A major U.S. government study on rats has found a link between cellphones and cancer, an explosive finding in the long-running debate about whether mobile phones cause health effects.

The multiyear, peer-reviewed study, by the National Toxicology Program, found “low incidences” of two types of tumors in male rats that were exposed to the type of radio frequencies that are commonly emitted by cellphones. The tumors were gliomas, which are in the glial cells of the brain, and schwannomas of the heart.

“Given the widespread global usage of mobile communications among users of all ages, even a very small increase in the incidence of disease resulting from exposure to [radio-frequency radiation] could have broad implications for public health,” according to a report of partial findings from the study, which was released late Thursday.

A spokesperson for the National Institutes of Health, which helped oversee the study, wasn’t immediately available for comment. Earlier in the week, the NIH said, “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 cellphone use.”

While not all biological effects observed in animals necessarily apply to humans, the National Toxicology Program’s $25 million study is one of the biggest and most comprehensive experiment into health effects from cellphones.

“Where people were saying there’s no risk, I think this ends that kind of statement,” said Ron Melnick, who ran the NTP project until retiring in 2009 and recently reviewed the study’s results.

Since mobile phones were launched commercially in the 1980s, the only widely agreed upon physical impact from cellphone radio-frequency energy is that it can heat human tissue at high enough levels. Cellphones are designed well below this thermal level.

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