June 1, 2011— -- Though a World Health Organization study concluded cell phones may cause cancer, some are wondering why, if there truly is a link, there has not been a significant worldwide increase in brain cancers.
The World Health Organization (WHO), whose International Agency for Research on Cancer announced the results of its year-long study Tuesday, estimates that there are 5 billion cell phone users globally, representing nearly three-quarters of the world's population.
However, the incidence and mortality rate of brain and central nervous system cancers has remained virtually flat since 1987, according to data from the National Cancer Institute.
The most compelling evidence cited by the WHO is a multi-country study that found people who used cell phones most often, an average of 30 minutes per day over 10 years, had a 40 percent higher risk for a rare brain tumor called a glioma.
The WHO also considered not-yet-released papers showing increased risk for another kind of cancer, acoustic neuroma, in the parts of the brain where cell phone radiation is strongest.
Roughly 30 older studies have tried and failed to establish any link between cell phones and cancer. This conundrum has been a hot topic since shoe-size phones hit the scene in the late 1970s.
One study even found those who used cell phones occasionally had a lower cancer risk than those who used old-fashioned land lines.
So what about the lack of rising numbers of brain cancers? Time is a major issue. The tumors involved take years, even decades, to develop, and some researchers say too few people have used cell phones long enough to affect worldwide numbers.
"The long-term consequences of putting radiation into brain we don't really understand," Dr. Keith L. Black of the neurosurgery department at the Cedars-Sinai Medical Center in Los Angeles told ABC News.
The WHO decided, in effect, to err on the side of caution.
"[The] IARC is saying that we should be cautious and think through what we do when we regulate exposures from cell phones," Dr. Peter Shields, chief of Georgetown University Hospital's cancer genetics and epidemiology program in Washington, D.C. told ABC News. "They follow the precautionary principle and want to maximally protect public health."
Meanwhile, the science is advancing. Researchers at the University of Utah established that the radiation dose is much higher inside the brains of 5- and 10-year-olds than in adults, a major concern as more children adopt cell phones.
Regulations are trailing behind the science.
In the U.S., the FCC set a maximum limit of 1.6 watts per kilo of body tissue. However, they did not test phones being carried directly in a person's pocket, just inside of belt holsters. So far, the recommendation continues to be to hold your phone about an inch away from your body.
ABC News' Katie Moisse contributed to this report.