Thursday, December 16, 2010
Dr. Mittleman at Rice University, provides a further discussion on the health effects of THz radiation
I'd be happy to write a short comment on the topic of the health effects of THz radiation. I'm certainly not the expert on this topic, as it has not been a focus of my own research in recent years. But I've kept up on it and can speak somewhat knowledgeably about what's known.
The key question here is whether irradiation by low-frequency electromagnetic waves can induce biologically significant effects in living cells, other than merely by heating them up. It should be pointed out that this question (and probably the answer) are just as relevant for microwaves as for terahertz radiation. All of these low-frequency waves are strongly absorbed by water. Since all biology happens in water, it's clear that we can affect cells (or even cook them) by putting enough energy into the surrounding water.
The hard part is determining if anything happens to the cells that is NOT merely the result of the water heating up. And that's the important question, from the standpoint of setting safety standards. If the only cell damage mechanism is thermal, then the minimum exposure limit for THz radiation would be set in the same way that it has been set for microwaves - we just avoid cooking people. On the other hand, if cell damage can occur at a lower power density via some other mechanism, then the exposure limits would have to be correspondingly lower.
The conventional wisdom in the physics community is that there simply cannot be non-thermal effects. The energy necessary to break a chemical bond is hundreds or thousands of times larger than the energy of a single terahertz photon. Thus, ultraviolet or x-ray radiation can lead to genetic mutations (think: too much sun exposure leads to melanoma), but radiation at lower frequencies (with lower photon energies) does not. If that conventional wisdom is correct, then the only danger from T-rays (or microwaves or radio waves or infrared radiation) is that it can induce thermal effects.
There have been a couple of proposals for mechanisms that would permit non-thermal effects from low-frequency radiation, most notably the recent paper by Alexandrov and co-workers (from Physics Letters A in 2010). This paper got a lot of press when it came out. It's a wonderfully inflamatory subject, after all. But there are many reasons to believe that the analysis in that article (which, after all, contains no experimental evidence) is woefully over-simplified, and therefore completely inapplicable to the real world. Most glaringly, the description neglects the absorption of THz radiation by the water bath in which the DNA chain resides. That's like being worried about the effects of sunlight on your brain tissue - if you ignore the opaque skull that entirely surrounds your brain then you might actually need to consider that question, but for those of us with skulls it isn't exactly a pressing concern. In my view, there is still no feasible proposal for a non-thermal mechanism.
Having said that, I do think that there is some experimental work that needs mentioning. I refer specifically to the recent work from the lab of Dr. Peter Siegel at CalTech (more precisely, NASA JPL). In these experiments, a collection of cells were irradiated with millimeter waves (not quite THz radiation), and certain changes were observed (e.g., induced cell poration, modulation of neuronal activity, etc.) These folks were as careful as possible to monitor local temperatures and eliminate the possibility of thermally induced effects. I would not call these results conclusive (neither would the authors), but they are suggestive that more work is needed.
If these experimental results are correct, then the burden will still remain upon the researchers to come up with an explanation of how it's possible. As far as I know, they've not even tried to explain anything yet, merely to observe and see what's going on. I have great respect for Peter, but I remain skeptical simply because there is no plausible mechanism to explain non-thermal effects.
Dr. Daniel Mittleman
ECE Dept., MS-366
6100 Main St.
Houston, TX 77005
Biographical InformationDr. Mittleman received his B.S. in physics from the Massachusetts Institute of Technology in 1988, and his M.S. in 1990 and Ph.D. in 1994, both in physics from the University of California, Berkeley, under the direction of Dr. Charles Shank. His thesis work involved the spectroscopy of semiconductor nanocrystals using laser pulses with durations of less than 20 femtoseconds, at wavelengths from 480 nm to 670 nm. He then joined AT&T Bell Laboratories as a post-doctoral member of the technical staff, working first for Dr. Richard Freeman on a terawatt laser system, and then for Dr. Martin Nuss on terahertz spectroscopy and imaging. Dr. Mittleman joined the ECE Department at Rice University in September 1996, where he is a Professor. At Rice, his research interests involve various aspects of spectroscopy, sensing, and imaging using terahertz radiation. Dr. Mittleman is a Fellow of the Optical Society of America.
Click here to see an up-to-date list of publications from the Mittleman group.
MY NOTE: THANK YOU VERY MUCH DR. MITTLEMAN FOR TAKING TIME TO HELP THE LAY COMMUNITY BETTER UNDERSTAND THz!