The results are not conclusive, and the overall relevance to human cell phone use is not at all certain. While animal studies can validate results of previous observational studies in humans, previous human studies have been inconsistent or only suggested at an association. The three largest studies used cell phone use data collected by questionnaire or cell phone bills. The largest study, the “Interphone” study was connected by a consortium of researchers in 13 countries. According to the CDC one analysis of the data found a statically significant, but modest increase in the risk of glioma among the participants who reported the highest cell phone use.
The “Danish Study” linked billing information from 358,000 cell phone subscribers with brain tumor incidence data from the Danish Cancer Registry and found no association with the incidence of glioma, meningioma or acoustic neuroma for the time frame of the study (according to the summary on the CDC website).
Finally, the “Million Women Study” was a study conducted in the United Kingdom using questionnaires. Though the original published findings reported an association with an increased risk of acoustic neuroma, this association disappeared after additional follow-up of the cohort.
Those studies, which track large groups of people over time, can look for associations between how many hours people said they used cell phones every day and the incidence of cancers in those people, but they can't prove a cause and effect relationship. In addition, the accuracy of recall studies are limited by recall bias, inaccurate reporting, morbidity, mortality, and participation bias, duration of exposure and finally changing technology. In less than 20 years radiation frequencies and signal modulation used in cell phones have gone from 2G (GSM or CDMA) to 3G (such as UMTS or CDMA2000) or 4G (LTE), which may have lower power outputs and different signal modulation.
Until now, the U.S. government’s official position is that the weight of scientific evidence hasn’t indicated health risks. Cell phones have been believed to be safe because scientists thought that the radiation from cell phones could only damage cells by heating human tissue. The Federal Communications Commission’s cell phone emission test ensures that the emission limit the heat output from the radio frequencies. However in 2011, scientists at the NIH found that low level radiation, held close to the head, could alter brain cells without raising body temperatures; the World Health Organization declared cellphone radiation was a group 2B possible carcinogen, but so is coffee and shift work. Then in 2015, German researchers reported that the same type of radiation emitted by cell phones could promote the growth of brain tumors in mice without raising body temperatures.
In the current study, scientists studied the effect of 2G (GSM or CDMA) radio frequency on rats over a two year period and compared the results to a control group of similar non-exposed rats. The NTP study controlled for heating, keeping the body temperature of the rats within 1° C (1.9° F. The rats were exposed for nine hours total each day, at intervals of 10 minutes on, 10 minutes off, for two years.
The researchers found that male rats exposed to the radiation had a higher incidence of two kinds of tumors. The tumors were gliomas, which are in the glial cells of the brain, and schwannomas of the heart. The association was not found with female rats.
This study raises more questions, but answers few. Consumer Reports has been concerned about cell phone radiation for years and did an excellent job of reviewing the available report. Consumer Reports recommends that you:
- Try to keep the cell phone away from your head and body especially when the signal is weak because phones may increase their power then to compensate.
- Text or video call when possible
- When speaking, use the speakerphone on your device or a hands free headset.
- Don’t stow your phone in your pants or shirt pocket.