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. In February 2018, in a draft report, it backed away from that relatively firm conclusion.
According to Dr. Jeffrey Shuren, the U.S. FDA's director for Devices and Radiological Health, such animal studies are important in the discussion, but it was not created to test the safety of cellphones in humans and could therefore not be used to draw conclusions for humans.
But the FDA has already taken issue with the findings.
The National Toxicology Program is an inter-agency program headquartered at the National Institutes of Health.
Results can't be compared directly to exposure humans experience when using a mobile phone, in part because exposure levels were greater for the studied rodents, John Bucher, senior scientist with the National Toxicology Program, said in a news release.
According to Bacher and his colleagues, animals that were irradiated by radio waves with a frequency of 900 and 1900 megahertz "clearly showed that these forms of radiation, encoded in the "mobile format", have carcinogenic activity". By contrast, people are mostly exposed in specific local tissues close to where they hold the phone.
The study used extremely high levels throughout the entire body and inexplicably found that exposed rats lived longer than those who didn't experience any radiation, Shuren said. The study originated during the Clinton administration, cost $30 million and involved some 3,000 rodents.
Since mobile phones became commonplace in the 1990s, some campaigners have claimed that the radio frequency radiation (RFR) emitted by the devices using 2G or 3G connectivity could be directly linked to cancer growth in humans. There were no such significant findings in the female rats.
He said the overall findings of the study - 384 pages devoted to rats, 260 to mice - had been conveyed to the Federal Communications Commission and the FDA.
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The final report doesn't change much that the researchers said in a preliminary report released in February.
These studies did not investigate the types of RFR used for Wi-Fi or 5G networks.
'The US IEEE-FCC human safety guidelines for wireless radiation which only protect against thermal heating or burning were scientifically false decades ago, ' said Dr Paul Heroux, a McGill University physicist and toxicologist.
For the study, male rats were exposed to high levels of radio frequency radiation like that used in 2G and 3G cell service - technology that hasn't been completely replaced as the industry moves to 4G and beyond. For the rats, the exposures started before birth and continued until they were about 2 years old.
In the studies, the rats were exposed to RFR starting from when they were in the womb. The highest level was four times higher than the permitted maximum. The final verdict: cellphone radiation may sometimes cause tumors in rats at high, continuous doses, but not in people.
The FDA was invited to the peer-review process for the NTP study, but didn't attend. Studies on human cell phone use often rely on questionnaires.
"We believe the existing safety limits for cell phones remain acceptable for protecting the public health".
"The safety of consumers is the wireless industry's first priority", CTIA, the trade association representing the USA wireless communications industry, said in a statement Thursday.