A new study, which finds an increased risk of certain cancers in animals exposed to cell phone radiation, could reignite concerns over the safety of wireless communication.

Some scientists expressed serious concerns about the study’s findings, however, noting that flaws in the way the research was conducted could have produced false results.

The study was performed by the National Toxicology Program, a federal agency located within the National Institutes of Health. Researchers found small increases in rare cancers in the brain and heart in male rats exposed to radiation from cell phones, compared to rats not exposed to this type of radiation. There was no increase in cancer among female rats, according to the study, released Friday.

Those findings echo conclusions from the International Agency for Research on Cancer, part of the World Health Organization, which in 2011 classified mobile phone use “as a possible carcinogen.”

The rats in the new study were exposed to “whole body” cell phone radiation for nine hours a day for two years. Between 2.2% to 3.3% of male rats exposed to cell phone radiation developed malignant gliomas, a type of brain tumor. Between 1.1% and 6.6% of male rats developed a type of tumor called a schwannoma in the heart.

None of the rats in the control group, who weren’t exposed to radiation, developed either type of tumor.

The report represents a “paradigm shift in our understanding of radiation and cancer risk,” said Otis Brawley, chief medical officer at the American Cancer Society, who wasn’t involved in the new study, in a statement. “For years, the understanding of the potential risk of radiation from cell phones has been hampered by a lack of good science. This report from the National Toxicology Program (NTP) is good science.”

The concern over cell phones and cancer stems from the fact that they emit low-frequency radiation. Because people hold their phones close to their ears, some have worried that this radiation could fuel brain tumors.

Yet the radiation given off by cell phones is fundamentally different than the ionizing radiation known to cause cancer. Unlike the radiation given off by atomic bombs, the non-ionizing radiation emitted by phones is too weak to damage the DNA inside cells. That fact has led many scientists to dismiss the notion that cell phones could cause cancer.

“The findings are unexpected; we wouldn’t reasonably expect non-ionizing radiation to cause these tumors,” Brawley said. “This is a striking example of why serious study is so important in evaluating cancer risk.”

The CTIA, which represents the cell phone industry, said the study’s findings conflict with other medical evidence. In a statement, the group noted that there’s been no increase in brain cancer since the 1980s, when cell phones first came into use.

Other scientists also rejected the study’s conclusions.

Michael Lauer, a researcher with the National Institutes of Health’s office of extramural research, said the study doesn’t prove that cell phones cause cancer. The study may have been too small to produce reliable results, said Lauer, in written comments published with the new study.

The researchers themselves acknowledge their study’s limitations.

Rats in the control group were unusually short-lived, for example, and had cancer rates far below what’s usually seen in lab animals, authors noted in the study. Those facts could skew the results in two ways. First, the unusually low brain tumor rates among unexposed rates could create the illusion of a difference in cancer risk that doesn’t actually exist. Second, the unexposed rats may not have lived long enough to develop brain tumors, authors noted.

Brawley said it’s not clear what the study reveals, if anything, about cell phone use by people. People who hold cells phones near their ears probably get the greatest radiation exposure on one side of their skull. The animals in this study were exposed to cell phone radiation across their entire body.

Brawley noted that rats in the study were exposed to very high signal strengths that were “near but below levels that would cause animal tissue to heat up.”

“Additional research will be needed to translate effects at these high doses to what might be expected at the much lower doses received by typical or even high-end cell phone users,” Brawley said. “Cell phone technology continues to evolve, and with each new generation, transmission strengths have declined and with it radio frequency exposures.”