However, until some more conclusive studies to prove or dismiss the risks, scientists recommend cell phone users not to exaggerate with mobile phones use.
The study saw the scientists place the rats and mice into special chambers and exposed to the radiation for up to two years in a way that mimics cell phone usage - 10 minutes on and 10 minutes off for a total of just over nine hours a day.
The rodents in the studies were exposed to radiation nine hours a day for two years, more than people experience even with a lot of cellphone use, so the results can not be applied directly to humans, says John Bucher, a senior scientist at the National Toxicology Program, during a telephone news briefing.
Six per cent of the male rats exposed to high doses of radiation developed tumours in their hearts.
In a statement to NBC News, the wireless industry said there's no evidence backing the study saying, "since the introduction of cell phones in the mid-1980s, the rate of brain tumors in the united states has remained stable". Whereas overexposure to ionizing radiation, which is emitted by an X-ray, can cause DNA damage and eventually, cancer.
The concern with this type of radiation is that it produces energy in the form of heat, and frequent exposure against the skin could alter brain cell activity, as some studies have suggested.
Yesterday, the National Toxicology Program at the US Department of Health and Human Services released two reports.
Bucher said the effect likely only showed up in the male rats because they were larger, and likely absorbed more radiation than the female rats or mice.
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In a counterintuitive result, male rats and mice exposed to radiation lived longer, and had lower levels of age-related kidney disease, than males not exposed to radiation.
Cellphone radiation quickly dissipates, so the risk, if any, would be to areas of the body in close proximity to the device emitting the radiation, Bucher said. It does not apply to 4G or 5G, which use different frequencies and modulation, he said.
The National Toxicology Program's studies are to undergo peer review at a March meeting.
"I think the reports don't go much further than what we reported earlier, and I have not changed the way I use a cellphone, no", said Bucher.
Some studies have found limited evidence of an increased risk of cancer from mobile phone use, according to the online notice from the National Institute of Environmental Health Sciences.
The NTP's Bucher, who helped lead the new studies, says he has no intention of changing his cell phone habits.
"Based on this current information, we believe the current safety limits for cell phones are acceptable for protecting the public health", director Jeffrey Shuren said in a statement.
Samsung and Apple did not immediately respond to requests for comment.