Two new studies exposed rats and mice to high levels of radio-frequency radiation — the type emitted by your cellphone. But researchers said there was little cancer risk for humans. Credit Victor J. Blue for The New York Times
Do cellphones cause cancer?
Despite years of research, there is still no clear answer. But two government studies released on Friday, one in rats and one in mice, suggest that if there is any risk, it is small, health officials said.
Safety questions about cellphones have drawn intense interest and debate for years as the devices have become integral to most people’s lives. Even a minute risk could harm millions of people.
These two studies on the effects of the type of radiation the phones emit, conducted over 10 years and costing $25 million, are considered the most extensive to date.
In male rats, the studies linked tumors in the heart to high exposure to radiation from the phones. But that problem did not occur in female rats, or any mice.
The rodents in the studies were exposed to radiation nine hours a day for two years, more than people experience even with a lot of cellphone use, so the results cannot be applied directly to humans, said John Bucher, a senior scientist at the National Toxicology Program, during a telephone news briefing.
The results, he said, had not led him to change his own cellphone use or to urge his own family to do so. But he also noted that the heart tumors in rats — called malignant schwannomas — are similar to acoustic neuromas, a benign tumor in people involving the nerve that connects the ear to the brain, which some studies have linked to cellphone use.
He said that nearly 20 animal studies on this subject have been done, “with the vast majority coming up negative with respect to cancer.”
Other agencies are studying cellphone use in people and trying to determine whether it is linked to the incidence of any type of cancer, Dr. Bucher said.
The Food and Drug Administration issued a statement saying it respected the research by the toxicology program, had reviewed many other studies on cellphone safety, and had “not found sufficient evidence that there are adverse health effects in humans caused by exposures at or under the current radio-frequency exposure limits.”
Why is there concern that cell phones may cause cancer or other health problems?
There are three main reasons why people are concerned that cell phones (also known as “mobile” or “wireless” telephones) might have the potential to cause certain types of cancer or other health problems:
- Cell phones emit radiofrequency energy (radio waves), a form of non-ionizing radiation, from their antennas. Tissues nearest to the antenna can absorb this energy.
- The number of cell phone users has increased rapidly. As of December 2014, there were more than 327.5 million cell phone subscribers in the United States, according to the Cellular Telecommunications and Internet Association. This is a nearly threefold increase from the 110 million users in 2000. Globally, the number of subscriptions is estimated by the International Telecommunications Union to be 5 billion.
- Over time, the number of cell phone calls per day, the length of each call, and the amount of time people use cell phones have increased. However, improvements in cell phone technology have resulted in devices that have lower power outputs than earlier models.
The NCI fact sheet Electromagnetic Fields and Cancer includes information on wireless local area networks (commonly known as Wi-Fi), cell phone base stations, and cordless telephones.
What is radiofrequency energy and how does it affect the body?
Radiofrequency energy is a form of electromagnetic radiation. Electromagnetic radiation can be categorized into two types: ionizing (e.g., x-rays, radon, and cosmic rays) and non-ionizing (e.g., radiofrequency and extremely low frequency, or power frequency). Electromagnetic radiation is defined according to its wavelength and frequency, which is the number of cycles of a wave that pass a reference point per second. Electromagnetic frequencies are described in units called hertz (Hz).
The energy of electromagnetic radiation is determined by its frequency; ionizing radiationis high frequency, and therefore high energy, whereas non-ionizing radiation is low frequency, and therefore low energy. The NCI fact sheet Electromagnetic Fields and Cancerlists sources of radiofrequency energy. More information about ionizing radiation can be found on the Radiation page.
The frequency of radiofrequency electromagnetic radiation ranges from 30 kilohertz (30 kHz, or 30,000 Hz) to 300 gigahertz (300 GHz, or 300 billion Hz). Electromagnetic fields in the radiofrequency range are used for telecommunications applications, including cell phones, televisions, and radio transmissions. The human body absorbs energy from devices that emit radiofrequency electromagnetic radiation. The dose of the absorbed energy is estimated using a measure called the specific absorption rate (SAR), which is expressed in watts per kilogram of body weight.
Exposure to ionizing radiation, such as from x-rays, is known to increase the risk of cancer. However, although many studies have examined the potential health effects of non-ionizing radiation from radar, microwave ovens, cell phones, and other sources, there is currently no consistent evidence that non-ionizing radiation increases cancer risk (1).
The only consistently recognized biological effect of radiofrequency energy is heating. The ability of microwave ovens to heat food is one example of this effect of radiofrequency energy. Radiofrequency exposure from cell phone use does cause heating to the area of the body where a cell phone or other device is held (ear, head, etc.). However, it is not sufficient to measurably increase body temperature, and there are no other clearly established effects on the body from radiofrequency energy.
It has been suggested that radiofrequency energy might affect glucose metabolism, but two small studies that examined brain glucose metabolism after use of a cell phone showed inconsistent results. Whereas one study showed increased glucose metabolism in the region of the brain close to the antenna compared with tissues on the opposite side of the brain (2), the other study (3) found reduced glucose metabolism on the side of the brain where the phone was used.
Another study investigated whether exposure to the radiofrequency energy from cell phones affects the flow of blood in the brain and found no evidence of such an effect (4).
The authors of these studies noted that the results are preliminary and that possible health outcomes from changes in glucose metabolism are still unknown. Such inconsistent findings are not uncommon in experimental studies of the biological effects of radiofrequency electromagnetic radiation (5). Some contributing factors include assumptions used to estimate doses, failure to consider temperature effects, and lack of blinding of investigators to exposure status.
How is radiofrequency energy exposure measured in epidemiologic studies?
Epidemiologic studies use information from several sources, including questionnaires and data from cell phone service providers. Direct measurements are not yet possible outside of a laboratory setting. Estimates take into account the following:
- How “regularly” study participants use cell phones (the number of calls per week or month)
- The age and the year when study participants first used a cell phone and the age and the year of last use (allows calculation of the duration of use and time since the start of use)
- The average number of cell phone calls per day, week, or month (frequency)
- The average length of a typical cell phone call
- The total hours of lifetime use, calculated from the length of typical call times, the frequency of use, and the duration of use
The statement, from Dr. Jeffrey Shuren, director of the F.D.A.’s center for devices and radiological health, also said, “Even with frequent daily use by the vast majority of adults, we have not seen an increase in events like brain tumors.”
The Federal Communications Commission sets exposure limits for radio-frequency energy from cellphones, but relies on the F.D.A. and other health agencies for scientific advice on determining the limits, the statement said.
For people who worry about the risk, health officials offer common-sense advice: Spend less time on cellphones, use a headset or speaker mode so that the phone is not pressed up against the head and avoid trying to make calls if the signal is weak.
Dr. Bucher noted that the radiation emitted increases when users are in spots where the signal is poor or sporadic and the cellphone has to work harder to connect.
In December, California issued advice to consumers about how to lower their exposure, including texting instead of talking, keeping the phone away from the head and body while streaming, downloading or sending large files; carrying the phone in a backpack, briefcase or purse, not a pocket, bra or belt holster; and not sleeping with the phone close to your head.
The two studies, involving 3,000 animals, are “the most comprehensive assessments of health effects and exposure to radio-frequency radiation in rats and mice to date,” according to a statement from the toxicology program, part of the National Institute of Environmental Health Sciences.
The studies extend partial findings released in May 2016, which found small increases in the incidence of tumors in the brains and hearts of male rats, but not female ones.
The new studies also found tumors in the brains and some other organs in the animals exposed to the radio-frequency radiation. But Dr. Bucher said those findings were “equivocal,” emphasizing that only the heart tumors provided evidence strong enough for the researchers to trust. Other possible effects need more research, he said.
Others felt that even the ambiguous findings were of concern. Joel M. Moskowitz, director of the Center for Family and Community Health at the School of Public Health at the University of California, Berkeley, said that based on the overall results of the study, the government should reassess and strengthen the limits it imposes on how much and what types of radiation cellphones can emit.
Scientists do not know why only male rats develop the heart tumors, but Dr. Bucher said one possibility is simply that the males are bigger and absorb more of the radiation.
The studies also found some DNA damage in the exposed animals, a bit of a surprise because scientists had believed that radio-frequency radiation — unlike the ionizing radiation in X-rays — could not harm DNA.
“We don’t feel like we understand enough about the results to be able to place a huge degree of confidence in the findings,” Dr. Bucher said.
A seemingly paradoxical finding that has also puzzled the researchers is that the rats exposed to the cellphone radiation actually lived longer than the controls. One possible explanation, Dr. Bucher said, is that the radiation may ease inflammation, and lessen the severity of a chronic kidney disorder that is common in aging rats and can kill them.
Asked if there was any chance that cellphone use could help people live longer, Dr. Bucher said: “The extrapolation to humans requires a number of steps that go beyond the realm of what we’re studying here. I don’t think that question is particularly answerable at the moment.”
The reports issued on Friday were considered draft versions released for public comment and a review by outside experts on March 26 to 28 at the environmental health institute in Research Triangle Park, N.C.