Scientists didn't find any increased risk associated with cell phone use, even among the heaviest users. And, in contrast with widely publicized Swedish research, scientists didn't find any correlation between brain tumors and the area of the head nearest the phone.
"I think it does provide some reassurance with regard to short-term cell phone use that it doesn't seem to be a hazard,'' said lead scientist Joshua Muscat of the American Health Foundation, a New York-based research organization.
His study, conducted by scientists from five U.S. institutions, appears in Wednesday's issue of The Journal of the American Medical Association. A second report released Tuesday, from scientists at the National Cancer Institute, will appear next month in The New England Journal of Medicine.
Muscat and his colleagues did find that one uncommon classification of brain cancer, known as neuroepithelioma, seemed to occur more often than expected in cell phone users. But because only a small number of people have this cancer, it's difficult to tell whether the finding signals a genuine risk or is simply a statistical wrinkle.
"At this point, we don't know if it represents something real,'' Muscat said. "This is not something we're immediately concerned about, but it does warrant further investigation.'' For this analysis, researchers interviewed 469 men and women with brain cancer about their cell phone use. The researchers also questioned 422 people who did not have brain cancer.
The government-funded investigators interviewed 782 people with brain and nervous system tumors, and 799 people without cancer. Studies published up to now have included smaller numbers of people and therefore had less statistical power to detect a problem.
"We've been waiting a long time for this,'' said Kenneth Rothman of the Epidemiology Research Institute in Massachusetts, who conducted one of the first studies on the mortality of cell phone users in 1996.
Wireless Technology Research, a group created by the cell phone industry, was the main source of funding for the American Health Foundation's study. But scientists said that wireless companies had no say in the study design, nor did they have control over the release of results.
Despite the encouraging conclusions, Muscat and others are not pushing the end button on the debate. While these studies may be the most important yet released, experts say, they are still only part of the research picture. Also, no one yet knows whether cell phones might pose any health hazards over the long term.
"This is reassuring,'' Rothman said, "but as the authors themselves say, it can't address the questions about cancers on down the road.''
Research soon after cigarettes came on the market could have been similarly comforting, Rothman noted. "People can smoke heavily and you wouldn't see cancers until many years later.''
The debate over cell phone safety began in the early 1990s, when a Florida man sued the industry, saying the phones had killed his wife. She had developed a tumor behind her right ear, where she always held the phone, he said. The case was dismissed for lack of scientific foundation.
All wireless phones emit radiation, but the biological effects of this radiation, if any, are unclear. The radiation from cell phones is on par with that of microwave ovens and television sets, not X-rays and ultraviolet radiation, which are known to cause cancer when people suffer too much exposure.
Since the Florida man's suit, researchers have published data on animals and people, most of which so far suggest that cell phones are not harmful in the short term. Rothman's studies have found one cause of death higher in cell phone users: motor vehicle accidents.
One study from Swedish researchers, while not showing any increase overall in brain cancer, did suggest that tumors were more likely to occur on the side of the head where people tended to use a cell phone. But that is not what their American colleagues found.
"One of them is a chance finding,'' Dr. Lennart Hardell of Sweden's Vrebro Medical Center said of the conflicting studies. "We can't tell which one.''
Like other experts, he said he was pleased to see these results finally released. "I don't think we can yet give a clean bill, but this is of course reassuring that there is no big risk'' from cell phone use.
But are cell phones safe? "I don't think we can tell that yet,'' Hardell said.
One other crucial piece of research will not be out for several years. An international team, sponsored by the World Health Organization, is conducting a large study that includes various countries with long histories of cell phone popularity. If that study, too, fails to find any cancer connection, then researchers will have to consider whether cell phones are worth the expense of more investigation, said Dr. Peter Inskip, the National Cancer Institute researcher who led the government study.
No one may ever know with absolute certainty whether cell phones cause cancer. Showing something is safe is always much more difficult than showing something is hazardous.
"You can't prove safety,'' Inskip said. When a product is safe, the study will have no positive findings. However, "you can always come up with possibilities to explain why not.''
For example, perhaps the type of phone makes a difference, or perhaps there is only an increased risk of a certain type of tumor, or perhaps the danger is too small to detect without a larger, more expensive study.
If people are still worried, they can always adjust their own habits – using a separate earpiece, for example, or restricting the time they allow their children to use a wireless phone. "Ultimately, it's going to come back to the individual,'' Inskip said. "What puts your mind at rest?''
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