WASHINGTON, Feb. 2 (Xinhua) -- Preliminary studies made by American scientists showed that high exposure to cell phone radiation resulted in tumors in tissues surrounding nerves in the hearts of male rats, though not in female rats or any mice.

The findings, reported Friday by the National Toxicology Program (NTP) under the U.S. Department of Health and Human Services, showed that the incidence of tumors, called malignant schwannomas, increased in male rats as they were exposed to increasing levels of radiofrequency radiation (RFR) beyond the allowable cell phone emissions.

The exposure levels used in the studies were equal to and higher than the highest level permitted for local tissue exposure in cell phone emissions.

Cell phones typically emit lower levels of RFR than the maximum level allowed.

Researchers exposed a total of 3,000 rats and mice in two studies to different levels of RFR for up to two years. Exposure levels ranged from low power to high power.

The low power level for rats was equal to the highest level permitted for local tissue exposures to cell phone emissions.

The animals were exposed for 10-minute on, 10-minute off increments, totaling over nine hours each day.

"The levels and duration of exposure to RFR were much greater than what people experience with even the highest level of cell phone use, and exposed the rodents' whole bodies. So, these findings should not be directly extrapolated to human cell phone usage," said John Bucher, NTP's senior scientist.

However, the tumors researchers saw in these studies are similar to tumors previously reported in some studies of frequent cell phone users, Bucher said.

Researchers also noted increases in an unusual pattern of cardiomyopathy, or damage to heart tissue, in exposed male and female rats. But there was little indication of health problems in mice related to RFR.

"These studies were complex and technically challenging, but they provide the most comprehensive assessment, to date, of health effects in rats and mice from exposure to RFR," said Bucher.

The studies used 2G and 3G frequencies and modulations, which are different from those used in the 4G and 5G networks.

"Cell phone technologies are constantly changing, and these findings provide valuable information to help guide future studies of cell phone safety."