The question of whether cellphone radiation is linked to tumours remains inconclusive after the partial release of a $25-million U.S study on rats.

Researchers at the National Toxicology Program bombarded rats with cellphone radiation at extremely high power ranges and studied aggressive glioma brain cancer and rare, benign schwannoma growths of the heart.

In the preliminary results, "low incidences" of tumours were found in 2 to 3 per cent of male rats. Female rats weren't affected.

Confusingly, after two years of testing, the control group of male rats who weren't exposed to any radio frequency radiation lived shorter lives than those exposed.

Interpreting findings involving animals is challenging, and the results can't be directly applied to humans. That's part of the difficulty of determining whether something causes cancer.

Some of the study's own reviewers had trouble accepting the results because of the quirks. 

"I am unable to accept the authors' conclusions," wrote outside reviewer Dr. Michael Lauer, deputy director of the National Institutes of Health's office of extramural research. "I suspect that this experiment is substantially underpowered and that the few positive results found reflect false positive findings."

The fact that the rats exposed to radiation survived longer than those that weren't "leaves me even more skeptical of the authors' claims," Lauer wrote. Other study reviewers also raised questions about the way the study was conducted and its conclusions.

For humans, the World Health Organization's fact sheet says: "No adverse health effects have been established as being caused by mobile phone use."

The WHO's International Agency for Cancer Research lists cellphone use and other radiofreqency electromagnetic fields as "possible" carcinogens, a category that also includes more than 260 other substances such as coffee, engine exhaust and talc-based body powder.

Distraction danger

"If cellphones cause cancer, they don't cause a lot of cancer," said Dr. Otis Brawley, the American Cancer Society's chief medical officer. 

Brawley said people should be far more concerned about "distraction caused by cellphone," which he said causes more deaths.

The toxicity question of cellphones is just half of the equation. Exposure is also key.

"I'm not particularly nervous about using my cellphone. On the other hand, from a public health point of view, this is an important issue," said Paul Demers, a senior scientist in prevention, screening and cancer control at Cancer Care Ontario.

"Given how much everybody is using cellphones, there's been a number of studies that have been carefully looking at brain cancer rates, and they simply have not been going up in any significant way." 

Demers also said cellphone technology has improved to emit orders of magnitude less radiation than they did decades ago. In part, it's because consumers want lighter phones with smaller batteries, he added. 

Some standard precautions to reduce exposure are:

  • Keep calls short.
  • Use hands-free devices.
  • Replace calls with text messages or video chats.

Parents should apply the same precautions to their children.

A 2015 report by the nonprofit group MediaSmarts suggested nearly a quarter of children in Grade 4 reported having their own cellphone. Ownership jumped to just over half in grade 7.

Matthew Johnson, director of education at MediaSmarts, said children aren't using their phones to talk — more and more the devices are used to take photos and to text-message.

Texting could be a positive in that it further reduces exposure compared with holding a phone to the head, Demers said. 

The National Toxicology Program researchers plan to release further findings on other parts of the body and in mice by fall 2017. Those findings also need to be carefully replicated to check their reliability and validity. 

With files from Associated Press and CBC's Christine Birak