The study also found some evidence that the high levels of radiation exposure were tied to the development of brain tumors in a small percentage of the male rats.
Moreover, the rat study examined the effects of a radio frequency associated with an early generation of mobile technology, one that fell out of routine use years ago.
What's more, the rodents were exposed to cellphone radiation - known as radio-frequency radiation - at greater levels, and for much longer periods, than what people experience, the researchers said. But he cautioned that the exposure levels and durations were far more significant than what people typically encounter, and thus can not "be compared directly to the exposure that humans experience".
In the NTP studies, the report notes, exposure was way more than what happens through average duration of cellphone usage.
Dr Bucher added: "We believe that the link between radio frequency radiation and tumours in male rats is real, and the external experts agreed".
Also, the highest exposure level in the studies was four times higher than the maximum power level permitted, the reports said.More news: Mario Segale, Inspiration for Titular Super Mario Bros. Character, Passes Away
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The U.S. Food and Drug Administration nominated cell phone RFR for study by NTP because of widespread public use of cell phones and limited knowledge about potential health effects from long-term exposure.
"We studied the effects of almost lifetime exposures to two different types, or modulations, of RFR (GSM and CDMA) used in cellular telephone networks in the United States in male and female rats and mice to identify potential toxicity or cancer-related hazards".
In should also be noted that the types of RFR used for Wi-Fi or 5G networks were not included in this investigation.
"A major strength of our studies is that we were able to control exactly how much radio frequency radiation the animals received - something that's not possible when studying human cell phone use, which has often relied on questionnaires", said Michael Wyde, Ph.D., lead toxicologist on the studies.
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. The RFR exposure was intermittent, 10 minutes on and 10 minutes off, totaling about nine hours each day. In addition, the studies will try to identify biomarkers that may indicate early effects of radio-frequency radiation exposure in rodents, such as changes in heart rate or molecular changes that might be predictive of cancer.
"Animal studies like this one contribute to our discussions on this topic, but we must remember the study was not created to test the safety of cellphone use in humans, so we can not draw conclusions about the risks of cellphone use from it", Dr. Jeffrey Shuren, director of the Food and Drug Administration's Center for Devices and Radiological Health, who was not involved with the study, said in a statement.
The Associated Press Health & Science Department receivessupport from the Howard Hughes Medical Institute's Department of Science Education.