An important new study has linked cell phone radiation to cancers in the brain and heart.

The new research was conducted on rats by the U.S. National Toxicology Program, which exposed rats to radiofrequency radiation that comes from cell phones for about nine hours a day for seven days a week. They found that the exposed rats were more likely to develop cancers, specifically malignant gliomas—a tumor of glial cells in the brain—and tumors in the heart.

The study was reviewed by experts at the National Institutes of Health (NIH), and the authors say more research on the link will emerge in the next couple years. There are some important caveats to the new report. A study in rats is never directly translational to humans. It does, however, give researchers evidence that can lead to further research on the impact cell-phone radiation has on people. The findings were also most statistically significant for male rats.

Other research has seen a link between cell phones and cancer, though research overall remains limited. The World Health Organization’s International Agency for Research on Cancer (IARC) classified cell phone use and other radiofrequency electromagnetic fields as a possible carcinogen in 2011. “This study in mice and rats is under review by additional experts,” the NIH said in a statement about the findings. “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 cell phone use.”

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Other studies have produced conflicting results. One cohort study in Denmark looked at billing information from 358,000 cell phone users and then compared it to brain-tumor data from a national cancer registry. That study did not find a link between the two. Another recent study published in May looked at incidence of brain cancer in Australia from 1982 to 2013 and did not find an uptick in cancer cases with the introduction of cells phones. Still, other government-funded studies have made connections between cell phones’ electromagnetic fields and changes in brain activity. And a June 2014 study found that radiation from cell phones can lower men’s sperm mobility by 8% and sperm viability by 9%.

The NIH says part of the reason research so far has been inconsistent is that there are various factors that can influence the results of a study. For instance, brain cancers are notoriously difficult to study due to their high mortality rates, and studies are also subject to issues like inaccurate reporting. There are also changes over time in the type of cell phones available as well as how much people use them.

The researchers say this new report is unlikely to be the final word on the possible risks of cell phone radiation, and more data from their research is anticipated to be released in fall 2017.

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