Backscatter is set up so that the majority of the energy is deposited in your skin. The problem comes from the fact that most of the data we've collected is with X-rays that deposit their energy evenly throughout your body's volume. The heuristics based on that data erroneously suggest that skin is one of the least cancer prone tissues. However, we know on the basis of microbiology that this isn't the case. It's only because the skin constitues a small fraction of the absorption cross section of the entire body that biases the numbers this way.

This is a "spherical cow" assumption applied by self-interested bureaucrats. Such an oversight wouldn't be allowed in a PhD thesis defense. Why is it allowed when applied to the health and well being of hundreds of millions of people?

Yes, the dose is low, but the geometry of the delivery is radically different. For something that affects so many, more testing is warranted.

The only real health concerns are for the workers who are exposed to them continuously.

I agree that that's also a major issue.

But instead of arguments, we can look at the evidence. The FDA is notoriously risk-averse, but it has done studies on the safety of backscatter devices and concluded they're safe: http://www.fda.gov/Radiation-EmittingProducts/RadiationEmitt...

According to the FDA, the dose to the skin is 89,000 times lower than the annual limit. If backscatter scanners caused harm, it would tell us something new about physics, biology, or both.

Are backscatter scanners an invasion of privacy? Yes. Are they a waste of money? Almost certainly. Do I wish airports got rid of them? Yes. But unless our understanding of physics and/or biology is very wrong, they're not dangerous.

Alpha particles only travel a few cm in air. Beta particles may travel around a metre. Unless someone works with radiation, I cannot imagine how anyone's skin is constantly bombarded by these types of particles.

Also, any form of ionising radiation is generally accepted to be A Bad Thing. To suggest that it could possibly be good seems ludicrous. Even UV rays are generally accepted to be harmful and I'm sure most people would understand that they're supposed to limit their exposure.

Edit: Or did you mean to say that dumping it in the skin might be better ('a good thing') compared to spreading throughout the body? Either way, it's not a medical procedure so there should be no need to use it at all imho.

Your interpretation is correct. I'm with you: all else equal, I'd prefer to be exposed to no extra ionizing radiation. I was just pointing out that many of the health arguments seem like motivated cognition to me, and showing how one can be reversed.

>Unless someone works with radiation, I cannot imagine how anyone's skin is constantly bombarded by these types of particles.

I meant that our skin is constantly bombarded by ionizing radiation, not necessarily that it's constantly bombarded by alpha and beta particles. I'm not sure how much alpha or beta dose the average person gets. While the particles themselves don't travel far, they can be formed in many ways. As klodolph has said, radioactive decay is one source. Alpha and beta emitters show up in some surprising places: smoke detectors, anti-static coatings, and emergency lighting systems.

UV rays in sunlight are used by the body to synthesize Vitamin D - they can be good for you.

Our understanding of the relationship between low-level doses of radiation and cancer is frighteningly incomplete -- to the point where it's not even certain that the cancer rate doesn't go down when low level radiation increases. This is nothing more than an appeal to authority -- the FDA makes the policy, not the science.

Our understanding of the relationship between low-level doses of radiation and cancer is frighteningly incomplete

This is what especially steams me when I talk to "experts" about this. (In quotes because what most doctors know about radiation and cancer is pretty shallow in the grand scheme of things.) It's like all they hear is "someone afraid of radiation" and it's like they can't even comprehend the geometric argument I'm making and then go on to quote data and heuristics based on the wrong energy deposition geometry. Also, they tend to talk about the relationship between cancer and radiation dose as something that's well understood. Well, maybe it is for chest X-rays and other specific situations for which we have lots of data, but this situation is different from those in an interesting way -- one which should bring questions to a scientific mind.

Where the experts have a mental blind spot such that they can't even find or formulate a cogent argument that actually applies to what you're saying, there might be something that needs to be looked into.

Why do we think know that? Because of data from conventional X-rays that have a totally different geometric assumption. (Energy distributed throughout the body volume.) What we know of cancer at the level of mechanisms inside the cell suggests that the living part of skin that constantly divides to replace cells is more prone to cancer than other tissues.

> The FDA is notoriously risk-averse, but it has done studies on the safety of backscatter devices and concluded they're safe

The web page you pointed to just mentions "low dose" and compares the dose to "Naturally occurring ionizing radiation." Well, duh, I know this already. Most naturally occurring ionizing radiation deposits its energy evenly throughout the body. It doesn't address the new geometry of energy deposition. Show me one study that does! Show me one government document that does!

Again, it's "spherical cows" assumptions applied to public health! As a technically adept person, this should raise your suspicions! I'll grant you that the risk is probably low, but without more research into the same energy deposition geometry, you're handwaving public health.

* backscatter cancer +"geometry of the delivery"

* backscatter cancer +"geometry of delivery"

* backscatter cancer +geometry

I didn't find that's how anyone talked about the risks, and so on that basis alone, I found your argument more compelling. However, I did find information that furthers the anti-backscatter argument in the same way you argue the pro, that is quantitative studies:

"research studies have concluded that a small number of cancer cases would result from scanning hundreds of millions of passengers a year."

http://www.scientificamerican.com/article.cfm?id=europe-bans....

I looked further to try and find studies that the SA article vaguely mentioned, and I found this one by Columbia University: http://www.columbia.edu/~djb3/papers/radiol7.pdf

This paper doesn't talk about the geometry of exposure, but it does say that while the machines are basically safe, it also uses statistics to show that:

* "there will be some cancers induced by the associated radiation exposure" (billions of scans means definitely some cancer)

* "risks will be somewhat higher for children, for radiosensitive individuals, and, particularly, for aircrew and frequent fliers."

* these machines may cause cancers cause by malfunctions

So, if you believe the machines will kill a few hundred people or the terrorists will kill a few hundred people, shouldn't we just save our money, and our liberty? I don't think it's just an inconvenience - I think it's a bunch of needless cancers.

I know what a sievert is (your definition is incorrect) and I know different types of ionizing radiation affect different tissues differently. But the safety margin is so great (89,000!) that it would take a new mechanism of action for such a tiny dose of X-rays to cause disease.

Edit: Because you deleted your comment, other readers now have no clue what I responded to. :/

No, that's factually incorrect. You're thinking of the gray (Gy), which is 1 J/kg no matter what. 1 J of gamma absorbed is 1 Gy and 1 Sv, but 1 J of alpha absorbed is 1 Gy and 20 Sv. 1 Sv of neutrons is only 0.1 Gy.

It doesn't help that they are dimensionally equivalent, so there's a proliferation of unit converters which consider the conversion to be 1.0 always.

> Unless someone works with radiation, I cannot imagine how anyone's skin is constantly bombarded by these types of particles.

Depends. Some people have granite countertops in their kitchens which give off Radon, an alpha emitter. (Not all granite does this.)

Or to flip it around, I opt out because I happen to know something that the TSA doesn't: I pose no risk to the aircraft, and scanning me in no way makes the aircraft safer. I can therefore assert with absolute certainty that my opting out poses less risk to the travelling public than being scanned poses a risk to me. (Plus, I think I might secretly enjoy TSA agents awkwardly and apologetically rubbing the backs of their hands on my crotch.)

If those brief, minimal x-ray scans we get during TSA searches is worth considering, then I wish someone would do something about shielding us while we're flying, and getting a great deal more radiation...

We take risks every day and that's completely okay as long as the benefits make up for it. I'll take a 0.1% chance of dying for a million dollars, but I won't take it for a sandwich; I'll take a bit of radiation to fly, but not because of a half-assed security mechanism.

It's a classic false dichotomy. Specifically, a person can care about both issues simultaniously.

The problem is that we're not producing the radiation in the air -- it's already there. Radiation shielding is heavy and infeasible for use in current airplane technology. It's a very, very hard problem.

BackScatter scanners, in contrast, are optional. They are completely and utterly unnecessary. It's very easy to get rid of them; indeed it would have been easiest and cheapest to never have forced them upon an an unwilling community of travelers.

But you get _nothing_ back by going through the TSA scanners except getting your cells blasted by energy enough to mutate your DNA. Yes you get the same in flight, but you are getting some place pretty darn fast as a trade-off.

To continue the analogy. Imagine you go through a machine every morning that has a spring loaded hammer when you exit you house to leave. Most of these machine will do nothing to you, but every rarely those machine are known to release their hammer and smash limbs or cause injury otherwise. That is exactly the equivalent of TSA scanners. You get nothing back for the risk of injury.

What I was trying to (and failed to) do was focus on the fact that I'm getting radiated while flying - and I don't ever recall that being talked about in much detail, and certainly not by any airline staff. For example - do I get more radiation from an open window? Do they make window screens that can block radiation? Is there any research into reducing radiation exposure while flying? In much the same way we take a risk while driving, but now have Air Bags, ABS Breaks, tempered glass, break away wheel columns, etc... - I'm wondering if we're putting the same effort into the radiation risk while flying.

I just never heard much about it until the TSA added the backscatter machines.

Since you are part of the traveling public, you can further simplify your sentence to: "my opting out poses less risk to the travelling public than being scanned". That's like a 5-word trim! :)

I fly constantly for work, and I've watched the number of people opting out slowly drifting down till it feels like, well, just me.

If I'm getting my rights violated, I'm at least getting a reach around for it. Plus, even though I know it's terribly childish, being able to ask if they're happy that they grew up to make a living by touching my balls gives me a little bit of false empowerment in a crappy situation.

Ya, just can't get that satisfaction when walking through a scanner.

FWIW, we have a friend who sells mw equipment, and he refuses to go through them for safety reasons. So even if I weren't opting out for moral reasons, I wouldn't go through them.

* I'm sure glad no terrorists are smart enough to do this.

He should probably stop selling them then.

I've been given the intimidation routine ("You know this means I'm going to touch your private areas," and "Do I need to call a supervisor?") and the downplay routine ("it'll only take 3 seconds and it's completely safe"). It's always in that order. It's always the same; send the bully then send the supervisor. It take 5 minutes to go through that. I've taken to simply standing in silence with my arms outstretched and legs apart.

Everywhere else is easier.

"Call someone, because as is my right I would like to request a private screening room, please."

Flying out of SFO (where they have MMW machines, at least in the terminals I use), there's another person opting out about half the time. If I get a chance, I'll ask them why. It's almost always a matter of principle, as it is with me.

Bravo. I'm going to use that.

"So wait, 3oz of fluid in a 3oz container is okay.. but 3oz in a 5oz container is not? Is it more dangerous for some reason?"

#fuTSA

>"So wait, 3oz of fluid in a 3oz container is okay.. but 3oz in a 5oz container is not? Is it more dangerous for some reason?"

As much as I dislike the current set of rules, I feel that if you going to have rules, then you need to make rules which are enforceable. "Containers 3 oz or less" is an enforceable rule[1]. Is it dumb? Yes, because a 3.1 oz container is no more dangerous (certainly not more than around 3% more dangerous) than a 3.0 oz container. "Containers with less than 3 oz of liquid" is not not an enforceable rule. How can the TSA person quickly assess the volume of liquid in the container? He or she can't, which is the problem. You could, of course, give the TSA agent the discretionary authority to allow larger containers with "about 3 oz of liquid," but that's just begging for an equal-protection lawsuit.

[1] Actually, it's not, because, as far as I can tell, the TSA determines the volume of a container by the declared volume on the label. If you really want to bring 5 oz of shampoo, you could probably just make a funny-shaped bottle with a label that says "Shampoo, 2.9 fl oz". Though it would be pretty easy to measure the volume of the entire container via water displacement ("Mr. President, we cannot allow... a thick-walled-container gap!")

I one brought through security the remains of a soft-drink with ice cubes, that I'd transferred into a bottle. They made me drain the liquid that had melted, but the ice-cubes themselves weren't a problem.

If I'm by myself or somehow end up with body scanner as the only option, I always opt-out. I guess part of the theater is designed to embarrass me, but I wonder why I should be more embarrassed than the dude squatting down below me to give me the feel in front of everybody (and no, I don't want a private screening, get down there and feel me up right here in the open). Like someone else suggested, I think they're as embarrassed as I am (or am supposed to be).

Even in hospitals, which have much stricter standards of safety and training have had accidents[1] where patients have been administered overdoses of radiation (some to lethal effect).

Just give the TSA another 10 or 20 years.. I would not be at all surprised to hear of major lawsuits from travelers who've been seriously injured by these machines, due to TSA incompetence in purchasing junky machines, maintaining/calibrating them poorly, or not training their employees to use them properly.

[1] - http://www.nytimes.com/2010/12/29/health/29radiation.html?_r...

http://en.wikipedia.org/wiki/Fourth_Amendment_to_the_United_...

The US Court of Appeals disagrees.

FYI in the US, the boarder is defined as a 100 mile wide swath of land around any actual boarder. Live within 100 miles of a cost line? Congratulations, the 4th Amendment boarder exception applies to you.

http://www.aclu.org/technology-and-liberty/fact-sheet-us-con...

These machines are none of those things. Why should I give them the benefit of the doubt?

At the very least, people in hospitals tend to stay there longer. The radiation burns from Therac-25 took up to several days to become visible. Any damage that these machines may cause is likely going to be much more subtle, making the situation even worse.

Is that what you wish to imply?

A risk, no matter how unlikely or small, is never worth taking if there is no payoff.

But we really don't know that, because no studies have been done using this equipment. In addition to concerns that the radiation, while lower in total than that absorbed during a flight, is absorbed over a much smaller volume of tissue, adequate validation that the machines can't break in dangerous ways hasn't been performed (and even the most basic detection mechanisms for failure such as having the operators wear dosimeters have been neglected or banned outright). It violates every principle of radiation safety.

The little bit I've seen came from a recent visit to a small regional airport in a swing state which I'll keep anonymous. This airport serves ~4000 passengers per day. The woman had a piece of paper where she kept tally of opt-outs. At mid-day there were ~40 opt outs (including mine). So 80/4000 = 2% opt-out rate I'd say is higher than I was expecting!

Did I parse that right?

This is possibly only true if there are no problems with the machines. If the machine is functioning properly, an X-ray beam will quickly scan you from head to toe. But if the scanner gets stuck (which happens from time to time), all of the dosage is concentrated in a single part of the body.

The question I have is this: Are the machines preventing 6 to 100 injuries or deaths from a terrorist attack every year?

If so, then perhaps the extra cancer cases are worth it. But if not -- if the scanners do not reduce the risk of terrorist attacks enough to offset the very small increase in cancer cases, then the machines are a waste.

Aren't you still being searched by hand when you opt out, though? It that much different?

That, and I flatly do not trust these machines and who's administering them. It's a matter of when, not if someone (or lots of someones) gets a bad dose of radiation through these things.

1) this level of dose is thought safe if absorbed throughout the body; 2) there is no consensus on whether this level is safe if absorbed solely in the skin; it sounds like the question simply has not been studied.