(CNN) - Cell phone radiation and a potential link to cancer risks have left consumers and scientists alike scratching their heads since mobile phones became widely used in the 1990s.
Some studies have failed to show a link between radio frequency from cell phones and certain health problems, such as increased risks of tumors, while others suggest the opposite.
Now, two much-anticipated reports released Friday by the US Department of Health and Human Services' National Toxicology Program add to the cell phone conundrum.
The comprehensive research reports detail findings from two large animal studies -- one in rats and one in mice -- that link high levels of cell phone radiation to some evidence of carcinogenic activity in male rats, including a rare type of tumor called a schwannoma in their hearts. There were no such significant findings in the female rats.
Similarly, no significant findings emerged in the mouse study, according to the reports.
The studies were a part of the National Toxicology Program's 10-year, $25 million assessment of radiation exposure and potential health effects.
"One of the things that we found most interesting about our findings was that the malignant schwannomas -- even though they occurred in the heart and not in the head of these animals -- were in fact schwannomas," said John Bucher, a senior scientist at the National Toxicology Program and one of the authors of the reports.
"These experimental animal studies are but one approach to understanding whether exposures to radio-frequency radiation pose a risk to human health," he said, adding that studies are continuing at the National Toxicology Program to examine changes on the molecular level in tissue samples from the rodents.
He added, "I have not changed the way I use a cell phone."
Although tumors and biological changes were found among rodents in labs, it remains unclear whether similar findings would emerge in humans, and more research is needed, Bucher said.
The US Food and Drug Administration notes that cell phones emit low levels of radio-frequency energy that are non-ionizing and thus not considered strong enough to permanently damage biological tissue including DNA.
Cell phone safety confusion and controversy
The National Toxicology Program studies involved about 3,000 rodents in all, Bucher said.
The animals were exposed to radio-frequency radiation levels equal to and higher than the highest level currently allowed for mobile phone emissions. The researchers tracked the health of the animals from in utero to two years after their birth.
A 2-year-old rat would be somewhat comparable to a 70-year-old human, Bucher said.
The researchers divided the rodents into two groups based on radio-frequency radiation levels, low or high, and exposed their entire bodies to radio-frequency radiation for 10-minute increments totaling to about nine hours a day over the two-year period.
"It's important to consider the magnitude of the exposures to the animals in these studies in relation to what one might typically receive from using a cell phone," Bucher said. "The lowest energy level of the radio-frequency radiation we studied was similar to the highest level currently permitted for cell phone emissions."
Among the male rats, the researchers found tumors in about 6% of those in the highest radiation exposure group, Bucher said. That percentage "exceeded the mean historical incidence (0.8%), and exceed the highest rate observed in a single historical control group (2%) of completed peer reviewed studies," the researchers wrote.
The researchers also found that the male rats in the high-exposure group appeared to live longer than the other rats, but more research is needed to determine why and how that may be relevant to the study results.
Overall, the findings "don't go much further than what we have reported earlier," he said.
In 2016, the National Toxicology Program released preliminary data indicating that high levels of cell phone radiation increased brain tumor growth in male rats.
Yet "after reviewing all of the data from these studies, the evidence for increased malignant schwannomas in the hearts of male rats is the strongest cancer finding in our study," Bucher said.
"In our complete evaluation, we again had a lower level of certainty that small increases in the numbers of male rats with tumors in the brains were associated with exposures to cell phone radio-frequency radiation," he said. "These findings are termed 'equivocal evidence of carcinogenic activity,' meaning it was unclear if the tumors were related to the exposures."
In 1999, the FDA nominated cell phone radio-frequency emissions for toxicology and carcinogenity testing.
"It's important to understand that -- as is commonly done in these types of risk assessment studies -- the study was designed to test levels of radio-frequency energy exposures considerably above the current safety limits for cell phones to help contribute to what we already understand about the effects of radio-frequency energy on animal tissue," Dr. Jeffrey Shuren, director of the FDA's Center for Devices and Radiological Health, said in a statement after the release of the reports.
"The current safety limits are set to include a 50-fold safety margin from observed effects of radio-frequency energy exposure. From the FDA's understanding of the NTP results, male rats that showed carcinogenic activity were exposed to a radio-frequency energy exposure rate that is much higher than the current safety standard," he said.
"Looking at the results in animals, the conclusions still require careful discussion, as our preliminary understanding of the NTP results is that the study found mostly equivocal, or ambiguous, evidence that whole-body radio-frequency energy exposures given to rats or mice in the study actually caused cancer in these animals."
In March, the National Toxicology Program will hold an external expert review of the complete findings from the rodent studies and is accepting public comments.
'It's incumbent on all consumers to pay attention'
Jerry Phillips, a biochemist and director of the Excel Science Center at the University of Colorado Colorado Springs, devoted about a decade to examining cell phone radiation during his career and conducted some of the earliest studies on the potential health impacts of cell phones, he said.
Regarding the new National Toxicology Program reports, "this keeps adding to the body of evidence that says you really have to be a little careful when we start talking about human safety and exposure to radio-frequency fields," said Phillips, who was not involved in the reports.
The early research Phillips conducted, as a scientist previously with the US Department of Veterans Affairs' Pettis VA Medical Center in California, was funded by the telecommunications company Motorola, he said.
"They funded us to do some whole animal studies, similar to what NTP did but with a different premise, and they also funded us to do some other studies with cells, just isolated cells, and that included a study to look at DNA damage in response to exposure to two of their cell telephone signals," Phillips said. "The study was based on the presumption that this type of radiation, this low-energy non-ionizing radiation, was not itself going to be carcinogenic."
Those study results suggested that cell phone radiation possibly could be a co-carcinogen: "That is, if you have another cancer-producing agent, the radiation would help promote the formation of tumors," he said.
The new National Toxicology Program study suggests otherwise -- that it could be carcinogenic, he said.
As scientists continue to seek answers to the many questions that remain concerning cell phone radiation safety, Phillips said, he thinks all consumers should monitor the research findings to come.
"I think it's incumbent on all consumers to pay attention to any potential adverse effects of any of the products that we rely on until more research is done," he said.
CTIA, a trade association representing the US wireless communications industry, said in a statement in response to the latest National Toxicology Program reports that the safety of cell phone users is important to the industry.
"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 scientific community will consider the NTP draft reports in the context of the many other scientific studies conducted over several decades," the statement said.
"NTP issued partial results of its rat study in June 2016 and since then the Federal Communications Commission, the Food and Drug Administration, the World Health Organization, the American Cancer Society and numerous other international and US organizations and health experts have maintained their long-standing conclusion that the scientific evidence shows no known health risk due to the (radio-frequency) energy emitted by cell phones," the statement said.
Cell phone radiation research also has drawn criticism from animal advocacy groups.
"10 years, $25 million tax dollars and 3,000 animals is how much Uncle Sam has wasted for contrived cell phone radiation tests that are irrelevant to humans. We're working with Congress to improve the federal program responsible for replacing these kinds of cruel, expensive and inefficient animal tests with superior alternatives that actually get results for tax payers," Justin Goodman, vice president of the taxpayer watchdog group White Coat Waste Project, said in a written statement.
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