The past two decades have brought rapid advances in technology that have greatly influenced our daily lives. But have these technologies introduced new health risks viagra professional along with new conveniences? Unfortunately, the development of electronic gadgets seems to be outpacing the development of techniques for measuring their effects on our bodies. As a result, it remains difficult to directly test if a new electronic device is dangerous. These limitations have been particularly apparent in our continued efforts to determine whether cell phones harm our brains. Most recently, a study showed that cellular phones result in an increase in brain glucose metabolism on the side of the head where the user is holding the phone1. This finding attracted a great deal of attention from mainstream media2, but no one is certain how to interpret the results. This begs the question: why are these findings difficult to understand, and why have we been unable to conclude once and for all whether cell phones are safe or harmful?
Why are people concerned about cell phones?
Cell phones are everywhere in the United States, with 96% of the population using cell phones as of December 20103. But these familiar devices provide more than just constant phone and email access; they also emit energy waves known as radiofrequency-modulated electromagnetic fields (RF-EMFs). The term EMF describes the energy waves generated by electrically charged objects. An EMF can be described as a wave; thus, an RF-EMF, also referred to as a radio wave, is an EMF with a particular wavelength. We also use EMFs in other technologies: microwaves use EMFs similar to those used by cell phones, while X-rays use EMFs with much shorter wavelengths. So far, we know how to accurately describe the physical properties of EMFs. Yet, we are still uncertain how particular types of EMFs, such as the radio waves emitted from cell phones, affect human health.
One noticeable effect of radio waves is the transfer of heat. We take advantage of this property when we use a microwave. However, the minimal amount of heat released by cell phones is unlikely to pass through the skull and heat the brain to damaging levels4. Instead, if cell phone radio waves affect the brain, it is more likely to result from their ability to affect the charged particles. Our brain works by using charged particles to activate cells, and so cell phone radio waves have the potential to alter brain activity by affecting the charged particles in our brains. But do cell phones emit enough radio wave energy to influence brain activity?
Why are we limited in determining whether cell phones harm the brain?
Some animal studies have shown adverse effects of radio waves on the brain5, but it is unclear what these findings mean for humans. So far, direct studies on humans have been inconclusive. This is because we currently lack the tools to study human brain activity with the same detail that we are able to study brain activity in other organisms (such as non-human primates, rodents, and flies). Studies of the human brain are limited to non-invasive technologies (technologies that do not involve opening the skull) such as functional magnetic resonance imaging (fMRI), electroencephalograph (EEG) recording, and positron emission tomography (PET). These tools enable us to obtain coarse measurements of brain activity, but they lack the precision and sensitivity of the more invasive methods used to study the brain activity of animals in the laboratory.
EEG recordings are measured using a group of electrodes that are positioned on the surface of the scalp. These measurements are useful in determining when brain activity occurs, but they offer little indication of where the activity occurs within the brain. Importantly, EEG recordings can only be used to detect brain activity near the scalp. Therefore, they cannot be used to detect changes in more internal regions of the brain that might be affected by cell phone radio waves.
Two commonly used brain imaging techniques are fMRI and PET. Unlike EEG, these techniques provide information about the location of brain activity. Unfortunately, fMRI and PET cannot tell us precisely when brain activity occurs; these methods detect activity over the course of seconds, while the brain operates on a millisecond timescale. Importantly, neither approach directly measures the activity of brain cells. fMRI detects changes in oxygenated blood flow throughout the brain, with the idea that active brain cells will require more oxygenated blood than inactive brain cells. PET detects areas of increased uptake of sugar (in the form of glucose) from the bloodstream, which cells use for energy. When cells become more active, they require more glucose, and so regions with increased glucose uptake may correspond to regions of increased brain activity6.
Currently, it is not certain precisely what fMRI and PET measurements tell us about brain activity. Because fMRI and PET provide indirect measures of brain activity, they could also reflect cell processes that require energy but do not qualify as brain activity. Until we better understand the relationship between fMRI and PET and the activity of brain cells, we cannot be certain what these imaging techniques convey about the brain state.
What do we know about the effects of cell phone radio waves so far?
Because of the limitations of current imaging techniques, studies of how cell phone radio waves affect the brain have been difficult to interpret. So far, we know that the brain can absorb radio waves emitted by cell phones7, and the wavelengths absorbed are within a range that could alter brain activity8. The study mentioned at the beginning of this article used PET to show that cellular phones result in an increase in brain glucose metabolism on the side of the head where the user is holding the phone1 (Figure 1). However, we cannot conclude much from this finding because, as described above, it is not yet possible to determine what increased brain glucose metabolism indicates about brain activity. And so, questions remain as to what, exactly, is being altered in the brain when it is exposed to cell phone radio waves.
Figure 1. PET image from one human brain. The subject was holding two cell phones, one on each ear. In the “Cell phone on” condition shown in the left part of the figure, only the cell phone on the right ear was active. The white arrow points to a red area of the image that indicates an increase in the rate of glucose metabolism on the side of the active cell phone. The left side of the brain, which was exposed to the inactive cell phone, does not show this same effect. Adapted from Volkow et al., 2011, Figure 2.
Together, studies of how cell phone radio waves affect the brain have thus far failed to determine whether there is or is not potential for harm. Until we learn more about how to measure human brain activity, testing the health effects of new electronic devices will continue to be a challenge. Advances in this area likely will come from animal studies, which allow for more invasive and precise measurements of brain activity.
Allison Baker is a graduate student at Harvard Medical School.
References
1. Volkow, N.D., Tomasi, D., Wang, G., Vaska, P., Fowler, J.S., Telang, F., Alexoff, D., Logan, J., and Wong, C. (2011) Effects of Cell Phone Radiofrequency Signal Exposure on Brain Glucose Metabolism. JAMA 305(8): 808-813.
2. Murphy, Kate. “Cellphone Radiation May Alter Your Brain: Let’s Talk.” New York Times 30 March 2011: B9. Print.
3. CTIA, The Wireless Association. Consumer info. <http://www.ctia.org/consumer_info/index.cfm/AID/10323>
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5. Hyland, G.J. (2000) Physics and Biology of Mobile Telephony. Lancet 356(9244): 1833-1836.
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7. Schönborn, F., Burkhardt, M., Kuster, N. (1998) Differences in energy absorption between heads of adults and children in the near field of sources. Health Phys. 74(2): 160-168.
8. Kleinlogel H, Dierks T, Koenig T, Lehmann H, Minder A, and Berz R. (2008) Effects of weak mobile phone–electromagnetic fields (GSM, UMTS) on event related potentials and cognitive functions. Bioelectromagnetics 29(6): 488-497.