A review of existing research on the topic, published online Oct. 13 in the Journal of Clinical Oncology, discerned no overall link. But when the spotlight was turned on only the more methodologically rigorous studies, a potentially harmful association was found.
Combined with similarly murky conclusions from earlier research, this leaves the world's four billion cell phone users with no clear indication of what risk, if any, they are taking when they converse on the go.
"We cannot make any definitive conclusions about this," said one expert, Dr. Deepa Subramaniam, director of the Brain Tumor Center at Georgetown Lombardi Comprehensive Cancer Center in Washington, D.C. "But this study, in addition to all the previous studies, continues to leave lingering doubt as to the potential for increased risk. So, one more time, after all these years, we don't have a clear-cut answer."
"What makes me worry," she stated, "is that the higher quality studies [seen here] did indeed show an association."
Joel Moskowitz, the study's senior author, said that "clearly there is risk." He's director of the Center for Family and Community Health at the University of California, Berkeley, School of Public Health.
"I would not allow children to use a cell phone, or I at least would require them to use a separate headset," Moskowitz said. "It seems fairly derelict of us as a society or as a planet to just disseminate this technology to the extent that we have without doing a whole lot more research of the potential harms and how to protect against those harms. Clearly, we need to learn a whole lot more about this technology."
Some in the technology industry disagree.
"The peer-reviewed scientific evidence has overwhelmingly indicated that wireless devices do not pose a public health risk," John Walls, vice president of public affairs for CTIA-The Wireless Association, said in a prepared statement.
"In addition, there is no known mechanism for microwave energy within the limits established by the [U.S. Federal Communications Commission] to cause any adverse health effects," he said. "That is why the leading global heath organizations such as the American Cancer Society, [U.S.] National Cancer Institute, World Health Organization and the U.S. Food and Drug Administration all have concurred that wireless devices are not a public health risk."
For the new study, Moskowitz and his fellow researchers in South Korea searched medical bases for the keywords "mobile phones," "cellular phones," "cordless phones" and "tumors" or "cancer." They included 23 case-control studies, involving 37,916 total participants, in their final analysis.
When the studies were pooled, no risk was seen between mobile phone use and brain tumors, either benign or malignant. But a subgroup of studies that employed more rigorous methodology -- most conducted by the same research team in Sweden -- reported a harmful effect, whereas a set of less rigorous studies -- most funded by an industry consortium -- found a protective effect.
Specifically, the more robust studies found that using a mobile phone for a decade or longer resulted in an 18 percent increased risk for developing a brain tumor.
Some studies also showed that brain tumors were more likely to appear on the side of the brain where the cell phone was used.
According to the American Cancer Society, nearly 21,000 malignant brain or spinal cord tumors are diagnosed in adults in the U.S. each year, while 3,800 such tumors are diagnosed in children.
Moskowitz also believes that there's potential for harm to other areas of the body -- the genitals, for example -- when the phone is carried in a pocket.
With so many people worldwide using cell phones, even a small risk could translate into many illnesses and deaths, he said.
"We need to do a whole lot more research because the stakes are really high and there seems to be suggestive evidence that you better be careful about this, especially in children, who have developing tissue and smaller brain and skull sizes," Moskowitz warned.
Subramaniam seemed to agree.
"I do encourage people to use the speaker phone or a hands-free device if they can, and I definitely do not encourage children to use cell phones because then there's a much longer lifetime risk of exposure," she said.
"In my opinion," she said, "the question remains unsettled -- and unsettled always carries with it likelihood that we might find an association."
A report last year from the National Research Council, the main operating agency of the National Academy of Sciences and the National Academy of Engineering, and compiled at the request of the U.S. Food and Drug Administration, called for more research into the risks posed by long-term cell phone use, rather than the more commonly studied short-term risks. It urged that such research focus on the health of children, pregnant women and fetuses as well as workers subject to high occupational exposure.