Concerns over whether cellphones can cause cancer have been around for years. Now, the issue is being raised yet again, as government researchers release the results of a major study that found evidence linking high levels of cellphone radiation exposure to certain types of cancer in rodents.
But you probably don't need to be too worried about these results, for one important reason: You are not a male rat.
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.
What's more, the rodents were exposed to cellphone radiation — known as radio-frequency radiation — at greater levels, and for much longer periods, than what people experience, the researchers said.
"The exposures used in the studies cannot be compared directly to the exposure that humans experience when using a cellphone," John Bucher, a senior scientist with the U.S. National Toxicology Program (NTP) and co-author of the study, said in a statement. Bucher added that the mice were also exposed to radiation across their whole bodies, which is not what happens in people, who instead receive only local exposure to the specific area where they hold the phone. [10 Do's and Don'ts to Reduce Your Risk of Cancer]
Finally, the study examined the radio-frequency radiation used in 2G and 3G cellphones, which were standard at the time the study began but are no longer used routinely.
Still, the researchers said their findings question the long-held assumption that the radio-frequency radiation used by cellphones poses no health concern. They plan to conduct further studies to investigate the issue.
What did the new study find?
The study cost $30 million and took more than 10 years to complete. It's the most comprehensive analysis of the health effects in animals exposed to the radio-frequency radiation used in 2G and 3G cellphones, the researchers said.
For the study, the animals were housed in special chambers so the researchers could control how much radiation they received. The animals were exposed to a total of 9 hours of radiation per day, in 10-minutes sessions. The radiation began in the womb or early in life and lasted for up to two years, which is most of the animals' lifetime.
The lowest level of radiation was equivalent to the maximum level that cellphones are allowed to emit in the U.S. But the researchers noted that a typical cellphone user rarely ever reaches this level. And the highest radiation level used in the study was four times higher than the maximum level allowed in people.
The researchers found "clear evidence" of a link between the radio-frequency radiation at the highest levels and the development of heart tumors, called malignant schwannomas, in male rats.
The study also found some evidence that the high levels of radiation exposure were tied to the development of brain tumors in a small percentage of the male rats.
Unexpectedly, the study also found that, overall, the male rats exposed to the cellphone radiation lived longer than the rats who were not exposed to the radiation. This may be because the male rats exposed to radiation were less likely to develop chronic kidney problems, which are a common cause of death among older rats, the researchers said.
No conclusions for humans
"Animal studies like this one contribute to our discussions on this topic, but we must remember the study was not designed to test the safety of cellphone use in humans, so we cannot draw conclusions about the risks of cellphone use from it," Dr. Jeffrey Shuren, director of the Food and Drug Administration's Center for Devices and Radiological Health, who was not involved with the study, said in a statement.
The statement also noted that the study did not find evidence of a true "dose response," meaning there was no clear relationship between the doses of radiation the animals received and their rate of tumors.
Overall, "the totality of the available scientific evidence continues to not support adverse health effects in humans caused by exposures at or under the current radio-frequency energy exposure limits," Shuren said. "We believe the existing safety limits for cellphones remain acceptable for protecting the public health."
The NTP researchers are planning future studies on the effects of newer technologies, and these studies will use different methods so that they will be completed in weeks to months, rather than years. 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.
"If scientists can better understand biological changes in animals, they will know more about what to look for in humans," the NTP said in its fact sheet on the study.
Originally published on Live Science.
Live Science newsletter
Stay up to date on the latest science news by signing up for our Essentials newsletter.
Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.