I've had a change of heart. After announcing previously that I would forgo additional back-and-forth from scientists about the hazards, or safety, of new TSA scanning machines, I've received enough interesting mail that I think I should offer at least one more installment.

The two original disputants -- a physics professor, and a biophysicist/enzymologist -- were both skeptical of the TSA's overall screening strategy. But they differed on which kind of machines might pose hazards, and why. Here are some additional views. If you read to the end, you'll see some distinct areas of disagreement -- and also some interesting similarities.

First, from a professor at a major research university:

>>I'm not sure whether the continuing lunatic exploits of the TSA have been driven more by terrorized bureaucrats or greedy lobbyists, but in either case I despair for the future.

I'm a tenured Professor of Theoretical Physics at [a major university], and I've been on the faculty for [several decades]. I'm not sure I agree with everything your physics correspondents have written over the past few days, but I certainly agree fully with one thing: In spite of the nominal radiation limits cited for the xray and submillimeter wave AIT machines, there is absolutely no assurance of the amount of radiation actually delivered to an individual subject. Perhaps if each person were issued a dosimeter print-out or a radiation technician were on the job full-time at each machine, there would be some reason to trust the technology. At present the TSA's attitude seems to be, gee, what a neat technology, let's radiate the entire population and look for problems later. The situation is bizarre, particularly in view of the stringent safety precautions mandated in other parts of American life. Um, thanks, but the radiation machines are not for me or my family.

By the way, I can think of multiple inexpensive ways to evade the strip search technologies, some of which I find were already discussed in the Journal of Transportation Security article. I hope that GAO teams, but not our enemies, are hard at work on exposing the flaws.<<  

I wrote back to this professor saying that I planned to quote his note -- and confirming that I should not use his name. He replied:

>>I guess it would probably be best if you did not mention my name or university.  I'm
not at all concerned as a faculty member, but I am also [an administrator], and I'm not sure I should comment openly on the TSA while I have the administrative job.

I've never found myself in the position of feeling so strongly on an issue and yet
hesitating to speak out because of my job.  You can be sure I admire pilot Chris Liu :)

More after the jump, largely on the question of how public agencies (like the TSA) and members of the lay public (like us, in the scanning queues) can reach sensible decisions about matters on which experts disagree.

From another physicist:

>>I too, am a holder of a PhD in Physics, and have to conclude that the professor/first writer is correct. If anything he/she is guilty of making the discussion accessible to laymen, which can leave one open to an attack on the details, but the points are largely correct.

My main point on writing though, is to plug Robert Park, who was cited by your first writer. He was head of the Department of Physics and Astronomy when I was a grad student at the University of Maryland. He now spends his time debunking junk science and writes a weekly column,http://bobpark.physics.umd.edu/index.html. I highly commend this to your attention. Alas, he has not weighed in on this particular topic but past writings leave me to conclude he would also support the first writer and also not go through a scanner.

One more tidbit, I am led to understand the scanners made by L3-Comm are supposed to be millimeter wave imagers, which would be safe. The Rapiscan scanners utilize x-ray radiation, which would be arguably unsafe. Also alas, you can't see which type is being used at the American terminal at Los Angeles International Airport.<<

Now, an organic chemist, on the difficulties of assessing "expert" views:

>>I enjoyed reading the "Scientist-on-Scientist" smackdown article. As it is, I have a Ph.D. in organic chemistry, so I was amused to read this quote from Scientist #2 (the biophysicist) on your post:

"Microwave energies can cause beaucoup molecular change; hell, I've used (um, well, I told my undergrad to use) a microwave source in an organic synthesis, so I'm real sure the emailer doesn't know what he's talking about. I mean, it can't _ionize_ (easily). Big whoop; most of biology is acid/base chemistry."

Our biophysicist has blundered into one of the mysteries of the last ten or so years in organic chemistry that has only recently been solved, namely: when you're running a chemical reaction in a microwave, it goes faster than if you were to use simple heating (a flame, or a electric warmer, etc.) Why is this? The simple answer is: faster heating, while the alternative answer is: it's the microwaves doing something special!

There was a lot of back-and-forth in the chemical literature until a professor in Germany made a microwave radiation-proof vessel (one that would be heated by microwaves, but would not let any of the microwave radiation into the reaction); the test reaction within showed no difference from a reaction that was heated via non-microwave means. It's covered here by the most influential chemistry blogger, Derek Lowe.

In a narrow sense, our biophysicist is confusing things. The "beaucoup molecular change" was happening because of heating, not because of the microwaves.

In a slightly broader sense, this doesn't make the "cell phones cause cancer" people go away. They're probably happy to tell you that the cancer could be caused by localized heating, which came from the microwaves in your cell phone.

In the broadest sense, this is part of the reason why science communication is so difficult: as you said, it is basically impossible for an average person to judge competing claims, especially by scientists with impressive-sounding credentials in fields that are slightly tangential to the actual matter on hand. Many of their arguments are from first principles of other sciences that they're not entirely expert in themselves: note the biophysicist talking organic chemistry, and the physics professor talking molecular biology.

This is why I haven't trusted any of the various people who've opined on the subject, because I'm waiting for an actual subject-matter expert:probably a professor of medical physics or radiation dosimetry at a major US medical school. Journalists, unfortunately, haven't decided to contact one of these folks; I don't quite know why.<<

From another physicist, on the challenge of talking rationally about risk -- especially when radiation is involved:

>>The recent back and forth about radiation from airport scanners shows how easy it is to get off track.

The average radiation dose we pick up each day walking around is about 6-12 microsieverts (don't worry about what a Sievert is, but you could look it up) from cosmic rays and crap in the ground, more if your basement leaks radon.  If you take a two hour flight, double that, because cosmic ray intensity increases with altitude.  The details can be found at a NOAA site -- and NOAA is interested in this, because solar flares can really up the dose, and it is actually an issue for aircraft crews, and esp for those in the Space Station.

The dose from a well adjusted scanner is 0.02 microsieverts.  Put into context the additional dose from the scanner is about 300 to 600 times less than the dose you will pick up from getting on the plane in the first place, if there are no solar flares taking place.  Assume that the machine was putting out ten times more X-Rays than it was supposed to, that is still 30 to 60 times less than the dose you are picking up by getting on the plane. 

Using the risk factor of 0.05 cancer cases per Sievert (see the NOAA web site) the risk to a person from a maladjusted machine would be 0.05 x 0.2 x 10^-6 = 1 in a 100 million.  For a well adjusted machine that would be 1 in a billion.  If you worry about that, don't cross the street. [This is from] a physicist who plays in chemistry and messes around with lasers, multiphoton processes, microwave generators, ionization and more.  Frankly the stuff you were fed is all spinach.<<

In the same vein:

>>Your physicist respondent is correct.

However, once the plane goes up to 35,000 feet the level of background radiation (mostly high energy gamma) will be far higher than that of the scanner.

If you are smart you will never go skiing near granite rock that often quite radioactive and also high up.

It is far better to live in a cave at sub zero elevation and only come out at night.

For a couple of million years animals and humans have evolved to manage radiation damage - natural levels of course.

Back in my graduate school days we used to get rid of carbon 14 spills by painting over them that was enough.

If C-14 gets inside you that is a different and more serious matter.

To avoid radiation better to take the bus or train.<<

And, similarly:

>>The issue of airport scanner radiation exposures (per your Dec 26 post) needs to be placed within the context of the reality that every single person passing through those scanners is already on their way toward an elevated dose of REMs for the day, by virtue of their flying at altitude. So, a double dose, in short order.
 
It will be an interesting epidemiological experiment, to be followed no doubt by an interesting legal experiment.<<

And to end, for now, with a flourish, here is a very detailed response to a point contested the first time around -- namely, what kind of radiation could inflict what kind of molecular damage:

>>Regarding the two principals, I do think they both have some merits in their arguments.

Here are the typical scales of strong chemical bonds and the weaker ones such as Hydrogen bond and van der Waals force.

Intramolecular force (strong chemical bonds)
--------------------
Ionic lattice        : 800 - 3000 kJ/mol ; 8.29 - 31.09 eV
Ionic bond         : 800 - 3000 kJ/mol ; 8.29 - 31.09 eV
Covalent bond  : 150 - 1000 kJ/mol ; 1.55 - 10.36 eV

Intermolecular force
--------------------
Hydrogen bond       :   1 -  155 kJ/mol ; 0.01 -  1.60 eV
van der Waals force :   2 -    9 kJ/mol ; 0.02 -  0.09 eV
As the physics professor pointed out, strong chemical bonds are in the UV range and thus vulnerable to UV or X-ray radiation. Peptide and amide bonds have a characteristic absorption peak between 230 - 190 nm, right in the middle of UVC band, and correspond to the energy level of 5.39 - 6.52 eV, just like any other typical covalent bonds. This suggests deamidation without the help of enzymic reactions or other extrinsic factors should behaves similar to typical covalent bonds.

The complicated part where our biophysics professor might be correct is hydrogen bonding and other (weak) intermolecular forces.

Hydrogen bonds is known to play an important role in the 3-D structures of proteins and nucleobases, and these specific shapes determines their physiological or biochemical roles.

In the secondary structure of proteins, the difference between alpha-helix and beta-sheet is determined by locations where hydrogen bonds are formed. Similarly these hydrogen bonds also plays a role in the tertiary structure of protein. Prion with its beta-sheet structure is of course the most well known example of mis-folded proteins.

The double-helix structure of DNA is another example with hydrogen bonds formed between the base pairs that link the two complementary strands.

The energy level of tens of meV up to eV for intermolecular forces corresponds to FIR MIF and NIR of EM spectrum, though not as small as microwaves (1 m - 1 mm ; 0.3 - 300 GHz ; 1.24 mueV - 1.24 meV ; 0.00012 - 0.12 kJ/mol) it is indeed much smaller than covalent bonds and UV or X-ray radiation that everyone is talking about.

Now, I don't know whether there is any hard evidence between low-energy radiation (500 meV to 10 meV ; 2.48 mum to 124 mum ; 121 THz to 2.42 THz) and protein mis-folding or poly-peptide structure alteration that may lead to any clinical significance but it seems at least theoretically possible to modify protein physiological or biochemical properties without breaking any covalent bonds or causing any mutation in nucleobases.<<

If you're still with me, there are more to come, but at the moment I'm worn out. I'll save the next couple of items for another installment, after which I will pronounce on the Public Policy Significance of it all. Sincere thanks to readers for taking such care in explanation.

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