Hacker News Reader

Comment by driverdan | Hacker News Reader

Backscatter doesn't pose any health risk to travelers. You get a much higher dose of radiation from the flight. The only real health concerns are for the workers who are exposed to them continuously.

X-ray, mm-wave, magic pixie scanners, I don't care what type they are, I'm still opting out because it's a blatant violation of the 4th amendment (when mandated, staffed, and managed by the gov).

I'll be celebrating when body scanners and other security theater is ended entirely. I'm not getting my hopes up.

View on HN

> Backscatter doesn't pose any health risk to travelers. You get a much higher dose of radiation from the flight.

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.

There's all kinds of FUD surrounding these devices, and it seems like many with health concerns are engaging in motivated cognition. For example: if the dose is concentrated in the skin, couldn't that be a good thing? Our skin is constantly bombarded with ionizing radiation. UV rays, alpha and beta particles, etc. When it comes to these sorts of insults, the skin is one of the most resilient organs.

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.

> "For example: if the dose is concentrated in the skin, couldn't that be a good thing? Our skin is constantly bombarded with ionizing radiation. UV rays, alpha and beta particles, etc"

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.

>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?

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.

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

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

> Yes. But unless our understanding of physics and/or biology is very wrong, they're not dangerous.

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.

> Yes. But unless our understanding of physics and/or biology is very wrong, they're not dangerous.

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.

> When it comes to these sorts of insults, the skin is one of the most resilient organs.

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.

I agree with your parent comment based on other information (details below), but I think his argument is wrong, or at least unsubstantiated. I was curious where his information was coming from, so I searched for some of his vocabulary. I googled for these things:

* 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.

It's not a "spherical cow" assumption. It's a scaling issue. Deposing radiation over the a membrane (the surface of the skin) is different to depositing it throughout a solid body. Please don't say "spherical cow", as it's a bad analogy.

As others have pointed out, there is little basis for your assertion that "backscatter doesn't pose any health risk to travelers." As my father (a physician) enjoys pointing out: we used to use X-rays to treat acne -- they were thought to not pose any health risk either! More generally, the history of technology is littered with arrogant overshoots where risks were dismissed until evidence became overwhelming; while one cannot conclude that backscatter is at a level to be dangerous, one can certainly not conclude that it "doesn't pose any health risk."

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.)

The radiation one received to treat acne was substantially more than the amount of radiation one received in the car ride to the doctor's office.

Then why does anyone have acne?

Because we don't zap people with lots of radiation to treat it anymore.

I'm completely comfortable with people who are concerned about the radiation we are getting from "Back Scatter X-Rays" - but I'm just wondering where they heck they were when trying to protect us from the radiation we get while flying?

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...

Yes, you are exposed to radiation while flying -- significantly more than when passing through body scanners -- but it's not a risk with no benefit: you're flying. Body scanners, on the other hand, are a risk (I'll grant, a tiny one) with absolutely no reward as they do nothing to actually increase the security of flights.

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.

Your reply reminds me of the post-9/11 "get some priorities" troll: http://everything2.com/user/NotBridgetJones/writeups/slashdo...

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.

And you also expose yourself to dangers of breaking a leg when you go on a hiking adventure. When you drive you also are very vulnerable. Every time you step into the shower you can slip, snap your spine or hit your head and remain paralyzed or dead. But you know what makes those things different than TSA? -- You get something back in return for that risk. You have fun hiking, you get to work faster, you get cleaned so you don't repulse friends and strangers with BO.

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.

Great analogy - and I'm sorry if I didn't make it clear that I am in agreement with those who are assessing our risk from TSA backscatter radiation, and trying to do an assessment of the risk/reward function.

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.

> my opting out poses less risk to the travelling public than being scanned poses a risk to me

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'm glad to hear that others still opt out.

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.

I opt out 100% of the time.

Me too. I make it an ordeal for the TSA agents by applying simple logic to their absurd rules:

"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

And one 6oz container isn't ok, but two 3oz containers is?

My favorite part is that empty bottles are always allowed, regardless of size. So you can have a six-ounce bottle with two ounces of liquid in it, and an empty three-ounce bottle, and that violates the rules, but if you pour the contents if the first bottle into the second, it's suddenly allowed, even though you have all of the same materials as before: bottles, liquids, etc. Totally irrational.

That's why it's called security theatre, not security.

Equally amusing - so long as a liquid is frozen, you can bring as much as you like in any size container, because now it's a solid.

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.

I think they are trying to make it harder for you to mix things.

Hogwash. I've taken quart-size lexan containers through security consistently, and then fill them with water on the secure side.

I think they are trying to avoid arguments over whether there's 2.9oz or 3.1oz of liquid in that 5oz bottle.

It's not that it's dangerous, or not, but "something must be done" when someone, somewhere, at some point, mis-used a liquid. It's not really meant to make sense.

I opt out, too.

>"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!")

My wife and I opt out every time. Whenever possible, we just aim toward the line that's using the old metal detectors* (since the new machines are either slow or prone to breaking, this is an option most of the time). But when the only option is the new machines (backscatter or mw), we opt out.

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.

"FWIW, we have a friend who sells mw equipment, and he refuses to go through them for safety reasons."

He should probably stop selling them then.

Free pre-flight massages. Why wouldn't you opt out?

Never thought of it this way.

I opt out 100% of the time. Instead of me just feeling violated (by going through the scanner) I think it's important that the TSA feels violated too. Most of the time the person doing the opt-out is humiliated as well. I also think it's important for Americans to see it.

The TSA at LAX really tries to talk me out of opting out. Maybe that's why.

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.

> "Do I need to call a supervisor?"

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

I've always preferred to do it in public. If there are any shenanigans, I want non-TSA witnesses.

I always refuse the private room as well, not so much because of the witnesses but since it seems to me to be a much more efficient protest to have American citizens see a guest in their country being molested by their security personnel.

Ditto.

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.

I agree with you, but unfortunately, when I travel it's almost always for pleasure and I have my child with me. I didn't opt out last time because I didn't want him to have to deal with someone getting personal with him (he was 10 at the time).

I and my family opt-out 100% of the time. Part of it is due to the unknown risks (however low the chance may be), part of it is on principle (violation of rights), and part of it is to have everyone else see in full-view that others are opting-out (so others feel comfortable doing so as well).

>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.

Bravo. I'm going to use that.

I always opt out of both the new booths. Occasionally they ask why. I say I don't trust them, nobody should trust the security apparatus of the state, and they have no real concern for my safety or well-being. I also make a point of saying everyone needs to see my pat-downs, for me to be safe, and to show people how to stand for their limited choice.

I probably fly a little more frequently than the average citizen (once a quarter or so). I opt out 100% of the time. My reasons are are a little further down the tin-foil hat spectrum though... I'm not comfortable with the government having a highly accurate 3D model of my body.

yep i opt out and consider it my tax paid free massage. TSA Blogger Bob really should highlight this feature. Almost free sorry for the dude who does it but hey he is getting paid. Unfortunately they may wise up and go full ballsy because of the flak from not doing a proper frisk. As for all this talk of low dose rads try asking anyone who's ever had skin cancer.

I'd say your claim of no health risk is premature. Backscatter X-ray focuses the ionizing radiation on the surface of your skin and immediately underlying tissue, so the effective local dose on your skin is much higher than other types of X-ray which are distributed evenly throughout your body. This is of particular concern to men, whose reproductive organs are located near the surface of the skin, as X-ray exposure to reproductive organs in parents is associated with leukemia in their children. For medical X-rays, measures are taken to limit exposure to reproductive organs.

Indeed. I opt out, I will opt out my children. It is a unnecessary risk.

For families with children, I've noticed that most airports (I've been to anyway) now have a family security line, which uses traditional metal detectors instead of body scanners and doesn't try to make kids go through the scanners. The lines are also usually shorter and friendlier to the extra time it takes getting kids through the line. However, I'm usually slower than the kids because I have to get all my laptops out, belt and shoes off, etc.

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).

Do you really expect your typical underpaid, undereducated, and overworked TSA employee to properly calibrate, maintain, or use a backscatter machines?

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...

> it's a blatant violation of the 4th amendment

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...

I'm fairly sure that full body scans do not fall under the border search exception. For one, the exception only applies to international borders, and usually, these types of searches would be performed by Customs agents—not the TSA.

I'm pretty sure you're wrong about that. Have you ever been searched at a US/Canada land border? It's very unpleasant.

I'm pretty sure he means the border exception shouldn't apply in any way to an in CONUS flight. I.e. Oklahoma City to St. Louis, MO.

Even medical grade x-ray equipment that is seemingly well and regularly maintained operated by trained medical professionals in hospitals can go horrifically wrong.

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

I disagree with your emphasis of "hospitals", because I think of hospitals as much more dangerous and threatening than airports. Your odds per visit of being killed by hospital-acquired infections alone are hugely greater than all forms of death caused by air travel.

I emphasis that not because I think hospitals are particularly safe but because I think the damage a malfunctioning x-ray machine may cause has a higher chance of being recognized for what it is in a hospital.

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.

The Therac-25 wasn't plugged into a wall socket.

These machines are plugged into wall sockets, therefore they must be safe.

Is that what you wish to imply?

Yup.

The thought that the manufacture of these machines may have displayed similar hubris and dismissiveness is frightening.

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

I really hope you aren't being sarcastic, as that would be funnier. Here's how to make an x-ray machine strong enough to work with standard film that you can plug in the wall and uses old tv parts. Be careful in doing this as dosimeters apparently are required. http://www.belljar.net/xray.htm

Speaking of radiotherapy overdoses, gnosis above provided an article about some more recent cases: http://www.nytimes.com/2010/12/29/health/29radiation.html?_r...;

> Backscatter doesn't pose any health risk to travelers.

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.

I'm with you on the whole government intrusion thing. I haven't flown since 9/11 because all of the security BS. My career has probably taken a hit, but we all gotta take a stand somewhere.

Has anyone seen data on # of opt-outs?

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!

It doesn't pose a non-trivial health risk, but IIRC there's no decade of air travel where the number of deaths per flight from terrorists was higher than the number these x-rays are expected to cause.

Too many negatives in this sentence. Trying to parse: "It may pose a real health risk; but, iirc, for each decade of air travel, the number of deaths by terrorists has been less than the combined impact of these new x-rays."

Did I parse that right?

> Backscatter doesn't pose any health risk to travelers.

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.

That sounds like a serious accusation. Can you provide a citation?

About radiation: Let's say that the article is correct and the radiation risk is very low -- 6 to 100 additional cancer cases per year among the millions that fly.

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.

> I'm still opting out because it's a blatant violation of the 4th amendment

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

Given the choice between getting felt up through my clothes and having a naked picture taken, i'll take the feel up every time.

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.

And not just a naked picture taken. A naked picture taken of you and potentially stored on disk.

It's a naked picture of you that doesn't really look anything like you and contains no identifying metadata. Imagine if the last 100 naked pictures from Logan ended up on the internet. Would anybody be able to identify them?

Maybe not, but they also have a list of exactly who was there at the time. We cannot examine the software, so we have no guarantee that it really blurs face and privates. Don't you think they might be able to do some analysis with either a full-body or partially blurred collection of scans over time?

It's not that it's different; it's a form of civil disobedience (which isn't even that disobedient) by requiring more time/energy/work on the part of the TSA agents. If enough people opt out, the line slows down, and the machine gets turned off.

appleflaxen put it well. Opting out is a method of showing TSA agents that some people think about these things and care. It also puts more demand on their limited time and resources. If everyone opted out they'd have to get rid of the procedure all together.

Poison is about dosage and duration. The dose during the flight may be the same, but the duration certainly is not.

The reason for the controversy is

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.

Reply on news.ycombinator.com