Two major reports issued Friday by federal scientists found that there was some evidence the type of radiation released from cellphones can cause certain forms of cancer in male rats, but the reports — which include many inconclusive findings — are not likely to quell the debate over whether cellphone radiation poses a threat to human health.

Researchers at the National Toxicology Program found that there was “some evidence of carcinogenic activity” from cellphone radiation in male rats. About 6 percent of the rats exposed to the highest level of radiation studied developed schwannomas in their hearts, whereas there was no evidence of schwannomas in a group of rats that were not exposed to radiation, according to John Bucher, a senior scientist at the NTP.

The schwannoma example was the most compelling evidence in the studies that the radiation might be a carcinogen, Bucher said.


Scientists also measured rates of tumors in female rats. There was only “equivocal evidence of carcinogenic activity” — some rats developed gliomas in the brain and a type of tumor in the adrenal gland but, statistically speaking, it was unclear if the tumors were associated with the radiation exposure.

Likewise, in both female and male mice, the researchers only found “equivocal evidence of carcinogenic activity” from radiofrequency radiation.

The new reports were based on data from two years of study pursued in a quest to determine whether cellphones — our main source of exposure to radiofrequency radiation — are emitting waves that increase our risk of cancer. Many studies have found no connection between the type of radiation and cancer, but the International Agency for Research on Cancer designates radiofrequency radiation as a possible carcinogen.

A rat that is 2 years old is roughly equivalent to a 70-year-old person.

As part of the research, the rodents were divided into different study arms that were exposed to different levels of radiation. The lowest level of radiation they were exposed to was about the maximum level cellphones are permitted to emit, and the rodents were exposed to nine hours of radiofrequency radiation every day for two years, which Bucher made clear was much more than a regular cellphone user is going to be exposed to.

“It’s a situation that allows us to express a potential biological event if one is going to occur,” Bucher said. “The message is that typical cellphone use is not going to be directly related to the kind of exposures we use in these studies.”

If radiofrequency radiation is indeed a carcinogen, Bucher said, it is likely a weak one.

While the researchers found that certain types of cancer were diagnosed at statistically significant higher rates in animals exposed to radiation, there was not a significant difference in the incidence of other types of cancer. And just because researchers discover an association in rodents does not mean the results would translate to humans.

Some of the tumors the scientists identified in the rodents are similar to those found in humans as part of epidemiological studies involving people and cellphone use.

The new reports are still considered drafts, and the NTP plans to have outside experts review them next month.

The NTP released some preliminary rat data in May 2016, and it found evidence then that radiofrequency radiation raised the risk of malignant gliomas in the brain as well as heart schwannomas in male rats. In the updated study, there was not a statistical significance in the incidence of gliomas in exposed rats compared to control rats.

Curiously, the study found that male rats in the radiation arms of the study had higher survival rates than the rats in the control arm. Whereas only 28 percent of unexposed male rats survived to the end of the study, 48 percent to 68 percent of the rats in the different exposure arms survived the full two years. The researchers said the control rats experienced higher rates of kidney damage than the rats exposed to radiation, but that could just be because of chance, not as a result of the differences in radiation levels.

Bucher said he has not changed how he uses his cellphone since the 10-year, $25-million study began.

NTP, which is part of the National Institutes of Health, started studying radiofrequency radiation from cellphones after a request from the Food and Drug Administration almost 20 years ago. (The Federal Communications Commission establishes how much radiofrequency radiation cellphones are allowed to emit, but relies on health agencies for recommendations.)

In a statement, Dr. Jeffrey Shuren, the director of the FDA’s Center for Devices and Radiological Health, said the new NTP reports as well as the hundreds of other studies have “given us the confidence that the current safety limits for cell phone radiation limits remain acceptable for protecting the public health.”

Dr. Paolo Boffetta, who studies cancer epidemiology at the Icahn School of Medicine at Mount Sinai and who was not involved with the NTP reports, said that because the studies suggested radiofrequency radiation could have a small carcinogenic effect, researchers should keep studying the subject to determine the level of risk.

But, he added, “one should not just extrapolate these results to think there is a risk to humans.”

Kenneth Foster, a bioengineering professor at the University of Pennsylvania who was also not involved with the studies, noted that the reports did not generally show a clear connection between the dose of radiation and the number of animals that developed tumors, which “argues against a direct biological effect of exposure.” He said he agreed with the NTP officials that if anything, radiofrequency radiation might be a weak carcinogen, but that these results don’t offer certainty.

“Very iffy,” he wrote in an email.

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