Since mobile phones became commonplace in the 1990s, some campaigners have claimed that the radio frequency radiation (RFR) emitted by the devices using 2G or 3G connectivity could be directly linked to cancer growth in humans.
While some experts, including those at the Food and Drug Administration, debate the level of concern between "clear evidence" and using terms like "some evidence" of adverse risk, Theodora Scarato - executive director of the Environmental Health Trust - says even a small cancer risk could have wide implications on world health.
"Not carrying the phone in your trousers, or your bra or your shirt pocket, and not allowing children to use cellphones, because they're most vulnerable to this radiation; it penetrates deeper into their bodies", says Scarato.
The levels the rodents experienced were far higher than people are typically exposed to.
"We note NTP's own assessment that today's report can not be extrapolated to human cell phone usage, and the Food and Drug Administration's concurrence", a spokesman for the trade group said.
"We believe that the link between radio frequency radiation and tumors in male rats is real, and the external experts agreed", said John Bucher, Ph.D., NTP senior scientist.
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. On long calls, he said, he tried to use earbuds or find other ways "of increasing the distance" between the cellphone and his body, in keeping with advice issued to consumers about how to lower their exposure.
The toxicology program released a preliminary draft of the study findings in May 2016, saying the radiation had "likely caused" the brain tumors.
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In should also be noted that the types of RFR used for Wi-Fi or 5G networks were not included in this investigation.
Plus, FDA pointed out confusing findings from the rodent study - such as that the radiated rats lived longer than comparison rats that weren't exposed to the rays.
The animals were housed in chambers specifically designed and built for these studies. The RFR exposure was intermittent, 10 minutes on and 10 minutes off, totaling about nine hours each day. RFR levels ranged from 1.5 to six watts per kg in rats, and 2.5 to 10 watts per kg in mice.
The NTP researchers are planning future studies on the effects of newer technologies, and these studies will use different methods so that they will be completed in weeks to months, rather than years.
These studies will focus on developing measurable physical indicators of potential effects from RFR, including changes in metrics like DNA damage in exposed tissues.
In June, at a meeting of scientific counselors to the toxicology agency, Donald Stump, one of the members, anxious that the study "will be vulnerable to criticism that it was conducted using outdated technology".
"Based on our results, we are planning further studies to confirm that the experimental evidence continues to support this".
'The US IEEE-FCC human safety guidelines for wireless radiation which only protect against thermal heating or burning were scientifically false decades ago, ' said Dr Paul Heroux, a McGill University physicist and toxicologist. For more information about NTP and its programs, visit https://ntp.niehs.nih.gov.