High exposure to radiofrequency radiation resulted in tumors in tissue around the nerves in the hearts of male rats, but not of female rats or male or female mice, according to preliminary conclusions of two studies.

Draft reports on the two studies by the National Toxicology Program (NTP), which is part of the National Institutes of Health (NIH) in the United States, were released recently, pending a review by external experts that is scheduled to take place March 26–28. Members of the public may also submit comments.

The reports contain the remaining results of two large "toxicology and carcinogenesis" studies—one conducted in rats and the other in mice—of the effects of radiofrequency radiation (RFR) emitted by cell phones.

"The levels and duration of exposure," explains Dr. John Bucher, a senior scientist with the NTP, "to RFR were much greater than what people experience with even the highest level of cell phone use, and exposed the rodents' whole bodies."

High frequency radiation—like x-rays and gamma rays—and some higher energy ultraviolet radiation are known as ionizing radiation because they can knock out electrons and other charged particles from within atoms. They carry enough energy to damage DNA inside cells, which can give rise to cancer.

However, RFR is at the lower energy end of the spectrum and is not able to knock out charged particles and alter atomic structure, but it can cause atoms and molecules to vibrate. It generates heat if absorbed in large amounts by food, tissues, and other materials that hold water.

Thus, although RFR is not the type of radiation that can cause cancer by damaging DNA, there have been concerns that it might alter tissue in other ways that could lead to cancer.

Rats, mice exposed to different RFR levels

The NTP researchers note that the "predominant source of human exposure to RFR occurs through the use of cellular phone handsets."

For their studies, they constructed special chambers, in which they exposed the rats and mice to different levels of RFR.

Exposure occurred in a pattern of 10 minutes on, 10 minutes off, for a total of just over 9 hours per day and went on for 2 years.

Dr. Bucher says that 2 years of age in a rat or mouse is about 70 years of age in a human.

The RFR exposure levels ranged from around the maximum that is legally allowed for cell phones in the US to around four times that level.

The animals were exposed to the same "frequencies and modulations" as those of 2G and 3G signals that are used to make voice calls and send texts in the US. Later generations of RFR—such as 4G, 4G-LTE, and 5G—use different frequencies and modulations.

Schwannomas found in hearts of male mice

The tumors that the NTP researchers found in the hearts of male rats are of a type called schwannoma, which develops from the Schwann cells that form the protective and supportive tissue that surrounds peripheral nerves. This type of tumor is rarely cancerous.

The results showed that the incidence of schwannomas in the hearts of male rats went up as the animals were exposed to RFR levels that were "beyond the allowable cell phone emissions."

The researchers also found that these levels of RFR led to unusual patterns of damage in heart tissue in both male and female rats.

However, they found little evidence of health problems resulting from RFR exposure in mice.

Dr. Bucher says that the results "should not be directly extrapolated to human cell phone usage," but also notes that the tumors that they saw "are similar to tumors previously reported in some studies of frequent cell phone users."

'Conclusions still require careful discussion'

Dr. Otis W. Brawley, of the American Cancer Society (ACS), notes that while the preliminary results are "bound to create a lot of concern, the evidence for an association between cell phones and cancer is weak, and so far, we have not seen a higher cancer risk in people."

"But," he continues, "if you're concerned about this animal data, wear an earpiece."

He also points out that in response to questions at a recent press conference, Dr. Bucher said that the new data has not altered his own use of cell phones, and he has not advised his family to change their use of them either.

In a statement about the new data, Dr. Jeffrey Shuren, director of the Center for Devices and Radiological Health at the Food and Drug Administration (FDA), urges that the "conclusions still require careful discussion," and notes that their understanding is that the evidence is "mostly equivocal, or ambiguous," as to whether the RFR exposures "actually caused cancer in these animals."

He also highlights some "unusual findings" of the studies, including the fact that the rats that were exposed to RFR lived longer than the control rats that were not.

For those who are nevertheless concerned about exposure to RFR from cell phone use, the ACS advise that they keep the antenna away from the head—for example, by using speakerphone mode or a hands-free device—and send text messages in preference to making voice calls, except while driving.

Other ways to limit cell phone use include spending less time talking on them and using a landline instead, if available.

"Cell phone technologies are constantly changing, and these findings provide valuable information to help guide future studies of cell phone safety."

Dr. John Bucher