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With all due respect to Dr. Reiz, and the Dept. of Physics at ASU, I think I'll be taking my advice from the group of doctors from UCSF (a school well known and respected for its medical research) who wrote a formal letter to the president's science advisor. The letter is worth reading.

"Agard and several of his UCSF colleagues recently wrote a letter to John Holdren the president's science adviser, asking for a more thorough look at the risks of exposing all those airline passengers to X-rays. The other signers are John Sedat, a molecular biologist and the group's leader; Marc Shuman, a cancer specialist; and Robert Stroud, a biochemist and biophysicist."

Story here: http://www.npr.org/templates/story/story.php?storyId=1268330...

Letter here: http://www.npr.org/assets/news/2010/05/17/concern.pdf




I'll go with the physics prof. He did some calculations, admittedly imperfect, based on generally known principles of dosimetry, and got what seems like a reasonable order of magnitude result.

All the MD's at UCSF said was "oh noes, the possibility of radiation danger isn't ruled out precisely by a previously done study, it might be dangerous." I'd be very curious to know whether they would use this same logic on a less politically charged topic.


Even on non-politically-charged topics, that is pretty much the standard operating practice, at least in the more science-oriented parts of medicine. If you want to know, "is this practice safe?", you run a study to find out, rather than speculating from first principles (though you can use speculation from first principles to come up with reasonable hypotheses and narrow down how to best design a study).


Reasonable doesn't necessarily mean correct. In the face of specific questions raised by doctors with expertise in this specific field, is reasonable is good enough?

From the NPR article:

"The stated dose — about .02 microsieverts, a medical unit of radiation — is averaged over the whole body, members of the UCSF group said in interviews. But they maintain that if the dose is calculated as what gets deposited in the skin, the number would be higher, though how much higher is unclear."

The observation about how high a dose gets deposited is a very specific observation made by people who actively apply radiation to people on a daily basis.


In the face of specific questions raised by doctors with expertise in this specific field, is reasonable is good enough?

You are trying to appeal to authority. That's a logical fallacy.

Radiation transport, ionization mechanisms, and dosimetry are all fairly well understood topics. Not perfectly, but well enough that one can make reasonably good ab-initio predictions. The physicist is doing exactly that.

Now, ab-initio dosimetry calculations are sometimes off by largish factors, (e.g., 500%) due to reactions of certain tissues. But they are rarely off by orders of magnitude, which they would need to be in this case for us to actually care.

The doctors at UCSF are just scaremongering, regardless of their credentials.


Radiation transport, ionization mechanisms, and dosimetry are all fairly well understood topics. Not perfectly, but well enough that one can make reasonably good ab-initio predictions.

There are a lot of people that would disagree with this statement as it applies to the human body. I think its fair to say that there is no such thing as ab-initio studies when it comes to human reaction to radiation (at least none that are worth a lick).


"The doctors at UCSF are just scaremongering, regardless of their credentials."

Thats a very blithe statement to make about a group of professionals.

Here's the thing -- I know several people who have been treated for cancer at UCSF, and my understanding is that you couldn't ask for a better group of doctors/scientists. Their expertise is very well-regarded. So if taking the advice of a doctor who has made this his/her life's work is appealing to authority, then guilty as charged. If I'm going to try and make the best decision I can without understanding the topic as well as you seem to, I'm certainly not going to chose the people who generally work in the field over the the people with real medical experience. My plumber is a very smart guy who knows a lot about electrical, but I'm not going to hire him to re-wire my house. The devils is in the details, and I'm going with the guy who has a very good grip on the details.


can somebody please down vote him? (i'll pay back with interest when i get my downvoting ability :)


This part of their letter doesn't make sense:

Unlike other scanners, these new devices operate at relatively low beam energies (28keV). The majority of their energy is delivered to the skin and the underlying tissue. Thus, while the dose would be safe if it were distributed throughout the volume of the entire body, the dose to the skin may be dangerously high.

The main issue is that the limit for a smaller volume of the body is actually greater than the body as a whole. Radiation dose is somewhat similar to a density, in that it's measured in energy deposited per mass. Your body is better at dealing with a high dose in a small volume, than it is with a low dose in a large volume.

...these new airport scanners are largely depositing their energy into the skin and immediately adjacent tissue, and since this is such a small fraction of body weight/vol, possibly by one to two orders of magnitude, the real dose to the skin is now high.

This statement doesn't make sense either. Dose is J/kg, so the fact that the volume (read mass) is small is taken into account.

This part sounds the most interesting:

In addition, it appears that real independent safety data do not exist. A search, ultimately finding top FDA radiation physics staff, suggests that the relevant radiation quantity, the Flux [photons per unit area and time (because this is a scanning device)] has not been characterized. Instead an indirect test (Air Kerma) was made that emphasized the whole body exposure value, and thus it appears that the danger is low when compared to cosmic rays during airplane travel and a chest X-ray dose.

The problem with this statement is that Air Kerma is the quantity that you'd use to calculate the dose. It is only measured indirectly in the sense that you can set the detector to have the conversion coefficients built in (you actually measure the quantity Exposure).

It seems that the letter was probably written by people with concern, but not expertise. I just know enough to see some problems.




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