As cell phones and smart phones take over more communication, information and entertainment functions, concerns about their possible role in causing brain cancer are buzzing on the airwaves.

This year alone, several studies and review articles have posited tentative links between radiation produced by cellular phones and the development of brain tumors.

A team of researchers in northern Europe, however, has now combed through three decades of cancer registries and found no increase in the rate of brain tumors in the five to 10 years following widespread cell phone adoption in that region.

"We did not detect any clear change in the long-term time trends in the incidence of brain tumors from 1998 to 2003 in any subgroup," the researchers wrote in the paper, which was published online Thursday in the Journal of the National Cancer Institute (JNCI). The study looked for two of the most common types of brain tumors—gliomas, which are often malignant, and meningiomas, which are more often benign—in people ages 20 to 79 in Denmark, Finland, Norway and Sweden between 1974 and 2003. The researchers found that the frequency of these cancers was steady throughout the decades despite mobile use becoming common by the mid-1990s.

Even as cell phone use has soared worldwide, not every cell chatterer encounters the same exposure. Many smart phones, for example, put out higher levels of radiation, and the use of wired headsets can decrease the amount of energy emanated near the head. Nevertheless, says lead study author Isabelle Deltour of the Institute of Cancer Epidemiology in Copenhagen, "The Nordic countries are a good model for studying the association" due to broad early adoption of the technology and thorough medical records.

A lack of correlation in these countries or elsewhere, however, doesn't clear the air of doubt—even in the researchers' minds. "The scientific literature is unsettled right now about the association between mobile phones and brain tumors," Deltour says.

Part of the trouble in tracking down a connection between brain tumors and cell phones is the poor understanding of cancers themselves. "The etiology of brain tumors is largely unknown," Deltour says. Some people may have an increased genetic risk for brain tumors, but, she says, "Mostly, we don't know why people get them."

Deltour and others also note that many cancers can take more than a decade to take hold.

"I don't think five to 10 years does cover it," says David Carpenter, a professor of environmental health sciences and biomedical sciences at the University at Albany, State University of New York. "Brain cancers are slow growing, so the idea that you would be able to detect something after five years would be surprising," he says. "Time will tell, but likely the biggest increase will be after 20 years."

Many who study radiation's effects on the cellular level prefer to err on the side of caution. Martin Blank, an associate professor of physiology and cellular biophysics at Columbia University Medical Center's College of Physicians and Surgeons in New York City, notes cell phones are most likely having some impact on biological processes. Exposing cells to radiation, for one, increases their stress responses, he says. Both he and Carpenter endorsed a report [pdf] this August that advocated greater caution in the use of cellular technology.

Although the new JNCI study did not analyze individual cell phone usage—preferring instead to take a broader look at the population at large—other research has examined subgroups of cell phone users to try to discern different trends among more frequent use. Even these approaches, however, can be problematic, as Carpenter, who is also the director of the University at Albany's Institute for Health and the Environment, points out. Study subject's recall bias about how much one had talked on their mobile device can sway results.

Solving this dilemma, however, will require firmer data, such as records from cellular providers, Carpenter says. And industry cooperation, he notes, has not yet been mandated.

One forthcoming analysis, from the long-term international study known as INTERPHONE, promises to provide new data on tumor incidence and cell phone usage. "I think we are all looking forward to this new report from INTERPHONE," Carpenter says. Although the research phase has been wrapped up, the report has not yet been published. But many aren't holding out hope that it will end the debate once and for all. "We're probably 10 years away from having an answer," Carpenter says.

Meanwhile—unlike Blank, who says he doesn't have a cell phone (or any other wireless devices)—Carpenter confesses that he does have a BlackBerry, which he says he uses occasionally. "Common sense says that some caution is wise," he concludes.