In addition, users are spending more time on the phone — 50 percent more for an average U.S. customer in 1999, compared with 1996, according to industry (CTIA) statistics. Many people now use their cellular phone as their primary phone line. These changes in use patterns have occurred even since the study information was collected, making it difficult to say with certainty that using cellular phones is safe. Typical use now may well exceed the average among study participants. Since this research looked primarily at analog phones, digital phone use also needs to be examined. Such international studies are under way.
"So far, the research doesn't demonstrate that any adverse health effects exist," said Russell Owen, Ph.D., chief of the radiation biology branch at the U.S. Food and Drug Administration, which oversees the safety of radiation-emitting electronic products.
Previously, studies in animals linked the possibility of cancer to cellular phone exposure, Owen said. In an Australian study, mice genetically engineered to be more likely to get lymphoma developed twice as many tumors when exposed to radiation intended to mimic human exposure from cell phones. A couple of other studies have found changes associated with cancer in blood or other cells after exposure to cell phone radiation, Owen said. Researchers now are trying to assess the accuracy and reproducibility of these studies, he said.
The studies that found harmful effects, however, "still have methodological problems, so it's hard literature to interpret," said Dr. Black, the Harvard researcher. "But there is no agreed upon mechanism for cell-phone frequencies to cause mutations. There is such a mechanism for X-rays."
Of course, microwaves, which fall in the same frequency range as cell phone radiation, clearly can damage cells. The mechanism also is clear — heat. But cell phones — which emit only about one thousandth of the power of a microwave oven — produce very little heat. So researchers are trying to determine whether the phones could cause harm by some means other than heating, or through low levels of heat, delivered over a long period, Owen said.
The three large studies add to a collection of research that has found no short-term harm from cell phones, Owen said.
The two American studies, both released in December 2000, were based on interviews with U.S. hospital patients from 1994 to 1998 about cell-phone use.
The American Health Foundation study, published Dec. 20, 2000, in the Journal of the American Medical Association, compared the cell-phone use of 469 brain-cancer patients at five academic medical centers with that of 422 patients who did not have cancer.
The New England Journal of Medicine study released at the same time involved 782 patients at three medical centers who had brain cancer and 799 patients who had other ailments.
The Danish study, published on Feb. 7, 2001, in the Journal of the National Cancer Institute, was much larger and used a somewhat different method. Using the comprehensive Danish Cancer Registry database and the records of Denmark's two cell phone companies, researchers compared cancer rates among the general population with those of all 420,095 Danes who had cell phone service between 1982 and 1995. Cell phone users had no more of the kinds of cancers examined — leukemia and tumors of the brain, nervous system and salivary glands — than other Danes.
In all three studies, the investigators found that no matter how they analyzed the data, the people who used cellular phones were no more likely to have cancer than non-users.
For instance, the National Cancer Institute study compared those who were on the phone for 60 minutes or more a day or regularly for at least five years with people who used the phones less — and found no increased risk with greater use.
"It all suggests that nothing bad is happening from the very minor energy that cell phones emit," said Dr. Black, who is the Frank D. Ingraham Professor of Neurosurgery at Harvard Medical School
But the average exposure — more than 100 total hours for the cell phone users in the National Cancer Institute study and about three years of use for the American Health Foundation and Danish studies — was still low compared with what may be common use patterns in the future.
Some answers may be provided by a long-term international study that is being conducted under the supervision of the International Agency for Research on Cancer, based in Lyon, France, researchers said.
People who are still concerned about cell-phone safety can take several steps, according to the FDA and other cell-phone experts:
Learn how much energy is emitted by your phone.
Minimize time spent using the phone.
Use a headset and place the phone away from your body.
Switch to a model with a remote antenna outside the car.
You might be considering use of a device to block cell-phone radiation. However, the FDA does not evaluate or recommend so-called cell-phone shields, Owen said.
Under Federal Communications Commission safety standards, cell phones sold in the United States may emit no more than 1.6 watts of energy per kilogram of tissue, a measure known as the
Specific Absorption Rate — the amount of radiation absorbed by the body. That standard was set far below the absorption level demonstrated to cause any biological change in laboratory animals, said Robert Cleveland, Ph.D., senior scientist with the FCC's Office of Engineering and Technology.
The Specific Absorption Rate gives only the maximum emission from a phone, which occurs when the user reaches the outer limits of a transmission tower's range, Cleveland said. Emissions are lower near a tower, he said.
Dr. Black said he expects that long-term studies will demonstrate the safety of cell phones. Meanwhile, he is taking no special precautions.
"I use a plain old handset cell phone without an earpiece," he said, "so I guess I'm living the wild life. Or I suppose I'm practicing what I preach."
