The International Agency for Research on Cancer (IARC), a branch of the World Health Organization (WHO), has officially labeled cellphone usage "possibly carcinogenic," grouping it in the same risk category as lead, the pesticide DDT and gasoline fumes.
The WHO pronouncement doesn't come with new and definitive evidence of a link between cellphone use and brain cancer. It's based on the results from dozens of studies and simply points to the possibility of a link, the need for caution when using cellphones, and above all, the importance of further study.
Here's what we know so far.
Why do scientists think cellphones might cause brain cancer?
During phone calls and other data transmissions, cellphone antennas emit radio waves, a form of electromagnetic radiation, or light, which is invisible to the human eye. It is unclear how large doses of these radio waves affect biological tissues.
There are two types of electromagnetic radiation: ionizing and non-ionizing. Ionizing radiation — including X-rays, UV rays, and gamma-rays — packs enough energy to strip electrons off the atoms it encounters, leaving them positively charged, or ionized. It is well established that large doses of ionizing radiation can cause cancer; by dislodging electrons from atoms in DNA, it can break chemical bonds and cause cancerous mutations.
Non-ionizing radiation, on the other hand, such as visible light, microwaves and the radio waves emitted by cellphones, is not typically energetic enough to ionize atoms. This doesn't mean it has no effect on biological tissues, though. While non-ionizing radiation isn't powerful enough to break up atoms, it does heat them up. Keith Black, chairman of neurology at Cedars-Sinai Medical Center in Los Angeles, recently told CNN, "What [cellphone radiation] does is similar to what happens to food in microwaves, essentially cooking the brain." [ Read: What Everyday Things Around Us Are Radioactive? ]
Though the effects of this "cooking" are not well understood, some scientists believe it may spur the growth of brain tumors.
Is there a proven link?
No: the link between cellphone radiation and brain cancer is not proved. In fact, the majority of studies that have been conducted thus far have been unable to identify a correlation between cellphone use and the likelihood of developing brain cancer.
The "most significant study" (according to the National Cancer Institute) conducted thus far is the Interphone study, a large international research effort published in 2010 that analyzed cellphone use and cancer rates among 13,000 participants in 13 countries. The Interphone study — which was coordinated by IARC, the agency behind the WHO's "possibly carcinogenic" pronouncement — did identify an increased risk of a type of brain tumor called glioma among participants who had used a cellphone for more than 30 minutes a day for 10 years or more. Moreover, these subject's tumors were more likely to show up on the side of their heads on which they typically held their phones.
However, there was no correlation between cellphone use and cancer for users at lower levels of exposure. Elizabeth Cardis, an Interphone study co-author, said at a news conference: "We have not demonstrated that there is increased risk but neither have we demonstrated that there is an absence of risk. These findings of increased risk in the heaviest users suggest a possible association but we don't have enough scientific evidence."
Some studies have looked at whether there exists an overall rise in the incidence of brain cancer in recent years. According to the National Cancer Institute, "Incidence data from the Surveillance, Epidemiology and End Results (SEER) Program of the National Cancer Institute (NCI) … show no increase in the age-adjusted incidence of brain and other nervous system cancers between 1987 and 2007, despite the dramatic increase in the use of cellphones." [ Read: Cell Phone Radiation Spurs Brain Activity ]
In short, the results are inconclusive.
Why is it taking so long for scientists to come to a conclusion?
Part of the problem is the relatively short time cellphones have been omnipresent in society. According to the NCI, "The interval between exposure to a carcinogen and the clinical onset of a tumor may be many years or decades. Scientists have been unable to monitor large numbers of cellphone users for the length of time it might take for brain tumors to develop."
It could be that studies are only starting to observe the first cases of brain cancer that have resulted from cumulative cellphone use. Some believe the effects will become clearer as time goes on. Others argue that there has been plenty of time for a correlation between cellphones and cancer — if there is one — to emerge, as cellphone use first took off in Europe in the 1980s.
Is there a danger threshold with cellphone use?
Some cellphone users have interpreted the results of the Interphone study as meaning that there could be a danger threshold — an exposure level above which cellphone use is dangerous (and below which there is no danger). This is because the study found a spike in glioma tumor rates only among people with the highest level of cellphone use: those who had talked on cellphones for at least a half hour per day for a decade. For less frequent users, no correlation between frequency of cellphone use and cancer risk was apparent.
However, for other types of dangerous radiation, there is no danger threshold. With ionizing radiation, such as gamma-rays and X-rays, the more you are exposed to, the higher the likelihood of mutations occurring and, thus, the higher your cancer risk. The relationship is linear.
Do children have a higher risk of developing cancer due to cellphone use than adults?
Children are a special cause for concern as "cellphone use by children and adolescents is increasing rapidly, and they are likely to accumulate many years of exposure during their lives," according to the NCI.
If the dangers of other forms of radiation are a model for the danger of radio waves, children may be at a greater risk than adults. "Children's skulls and scalps are thinner. So the radiation can penetrate deeper into the brain of children and young adults. Their cells are dividing at a faster rate, so the impact of radiation can be much larger," explained Keith Black of Cedars-Sinai Medical Center. [Read: Cellphone-Based Medical Tools Boost Health Around the World ]
At least two European studies of cellphone use and cancer rates in children are in progress.
How can I minimize my radiation exposure?
Non-ionizing radiation is emitted by a cellphone's antenna, which is usually embedded inside the handheld device itself. The best way to minimize your exposure is to use a headset, which allows you to conduct phone calls without holding your phone to your ear.
The power radiated from the phone drops off rapidly with distance, so some researchers say you can significantly decrease your exposure by simply holding your phone away from your head slightly during phone calls. Some cellphone manufacturers recommend doing this. The Apple iPhone 4 safety manual advises, "When using iPhone near your body for voice calls or for wireless data transmission over a cellular network, keep iPhone at least 15 mm (5/8 inch) away from the body." Blackberry Bold tells users to "keep the BlackBerry device at least 0.98 in. (25 mm) from your body when the BlackBerry device is transmitting."
Until there's a scientific consensus on the matter, many scientists say you're better safe than sorry.
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Follow Natalie Wolchover on Twitter @nattyover.