Another reassuring piece of evidence that cellular phones do not cause cancer appears in this issue of the Journal. Researchers evaluated whether a cohort of nearly half a million cellular phone subscribers in Denmark had a higher incidence of cancer than the general population, with a particular interest in leukemia and cancers of the brain and salivary gland. The answer was a resounding no (see article, p. 203).
This article comes on the heels of two casecontrol studies reported in December by the American Health Foundation and the National Cancer Institute, both of which also found no association between cell phone use and the risk of brain tumors.
"Were trying to do everything we can to be prudent and evaluate this brand new technology that humans have never experienced before in their history," said David Savitz, Ph.D., an epidemiologist at the University of North Carolina at Chapel Hill, and member of NCIs Board of Scientific Counselors. "I dont see how you can see these studies as anything other than good news."
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The underlying impetus for these latest three studies is the exponential rise in worldwide cellular phone use during the last decade. Not only has the number of people using cell phones increased dramatically during the past 10 years (see Stat Bite, next page), but this trend appears likely to continue. According to the Cellular Telecommunications Industry Association, there are about 108 million mobile phone subscribers in the United States, and this number is increasing at the rate of about 46,000 new subscribers every day, or about one every 2 seconds. And Christoffer Johansen, M.D., Ph.D., of the Institute of Cancer Epidemiology in Copenhagen, Denmark, and first author of the Journal study, said that the rates for the Scandinavian countries are even higher; about 50% of Denmarks households own a cell phone, and an even larger percentage of Swedes and Finns are users.
The radiofrequency radiation emitted by cellular phones is a very low intensity, nonionizing radiation that has not been shown in previous studies to pose a cancer risk. In contrast, ionizing radiation, such as x-rays or gamma-rays, at high doses is known to cause cancer. The low-intensity radiation emitted by cellular phones, however, cannot break chemical bonds like ionizing radiation can. In addition, no substantial biological tissue warming occurs with radio frequencies emitted by cellular phones.
CaseControl Studies
The two studies reported in December are both casecontrol studies. The first, reported in the Journal of the American Medical Association by Joshua E. Muscat and colleagues from the American Health Foundation, involved 469 brain cancer (glioma) patients and an equal number of control patients. The second study, reported in the New England Journal of Medicine by Peter D. Inskip, Sc.D., and Martha Linet, M.D., of NCIs Division of Cancer Epidemiology and Genetics, involved 799 control patients and 782 brain tumor patients recently diagnosed with glioma, meningioma, and acoustic neuroma. Both studies involved exposures from 1994 to 1998a time period when analog phones were more commonly used than digital phones. Digital phones, which operate at a different frequency and power, are more commonly used today.
The degree of phone use for the casecontrol studies was measured by a structured personal interview in which participants were asked specific questions about when they first began using a hand-held cellular phone and their usual level of use. Neither study had a high percentage of high-level users, and very few people in either study had used their phone frequently for more than 5 years.
"I think that the casecontrol studies make a tremendous contribution to the brain cancer question," said Nancy Dreyer, Ph.D., at Epidemiology Resources Inc., in New Lower Falls, Mass. "They are the first real strong evidence that has come out on a substantial number of people with brain tumors. So while they dont answer every question, they make a tremendous contribution."
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One of the problems, however, is that cellular phones are an evolving technology. "[The studies] are reflecting an historical period where use was less prevalent and a technology that is not used anymore," said Savitz.
Dreyer agreed. "Not only has there has been an explosion in the number of cell phone users, but people are using cell phones a lot more now. We have a lot higher exposure levels now than when the studies were done, " she said.
Some of these same concerns exist with the Danish study. Although the 420,000 cell phone subscribers were followed from 1982 to 1995, the average period of follow-up was short3.1 years. More than two-thirds of the subscriptions began in 1994 and 1995.
"The real crux of the problem with all the cell phone studies that have been done so far, is there isnt a very long period of time in which people have been exposed," said Margaret Wrensch, Ph.D., an epidemiologist from the University of California at San Francisco. "If something is a very abrupt, acute effect, you might expect to see it. But if its something that you need to have the exposure over a fairly protracted period of time and at fairly high levels, youre not going to have enough people who have been exposed at those key levels."
One important advantage of the Danish cohort study compared to the casecontrol studies is that determining the level of exposure is not dependent on people, sometimes sick people, recalling their rates of phone use. The cellular companies files were used to determine exposures. However, how many people per cellular subscription used the phone cannot be determined.
"We need to get more precise information," Johansen said. "Its always a problem in a large cohort study to refine the exposures. I dont think we can correct for that."
Researchers in the Danish study were able to take advantage of the countrys high-quality cancer registry as well as its meticulous record keeping and data linkage. Not only were they able to evaluate several kinds of cancer, they will also be able to follow participants for many years as the technology changes and track the incidence of other diseases.
In 1968, Denmark established a Central Population Register, in which each citizen is assigned a unique 10-digit identification number. A nationwide cancer registry has operated there since 1942. Once the cell phone companies agreed to allow the Danish researchers access to their computerized files (which, according to Johansen, took a year and a half of negotiations), they were able to identify the subscribers and length of subscription. The Central Population Register provided them with the identification numbers of the subscribers that were then used to track cancer incidence rates from the Danish Cancer Registry.
For some scientists, such as Robert Park, Ph.D., a physicist at the University of Maryland, and the author of the editorial that accompanies the Danish study (see p. 166), the three most recent studies settle the safety issue of the phones. He is particularly impressed with the Danish report. "This is a really sound study. Its a large cohort. The database is reliable. This ought to put the issue to rest."
In addition, he has always been bothered by the fact that there is no biological mechanism that can explain how radio frequency radiation causes cancer. He does admit, though, that one of the shortcomings of all of the studies is that if cancer takes 20 years to develop, "were not going to see it in any of these studies."
In spite of the these caveats, one conclusion that does seem reasonable to draw is that the radiofrequency exposure from cell phones does not appear to act by promoting the growth of a pre-existing brain lesion. If this were true, Johansen points out that the intense recent use by a large number of people in the Danish cohort would have had adverse health effects.
Whether or not future studies with longer exposure times and digital phones will show cell phones to be safe, Park thinks there will always be some people that consider cell phones unsafe. In a commentary in the Nov. 25, 2000, issue of The Lancet, Philip P. Dendy, Ph.D., of Cambridge, England, reminded the readers that it is impossible to design an experiment to show that anything is safe. "The best that can be achieved is an analysis of the data that demonstrates that the risk is no greater than a specified figurefor example, one in a million."
He draws a parallel with natural gas. "This highly explosive substance is piped into millions of homes in the country. Is it safe? Of course not, but most people are prepared to live with the risk."
Johansen makes the same argument about the automobile. "We know its dangerous to drive a carthat is why we have safety belts and drunk driving laws. We take risks and do certain things to modify that risk," he said. "If we had found there was a 10% increased risk of brain cancer by using cell phones, people would consider that before using the phone. But many would still use it."
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