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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