It may seem easy to imagine that a cellphone — pressed up against your ear, bathing your brain in radiation — could be bad for you. But what does the science say? Part of a new report from a U.S. National Toxicology Program (NTP) study on the potential association between cellphone use and cancer has renewed attention to this uncertain relationship. In the study, released last week, researchers at the NTP, part of the National Institutes of Health, found that long-term exposure to high levels of this type of radiation might be linked with a small increase in the risk of brain cancer in male rats. But experts not involved in the research said these findings are not definitive. "The implications of this for the safety of mobile phone use is between questionable and nonexistent," John Moulder, a professor of radiation oncology at the Medical College of Wisconsin who was not involved in the research, said in an email interview with Live Science. According to the Pew Research Center, 90 percent of American adults owned a cellphone as of January 2014. The mass proliferation of this technology has led to concerns about the potential consequences of cellphone use, but much of the technology's health impact has remained uncertain. [9 Odd Ways Your Tech Devices May Injure You] What is known is that cellphones work by sending and receiving electromagnetic waves. The type of electromagnetic energy they use is similar to that of FM radio waves and microwaves. Along with visible light and heat, cellphone radiation is a kind of nonionizing radiation. Nonionizing radiation waves have a low frequency and low energy. Many studies have looked at nonionizing radiation, and there is no clear evidence that it can directly damage the DNA in human cells or cause cancer, according to the American Cancer Society. (In contrast, ionizing radiation, such as ultraviolet rays and X-rays, are well known to cause damage to cells.) Science has yet to find a definitive answer to the question of what impact, if any, cellphone radiation could have on people's bodies. At least three large studies — known as the Interphone study, the Danish study and the Million Women study — that included tens to hundreds of thousands of people looked at this relationship. These studies relied on surveys, mobile subscription records and health records to gather information about people's phone use and health status. None showed clear evidence of an association between cancer and cellphones, according to the findings. But there are problems in trying to study this topic. For example, people aren't always good at recalling how much time they spent using their cellphones, it's hard to directly link a behavior with the development of a disease and cellphone technology is changing constantly. The new study from the NTP had some advantages here because the researchers were able to control and monitor the exposure of the rats to the nonionizing radiation, and also measure their cancer growth. In the study, the researchers exposed groups of 90 rats and mice of each sex to three different doses of radiation for up to two years. In total, there were 12 groups of 90 animals. There were also two control groups, each of which contained 90 animals, that were not exposed to any nonionizing radiation. The researchers exposed the experimental groups to the same kind of radiation cellphones give off for approximately 9 hours each day for 106 weeks, starting while the rats were still developing in utero. The rat groups were exposed to specific absorption rate (SAR) levels of 1.5, 3 or 6 watts per kilogram (W/kg). SAR is a biological measure of exposure that shows how much radio-frequency energy is absorbed into a specific amount of tissue. According to the American Cancer Society, 1.6 W/kg is the upper limit of SAR allowed in the United States. [10 Things You Didn't Know About the Brain] The results showed that there was an increased incidence of tumors with increased exposure. Out of the six groups of male rats exposed to radiation, four groups had higher rates of malignant brain gliomas (a type of brain or spinal tumor) than the males that were not exposed. These groups had to two to three tumors per group. Every group that was exposed to the radiation had at least one case of a heart schwannoma, a benign tumor. In each group, there were between one and six schwannomas found.

The control group had none. However, these are only partial results, and they seem to offer more questions than answers, Moulder said. "[I]t is implied (but not clearly stated) that results for other tumor types were negative," he said. "If all other tumor sites showed no effect, then the statistical and biological significance of the glioma effect is much diminished." More generally, Moulder said the report didn’t clarify whether there was a statistically significant difference in the groups' rates of gliomas. These rats, regardless of exposure to radiation, would be expected to grow one to two tumors per group of 90, Moulder said.

This would make the glioma count in the exposed rats similar to those that were expected even in unexposed rats. He added that it’s possible that the unexposed rats didn’t grow tumors because they died earlier than the exposed rats, and the tumor growth may be related, in part, to aging. [6 Foods That Are Good For Your Brain] The report didn’t explain why the unexposed rats died earlier. It's also important to note that these effects were not seen in the female rats, and there was no information in the report about the effects on the groups of mice.

And it's not clear whether the results would translate to humans. Moulder added that the higher levels of radiation exposure were high enough to create heat stress in the animals, and that some studies have linked this to possible tumor development. (These levels are higher than what people would experience during cellphone use.)

Eight expert reviews were included in the appendix of the NTP report and brought up concerns similar to Moulder's. Moulder said that many of the issues with the NTP report are likely the result of it being a preliminary version that has yet to take into account the peer review comments. The researchers who conducted the study said they would address this feedback further in supplemental materials in the future. The researchers estimated that the complete report containing all of the related studies will be available by the end of 2017. - See more at: http://www.livescience.com/54934-cellphones-radiation-cancer-questions.html#sthash.0QjUlebt.dpuf