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major new study provides evidence of a possible link between cellphone exposure and cancer, at least in rats — findings that are likely to spark a fierce new debate about the 21st century’s most ubiquitous tech gadget.

When researchers exposed rats to the radiofrequency radiation emitted by cellphones, they saw higher incidence of two types of cancer: malignant gliomas in the brain and schwannomas in the heart. The increased risk was relatively small, but if the findings translate to humans — still an unknown — it could have a large public health impact, given the widespread use of cellphones worldwide.

The highly anticipated, $25 million study was conducted by the US National Toxicology Program. Partial results were released late Thursday, before full results were available, because of intense public interest, according to John Bucher, associate director of the NTP.

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“We feel that these findings are potentially of interest to the discussion over the cellphone safety issues,” Bucher said. “We felt it was important to get the word out.”

However, “the results from our studies are far from definitive at this point,” he said. For human health, “this may have relevance. This may have no relevance.”

The study had some head-scratching findings. For instance, it found that despite developing more tumors, male rats exposed to radiation for about nine hours every day also lived longer than a control group not exposed to radiation. In addition, it was unusual that no cancers occurred in the control group in this study. The incidence of malignant gliomas in male rats exposed to radiation — 2.2 to 3.3 percent — was within the range seen in nonexposed rats in previous studies, the authors said.

Still, the authors said that the brain and heart tumors observed in rats exposed to the radiofrequency radiation are similar to malignancies seen in some epidemiological studies of cellphone use. They say their findings “appear to support” the World Health Organization’s classification of cellphones as a possible carcinogen. (That’s the same classification given to coffee and talcum powder.)

Elisabeth Cardis, the lead investigator on the study that spurred that classification, said the NTP report provided “really quite novel findings.”

“A lot of people felt that the issue was closed; there’s nothing there and you can just move on. The release of these results now … really raises the question about whether, in fact, there is a real effect,” she said.

Population implications

But other population-level studies in humans have found no increased risk, and the incidence of brain cancer has not risen in recent decades as cellphone use has exploded.

“These data will probably raise the level of suspicion that there might be a problem. I didn’t see them as strong enough to say that there’s really a problem,” said Kenneth Foster, a bioengineering professor at the University of Pennsylvania who has studied radiation and health. “It’s really hard to know just how to interpret these results.”

He noted in particular the finding that male rats exposed to radiation lived longer than those not exposed to radiation.

Michael Lauer, deputy director of extramural research for the NIH, who provided comment to the NTP on the study, said that because of the small sample size, the study’s findings are suspect. “It is much more likely than not that this is not a true finding,” he said.

As recently as Wednesday, the NIH said the study was still under review by unnamed additional experts.

“It is important to note that previous human, observational data collected in earlier, large-scale population-based studies have found limited evidence of an increased risk for developing cancer from cellphone use,” the NIH said in a statement.

The agency did not immediately respond to a request for comment on the report’s release, which followed a Wednesday report by the website Microwave News that disclosed what the researchers had found.

The researchers have more data stockpiled that they haven’t reported. They say the rest of the results from the study will likely trickle out starting in late 2017.

Potential firestorm

The new study has the potential to start a firestorm. Until now, there have been conflicting results from other research about whether cellphones cause cancer, but the general takeaway from official authorities was that there is no definitive link — as the NIH statement reiterates.

Dr. Jonathan Samet, director of the University of Southern California’s Institute for Global Health, said one of the big takeaways is “we really need a more serious research agenda.”

“I think the public needs to know if there’s any risk,” he said. “I think this is something that needs to be taken as a need for further research and investigation. … We should not say there is no risk.”

The findings could jeopardize the conventional wisdom at a time when the number of Americans who own a cellphone has exceeded 90 percent in recent years.

“None of us expected them to find anything in this study. I’ve been quoted as saying it’s a total waste of money,” said David Carpenter, director of the Institute for Health and the Environment at the University of Albany.

The results have been long anticipated. An NIH official told Congress in 2009 that the results would likely be released in 2014, but their release appeared to be prompted only by this week’s leak.

“We’ve been waiting a long time for this study, far too long for this study,” said Joel Moskowitz, director and principal investigator at the Center for Family and Community Health at the University of California, Berkeley.

Still, he added: “The debate will keep going on, I’m sure. This is not going to be the definitive study.”

This story has been updated with new details about the study and previous research on cellphones.

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