In recent years, people have claimed to suffer from a rather odd disorder. It takes the form of electrosensitivity, and its victims complain of headaches, nausea, and a multitude of other symptoms. All of these symptoms are supposedly due to the electric fields produced by power lines, cellphone towers and, now, the terahertz (THz) sources used in some whole-body scanners. At the extreme end of the scale, there are those who believe that these low-frequency electric fields cause cancer.
There are two problems with these claims: there is no known causative mechanism that goes from low-intensity, low-frequency electromagnetic radiation to sickness. The second problem is that epidemiological studies have shown that the claimed links are mostly spurious. That is not to say that the people who claim symptoms don't experience symptoms, rather the cause of the symptoms is simply unknown.
Nevertheless, and in contrast to my rather dismissive introduction, it is always worth trying to come up with potential causative mechanisms and explore those to see if they are realistic. Perhaps the best candidate mechanism for THz and cellphone radiation to cause damage is through mechanically driven vibrational resonances.
The frequency of this radiation overlaps with low frequency vibrations that occur in proteins and DNA. DNA, for example, is a helical structure of two strands that are held together by the relatively weak attraction of hydrogen atoms for partners on the base pairs that make up the genetic code.
Vibrational modes are the mechanical motions of the atoms and whole sections of a molecule—they occur within a molecule. The highest energy of these modes are the stretching and contracting motions of hydrogen atoms attached to carbon atoms. We then cascade down in energy, going through stretching motions between pairs of carbon atoms, and the bending and stretching of entire groups of atoms relative to other groups. Finally, in the case of DNA, we get the collective motion of one strand relative to another.
These fall squarely in the THz and GHz range. Since hydrogen bonds are weak, might it not be possible for a sufficiently strong THz source to simply shake your DNA apart? To answer this question, Alexandrov and colleagues put together a mechanical model of DNA and found that with the right excitation, they could, indeed, cause your DNA to explode—actually, they refer to it as melting, but my policy is to never let being correct get in the way of hyperbole.
The basic result is similar to that of the Tacoma Narrows bridge collapse. Under the unremitting forcing motion of THz radiation, the DNA hydrogen bonds collectively stretch and contract, until they simply end up too far from each other, causing the DNA molecule to unzip. If this happened in a cell, it would in all likelihood cause problems.
But these results really weren't considered definitive because these models rely on a lot of parameters that relate things like how far apart molecules are and what the spring-like potential between them is. A rather suspicious number was the amount of force the researchers calculated was required to melt the DNA. Yet, other parts of the paper seemed to suggest that the DNA melting might occur spontaneously due to thermally generated fields and forces within a cell. It all just seemed a little... forced.
Swanson, from the University of Pittsburgh, decided to investigate the model more thoroughly. He began with the work of Alexandrov and friends, and showed that the bare model would, with the input of a small pulse of radiation, generate vibrational modes that melted the DNA. Having replicated the results of previous work, he then showed that this only happened for a very small parameter range. Indeed, if you simply take into account that different base pairs have different bond strengths, it was enough to damp the oscillations and save all life on Earth.
Having examined the parameter sensitivity, Swanson then looked at the probability, under ideal conditions, of a single skin cell's DNA melting. He came up with a number that is, you know, small: 10-40 per person per year. I think I will take those odds. He then calculated the amount of force a THz source that fell within legal irradiation limits would apply to DNA and came up 6 orders of magnitude short of the force calculated by Alexandrov and coworkers. And that was under optimal conditions.
Finally, he went on to note that, in a cell, DNA is subject to things like charge screening, which would reduce the force further—not to mention that water is a great absorber of THz radiation. To his considerations, I would also add that, normally, DNA is not free in a cell; it is usually wrapped up tightly in proteins, so it doesn't have much freedom of motion to vibrate. Indeed, it is probably more important to model the response of the protein than it is the DNA.
There is also one aspect to this that seems to have been missed. If the mechanism here would have been realistically possible, it would have been likely to destroy a cell. But dead cells don't carry mutations and thus are unlikely to cause cancer.
Physical Review E, 2011, DOI:10.1103/PhysRevE.83.040901
Physics Letters A, 2010, DOI:10.1016/j.physleta.2009.12.077
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