Last month, the International Agency for Research on Cancer, a branch of the World Health Organization, declared cellphone radiation "possibly carcinogenic to humans." The scientific evidence linking cellphone use to brain cancer isn't conclusive, the agency said, but there is some evidence that brain cancer rates are higher among people with the highest levels of cellphone exposure, and cellphone users should take precautions until more is known.
Now, some scientists are claiming that certain types of cellphones could be more "possibly carcinogenic" than others.
"I've been telling friends and family members to seriously consider switching to CDMA [cellphones] if they're using GSM cellphones," said Joel Moskowitz, the director of the Center for Family and Community Health at the University of California, Berkeley.
CDMA, or Code Division Multiple Access, is the type of cellular network used by the phone companies Verizon and Sprint. GSM, or Global System for Mobile Communications, is the type used by AT&T and T-Mobile.
There is accumulating evidence that cellphones that operate on GSM networks emit significantly more radiation than do cellphones operating on CDMA networks. This is not apparent when you look at a phone's specs, Moskowitz said, because phone companies are required to list only the "specific absorption rate" (SAR) — the measure of the rate at which energy from a radio frequency electromagnetic field is absorbed by the body — of a phone at its maximum radiation output. "The SAR can be misleading as it measures the maximum radiation a cellphone emits and does not reflect the average amount of radiation it emits," Moskowitz told Life's Little Mysteries.
Several recent studies have shown that CDMA phones normally emit a small fraction of their maximum radiation output, while GSM phones emit, on average, half the maximum, he explained. This comes down to the different radio frequency (RF) bands that the two networks operate on, and the different methods by which the two networks carry phone transmissions.
"When a GSM phone transmits, it immediately goes to the peak power, and then the power control circuitry ratchets down the power to an acceptable level," explained Mark McNeely, an electrical engineer at Exponent Engineering and Scientific Consulting services and co-author of a recent study comparing GSM and CDMA networks. "CDMA networks share the same frequency among many different phone calls, so all phones transmit at the lowest possible power level necessary to maintain the fidelity of the call." It's like people talking quietly at a party, he said.
The radiation spikes at the beginning of GSM phone calls means that they emit, overall, up to 28 times more radiation than CDMA phones, according to a study co-authored by McNeely and published last year in the Journal of Exposure Science and Environmental Epidemiology.
There are exceptions. If you have a CDMA phone in a rural area and the nearest CDMA cellphone tower is far away, then you have to broadcast at a radiation level that is equal to or greater than GSM to reach the tower, McNeely said. If there is a GSM tower much nearer to you, you might be better off going with a GSM network, he said; but in most parts of the country, where both CDMA and GSM towers are ubiquitous, CDMA phones will emit less radiation than GSM phones.
What does radiation do to the brain?
Although dozens of international studies have been conducted over the past decade, some of which point to higher incidences of certain types of brain cancers in people who use cellphones heavily, the negative side effects of cellphone usage remain undetermined.
A possible consequence of the higher radiation output of GSM phones was seen in a study published in the International Journal of Science Technology & Management in April. Researchers compared brain scans of people talking on GSM phones and CDMA phones and found that the former stimulated much more brain activity than the latter.
Although it's still unclear what that extra brain activity is, how it's caused or whether it's bad, other studies have also shown varying health consequences of using GSM versus CDMA phones. Of 37 studies that have examined GSM phones, 43 percent have found harmful biological effects from the phones — such as a decrease in the expression of genes that help suppress tumors — Moskowitz said, while only 15 percent of the 33 studies that looked at CDMA phones have identified harmful effects.
When reached for comment on the possible hazards of GSM phones, AT&T referred to previous statements by the Federal Communications Commission and the Food and Drug Administration that said the available scientific evidence shows no proven health risk of radiofrequency (RF) energy. The FDA states: "Although evidence shows little or no risk of brain tumors for most long-term users of cellphones ... people who want to reduce their RF exposure can: reduce the amount of time spent on the cellphone, and use speaker mode or a headset to place more distance between the head and the cellphone." T-Mobile did not return telephone calls or emails.
What to do?
GSM and CDMA networks work so differently that a phone built for one cannot operate on the other. Furthermore, AT&T and T-Mobile cellular networks, which are GSM, cannot simply switch and become CDMA networks. Given these facts, if you own a GSM phone, should you switch to a carrier that supports CDMA? Experts have mixed opinions.
It is difficult to say whether higher radiation output is bad, simply because the jury is still out on whether cellphone radiation is bad in the first place, says Ken Foster, a professor of bioengineering at the University of Pennsylvania who has been studying the effects of radiofrequencies for 40 years. The radiation level of cellphones — all cellphones — is so low it is considered "non-ionizing." It isn't powerful enough to knock electrons off atoms in cells and potentially change the structure of DNA molecules, which is the way that ionizing radiation (like gamma-rays and X-rays) causes harmful mutations. No one knows by what mechanism non-ionizing radiation, such as RF from cellphones, could possibly damage DNA, Foster says.
Though Foster grants that consumers could probably reduce their exposure by choosing CDMA rather than GSM phones, he doesn't think it's likely a higher radiation output actually makes GSM phones more hazardous than CDMA phones. The radiation levels of both phone types are so low, he said, that there is no known way they could harm DNA.
"If you take a shower at 160 degrees Fahrenheit, that could burn you. But I personally don't fear a shower at 65 degrees Fahrenheit more than one at 63 degrees — neither temperature is dangerous," he said. In his view, cellphone radiation on either type of network is as harmless as two cold showers of slightly different temperatures.
If cellphones are a biohazard, Foster said, that can't be related to the amount of the radiation they emit. "Presumably, some parameter other than [radiation output] would be involved." No one has identified what aspect of cellphone radiation is dangerous, Foster said, so there is no way of knowing whether GSM phones are worse for you than CDMA phones, or vice versa. [Read: What Everyday Things Around Us Are Radioactive?]
However, according to other scientists, there is some evidence that the potentially hazardous aspect of cellphone radiation may be the way in which transmissions are modulated — the way individual pulses of radiation are constructed out of a range of frequencies. The modulation pattern is different for CDMA and GSM phones, and some scientists think GSM pulse modulations may have adverse biological effects.
A review article in the April issue of the journal BioElectroMagnetics by Jukka Juutilainen and colleagues at the University of Eastern Finland suggested that specific types of RF modulation may well have biological consequences.
"While the majority of recent studies have reported no modulation-specific effects, there are a few interesting exceptions indicating that there may be specific effects from amplitude-modulated RF fields on the human central nervous system. These findings warrant follow-up studies," the researchers wrote.
According to Moskowitz, the study found that GSM phones contain radiation at a frequency of 8 hertz, or 8 cycles per second, which "is in the range of 'possibly carcinogenic' because our cells have processes on that frequency level, with which the phone radiation may be interfering," he said.
Foster, on the other hand, thinks there is no robust evidence that one type of modulation is more dangerous than the other. "To my knowledge, nothing shows a clear effect of pulse modulation," he said.
Wanted: more evidence
Do some mobile phone networks pose more of a health risk than others? Though some researchers suspect so, it is too soon to say for sure. "Clearly more comparative studies are needed," Moskowitz said.
At this point, all cellphone users should be cautious. "My first recommendation is to keep a safe distance from your phone. Text instead of calling. Use the speakerphone. Use a headset," Moskowitz said. Radiation levels fall off rapidly with distance — so rapidly that you can decrease your brain's exposure to a negligible level simply by keeping your phone antenna just a few inches away.
Moskowitz also thinks people should avoid keeping their cellphones on in their pockets. "There's accumulating evidence of a risk to sperm and male fertility," Moskowitz said. "People are forgetting where they're keeping their cellphones all day long."
Foster doesn't believe cellphone radiation poses a significant danger, but he still suggests that people take precautions if they're worried, just for peace of mind. "My best advice to consumers: If they are concerned about possible radiation risks from cellphones, use a hands-free kit, which actually does reduce exposure and costs very little."
Editor's Note: This article was updated at 12:16 p.m. EDT on April 29, 2021 to correct the spelling of Mark Mcneely's name.
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Natalie Wolchover was a staff writer for Live Science from 2010 to 2012 and is currently a senior physics writer and editor for Quanta Magazine. She holds a bachelor's degree in physics from Tufts University and has studied physics at the University of California, Berkeley. Along with the staff of Quanta, Wolchover won the 2022 Pulitzer Prize for explanatory writing for her work on the building of the James Webb Space Telescope. Her work has also appeared in the The Best American Science and Nature Writing and The Best Writing on Mathematics, Nature, The New Yorker and Popular Science. She was the 2016 winner of the Evert Clark/Seth Payne Award, an annual prize for young science journalists, as well as the winner of the 2017 Science Communication Award for the American Institute of Physics.