancer risk could have wide implications on world health.
The lowest exposure level used for the study was equal to the maximum local tissue exposure now allowed for cellphone users, the report states.
In June, at a meeting of scientific counselors to the toxicology agency, Donald Stump, one of the members, anxious that the study "will be vulnerable to criticism that it was conducted using outdated technology". For more information about NTP and its programs, visit https://ntp.niehs.nih.gov.