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Toxicology studies more than 10 years in the making have found that high exposure to radiation from radio frequencies used by cell phones was linked to tumors in male rats.

The National Toxicology Program, which released the results of its $30 million studies on Thursday, found that radio frequency radiation (RFR) similar to that used in 2G and 3G cell phones is associated with cancerous heart tumors in male rats. The studies also found evidence that the radiation was linked to tumors in male rats' brains and adrenal glands. It's unclear whether tumors observed in female rats, as well as male and female mice, were linked to radiation exposure.

"We believe that the link between radio frequency radiation and tumors in male rats is real," John Bucher, a senior scientist at NTP, said in a statement. He added that external experts agreed with the finding.

The studies examined radiation similar to that used in 2G and 3G cell phones because those networks were standard at the time the studies were designed. Both 2G and 3G networks remain in use for calls and texts. The studies didn't look into the kinds of RFR used for WiFi or 5G networks.

"5G is an emerging technology that hasn't really been defined yet," lead toxicologist Michael Wyde said in the statement. "From what we currently understand, it likely differs dramatically from what we studied."

The studies could be an important step to understanding RFR's impact on humans. But the exposures in the studies can't be directly compared to what humans experience, Bucher said. The rats and mice in the study were exposed to radiation across their entire bodies, while humans are usually exposed to radiation near where they keep their phones.

The exposure levels and lengths were also greater in the studies. The lowest exposure level was the same as the maximum exposure allowed for cell phone users, a power level that "rarely occurs with typical cell phone use," according to the NTP statement. The highest exposure level used in the studies was four times greater than the maximum power level allowed.

In the studies, the rats were exposed to RFR starting from when they were in the womb. Mice were exposed beginning at 5 or 6 weeks old. The animals were exposed for up to two years, for about nine hours a day (with 10 minutes of exposure and 10-minute breaks in between).

One of the strengths of the studies was that scientists could control how much radiation the rats and mice were getting, which isn't possible when studying how humans use cell phones, Wyde said. Studies on human cell phone use often rely on questionnaires. The RFR levels in the studies ranged from 1.5-6 watts per kilogram in rats, and 2.5-10 watts per kilogram in mice.

The National Toxicology Program will share the results of the studies with the US Food and Drug Administration and the Federal Communications Commission, which will then review the information as part of its monitoring of research on the potential effects of RFR.