This summer, Ronald Herberman, director of the University of Pittsburgh Cancer Institute, sent a memo to staffers warning them to limit their cell phone use and to use hands-free sets in the wake of "growing evidence that we should reduce exposure" to cell phone radiation. Among the possible consequences: an increased risk of brain cancer.

Five months later, a top official at the National Cancer Institute (NCI) told a congressional panel that published scientific data indicates cell phones are safe.

So what's the deal? Do cell phones cause cancer—or not?

It depends on whom you ask: Herberman, Robert Hoover, director of NCI's Epidemiology and Biostatistics Program, and other health officials recently clashed during a hearing before the House Subcommittee on Domestic Policy held to determine whether mobile phones are safe.

"Long term and frequent use of cell phones which receive and emit radio frequency may be associated with an increased risk of brain tumors," Herberman told lawmakers. "I find the old adage 'better to be safe than sorry' to be very apt to this situation."

Hoover, on the other hand, insisted that the pervasive technology was safe, testifying that "its effect on the body appears to be insufficient to cause genetic damage."

The debate became so heated at one point that Rep. Dennis Kucinich (D–Ohio), who called the hearing, snapped at Hoover for interrupting David Carpenter, director of the Institute for Health and the Environment at the University at Albany, State University of New York, as he argued there was enough evidence to warrant more scrutiny and a government warning of potential damage.

Cell phones use non-ionizing radiation, which differs from the ionizing radiation of x-rays and radioactive material in that it does not have enough energy to knock around—or ionize—electrons or particles in atoms. Cell phone radiation falls into the same band of nonionizing radio frequency as microwaves used to heat or cook food. But Jorn Olsen, chair of epidemiology at the University of California, Los Angeles, School of Public Health says that unlike microwaves, cell phones do not release enough radiation or energy to damage DNA or genetic material, which can lead to cancer.

Recent research suggests, however, that although short-term exposure is harmless, long-term cell phone use may be a different story. Three studies since 1999 indicate that people who have used cell phones for more than a decade may have as much as three times greater risk of developing brain tumors on the side of the head against which they most often hold their phone—an argument for, at the least, shifting ears regularly or, even better, using an earpiece or the speakerphone feature while chatting.

"For people who've used their cell phones for more than 10 years and who use their phone on the same side as the tumor, it appears there's an association," Lawrie Challis, emeritus physics professor at the University of Nottingham in England and former chairman of the U.K.'s Mobile Telecommunications and Health Research program, told during a recent interview.

Worldwide, one in 29,000 men and one in 38,000 women on average develop brain tumors each year, with people in industrial nations twice as likely as those in developing countries to be diagnosed with one, according to the World Health Organization's International Agency for Research on Cancer (IARC) in Lyon, France. If cell phone use does, in fact, triple the odds of getting cancer, these stats would suggest that over 60 years a man's risk of developing a brain tumor from cell phone use increases from 0.206 percent to 0.621 percent, and a woman's from 0.156 percent to 0.468 percent.

IARC in 2000 launched a study called Interphone, funded by the European Union, the International Union against Cancer and other national and local funding bodies. Interphone compared surveyed cell phone use in 6,420 people with brain tumors to that of 7,658 healthy people in 13 developed countries—Australia, Canada, Denmark, Finland, France, Germany, Israel, Italy, Japan, New Zealand, Norway, Sweden and the U.K.—to try to determine whether people with brain tumors had used their cell phones more than healthy people, an association that might suggest that cell phones caused the tumors.

The results are expected by the end of this year. "The interpretation of the results is not simple because of a number of potential biases which can affect the results," says project leader Elisabeth Cardis, a professor at the Center for Research in Environmental Epidemiology at the Barcelona Biomedical Research Park. "These analyses are complex and have, unfortunately, taken much time." Among factors that might skew the results: failure of participants—especially those with tumors—to accurately recall exactly how long and often they talk on their cell phones.

According to the U.S. Centers for Disease Control and Prevention (CDC), the average time between first exposure to a cancer-causing agent and clinical recognition of the disease is 15 to 20 years or longer—and cell phone use in the U.S. has only been popular for about a decade. (In 1996 there were 34 million U.S. cell phone users compared with more than 200 million today, according to CTIA–The Wireless Association, a Washington, D.C.–based cell phone industry group.)

Carpenter told the congressional panel that most of the studies that have shown an increased risk are from Scandinavia, where cell phones have been popular since the early 1990s. Herberman added that most of the research showing cell phones are safe is based on surveys of consumers who have used them for less than 10 years.

Despite a dearth of human studies, more than 400 experiments have been done since the early 1970s to determine how cell phone radiation affects animals, cells and DNA. They, too, have produced conflicting results. Some suggest that cell phone radiation damages DNA and/or nerve cells, others do not. At the hearing, Carpenter suggested that cell phones may increase the brain's production of reactive forms of oxygen called free radicals, which can interact with and damage DNA.

Contradictory findings could be a sign of poor study quality, according to NCI expert Hoover. But Jerry Phillips, a biochemist who performed cell phone research at U.S. Department of Veterans Affairs's Pettis VA Medical Center in Loma Linda, Calif., in the 1990s, believes that conflicting results are to be expected given the nature of the radiation being scrutinized.

Phillips says, for instance, that sometimes the body will respond to radiation by initiating a series of intrinsic repair mechanisms designed to fix the harmful effects. In other words, the effects from radiation exposure may be different in different people. And these varied responses may help explain the contradictory results, says Phillips, who is now director of the Science/Health Science Learning Center at the University of Colorado at Colorado Springs.

There is plenty of anecdotal evidence out there claiming a link between cell phone use and cancer: Keith Black, chairman of neurosurgery at Cedars-Sinai Medical Center in Los Angeles, says that the brain cancer (malignant glioma) that killed O. J. Simpson's attorney, Johnnie Cochran, was the result of frequent cell phone use, based on the fact that the tumor developed on the side of the head against which he held his phone. And in May, a week after Massachusetts Sen. Edward Kennedy was diagnosed with a glioma, The EMR Policy Institute, a Marshfield, Vt.–based nonprofit organization that supports research on the effects of electromagnetic radiation, released a statement linking his tumor to heavy cell phone use. But the NCI maintains that there is no definitive evidence that cell phones increase cancer risk.

In other words, the verdict is still out. "We can't rule out the possibility of risk," Nottingham's Challis says. "There hadn't been as much work in this area as is now demanded."