New York, Nov 2 (IANS): Exposure to high levels of radio frequency radiation (RFR) -- used in 2G and 3G cell phones -- can increase the risk of cancerous tumours in the heart, brain and adrenal gland, researchers have warned.

The study, led by the US National Institutes of Health's National Toxicology Programme (NTP), looked at the effects of exposing rodents to extremely high levels of radiofrequency throughout the entire body.

While high levels of RFR caused cancerous tumours in the heart (found very rarely in humans), brain and adrenal gland, of male rats, the findings on female rats were ambiguous.

"The exposures used in the studies cannot be compared directly to the exposure that humans experience when using a cell phone. In our studies, rats and mice received radio frequency radiation across their whole bodies," John Bucher, researcher from the NTP, said in a statement.

"By contrast, people are mostly exposed in specific local tissues close to where they hold the phone," Bucher added.

For the study, the team housed the animals in chambers specifically designed for the study.

Exposure to RFR began in the womb for rats and at 5 to 6 weeks old for mice, and continued for up to two years, or most of their natural lifetime.

However, the RFR exposure was intermittent -- 10 minutes on and 10 minutes off -- totalling about nine hours each day.

In addition, the RFR levels ranged from 1.5-6 watts per kilogram in rats, and 2.5-10 watts per kilogram in mice.

"We believe that the link between radio frequency radiation and tumours in male rats is real," Bucher noted.

Interestingly, the team found that rats exposed to whole body RFR lived longer than rats unexposed to any radiation.

"This may be explained by an observed decrease in chronic kidney problems that are often the cause of death in older rats," the researchers noted.

According to the US Food and Drug Administration (FDA), while animal studies contribute to discussions on the topic, "this study was not designed to test the safety of cell phone use in humans, so we cannot draw conclusions about the risks of cell phone use from it."

Since the exposure levels and durations in the studies were greater than what people experience, "we agree that these findings should not be applied to human cell phone usage", the FDA said on Thursday.