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Just curious, does anyone have some hard evidence that the MWBS is a cancer risk?

I ask this simply because anti-X activists have pushed junk science claims of the form "X causes CANCER" many times [1], so I'm a little dubious. I don't think fighting civil liberties violations with junk science is a useful tactic, if that is indeed what is happening here.

[1] For example, feminists pushed the "silicone gel implants cause breast cancer", anti-bioscience types push "GMO foods cause cancer". Drug warriors have pushed "pot causes cancer" and anti-abortion crusaders pushed "abortion causes cancer".




does anyone have some hard evidence that the MWBS is a cancer risk?

Ordinarily I'd (enthusiastically) go along with the standard that this implies: until proven otherwise, you ought to be able to do what you want with your body (and maybe even after it's proven unhealthy).

This situation is different. Because the crucial element of personal choice has been removed, the burden of proof must shift 180 degrees. It is incumbent on those forcing the test on us to prove beyond any reasonable doubt that there's no danger involved. And when there may be dangers (as with the woman in the OP who had been told to avoid X-rays), it's also responsibility of the authorities to provide work-arounds.

EDIT: fixed "healthy" in 1st para to "unhealthy" -- oops.


There's a letter here from a group of X-ray imaging and cancer experts recommending further study: http://www.scribd.com/doc/35498347/UCSF-letter-to-Holdren-co...


I'm no expert, but I have a basic understanding of radiation physics. And this letter reads like FUD:

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.

This statement is very shady. When a photon interacts with a cell, it has a certain (small) probability of causing a carcinogenic mutation. The probability that the radiation is safe is (1-P)^N, where N is the number of interactions [1]. The fact that it is all deposited in the skin is irrelevant to leading order [2], this simply makes skin cancer more likely than breast or liver cancer.

I.e., if the dose in the form of a chest x-ray caused a 1e-9 chance of skin cancer and 1e-9 chance of breast cancer, it instead causes a 2e-9 chance of skin cancer with comparably reduced breast cancer risk.

[1] I'm ignoring high field (nonlinear) effects, since if the field were strong enough to cause them, it would probably also vaporize the person being scanned.

[2] It's actually a good thing since many skin cells are dead or will soon die, and thus carcinogenic mutations will harm them less.


"When a photon interacts with a cell, it has a certain (small) probability of causing a carcinogenic mutation."

I'm also no expert, but surely there is a risk of more significant, irreparable genetic damage accumulating if several photons interact with the same skin cell, as opposed to those same photons interacting with several different cells throughout the body?


>I'm no expert

Well they are, so perhaps we should give their word a bit more credibility than yours?


I'm not asking you to take my word for anything. My reasoning is laid out and explicit. If I made a flaw, point it out.

But I guess it's much easier to criticize me for being a mathematical physicist rather than a medical doctor. After all, finding flaws in an argument takes work!


edit: s/mwbs/bs/g; per comment below

Well, X-rays cause cancer. BS uses X-rays So it is a cancer risk. And it is a little more founded than someone randomly saying' this causes cancer'.

In terms of risk...

This says dental x-rays are about 2-3 mrem: http://www.physics.isu.edu/radinf/dental.htm

This says BS is about .006-.009 mrem: http://science.howstuffworks.com/innovation/backscatter.htm

Keep in mind that under certain models (the 'no-threshold model'), any amount of exposure (even a tiny amount) to an X-ray is a slight increase in the chance that that person will get cancer.

Now you might think that because dental x-rays are about 300 times more powerful than BS, dental x-rays have a much higher chance to cause cancer. Sure, individually, my chances of getting cancer from a BS are much lower than my chances of getting cancer from a dental x-ray. It is even higher from daily background radiation, or from the radiation that you get from flying in an airplane.

But everything is about context. How many people fly each day? 2 million or so? How many people do we propose to put through the BS machine? All of them? (I assume the intent is to eventually replace the metal detectors with these). If you are putting 2 million people through the BS each day, that is actually large amount of radiation exposure. Probabilistically, someone will get cancer from the BS.

There are certain types of radiation that you can avoid (x-raying your foot for fun! backscatter machines), and there are certain types of radiation that you cannot avoid (background cosmic).

I don't see how the argument that it is 'nothing' in comparison to other forms of radiation really stands up, in that I could potentially avoid backscatter radiation, whereas I couldn't conceivably avoid the other types.

(This doesn't take into account other real issues, like 'what if the machine breaks down and we start blasting people with focused beams of radiation'... while trained to run the machine, I doubt that the TSA people are trained to maintain the machine, or would even know if something had gone horribly wrong with it).


(This doesn't take into account other real issues, like 'what if the machine breaks down and we start blasting people with focused beams of radiation'...

What if my flashlight breaks down and I start blasting people with focused beams of laser light? What if my cell phone breaks down and the microwaves melt the brains of everyone within 6 feet of me?

I don't know a great deal about the engineering of MWBS, but I see no reason to believe that your fear is any more likely than mine. Most machines just don't work that way. Do you have evidence that MWBS is different?


The situation is analogous to what happens when a CT machine malfunctions. The basic design of a CT machine has an x-ray source on a rotating ring. The patient lays down on a table and the table is moved through the center of the ring at constant velocity for the duration of the scan. The overall motion of the x-ray source relative to the patient is helical. If the movement mechanism jams and the table stops moving, the pattern of motion collapses into a circle and the patient receives an excessive radiation dose focused in the region of the body that is in the center of the ring when the table stops. The received dose is a function of the time until the operator notices the malfunction and stops the scan.

With a backscatter x-ray system, the person stays motionless and the x-ray source moves. If the x-ray source stops moving, the person receives an excessive dose wherever the source happens to be pointing when it gets stuck. The dose received is, again, a function of how long it takes for the operator to notice the malfunction.

With a microwave backscatter system, the radiation used is in the RF range and, therefore, not ionizing. However, if the microwave source stops moving because of malfunction, a portion of the body experiences higher than normal SAR. If the SAR is high enough, significant localized heating may occur, causing burns on the person.


It is hard to think that a flashlight or cellphone would have a power supply strong enough to do that much damage. (Although actually, metal halide lamps can sometimes cause UV radiation burns if they are damaged... http://archpedi.ama-assn.org/cgi/content/full/158/4/372 ).

As long as the backscatter machine doesn't run on Double A's, my fears are actually more reasonable than yours.

The backscatter machine presumably uses some sort of electrical plug to connect to power. It isn't inconceivable that it could receive more power than intended and produce more x-rays, or that something could go wrong with the shielding allowing x-rays to leak, or that something could go wrong with the x-ray bulb and we could produce the wrong type of radiation.

And it doesn't have to be 'burning your head off' to be a problem, it could simple produce an order of magnitude or two more radiation than intended, and be a health risk.


My desk lamp also uses an electrical plug to connect to power. It isn't inconceivable that it could receive more power than intended and blind me! By the way, an order of magnitude or two more radiation than intended would be equivalent to a dental xray.

Look, I'm not saying your fear is physically impossible. I'm just saying I can't think of a very many machines (radiation sources or otherwise) for which anything similar is a plausible fear. Most machines simply stop working when used outside their operating parameters.

Also, do you really think that if the backscattering machine emitted the wrong type of radiation (x-rays of a different frequency), the TSA agent operating the scanner wouldn't notice when his screen went blank (unless the detectors simultaneously malfunctioned to pick up the incorrect frequency)?


And if it does, it's your responsibility.

Your desk lamp doesn't emit X-Rays, you don't force droves of people to stand underneath it, and no oncologists ever told me to be wary of your desk lamp until it has received further study.

So, yeah. Not the same thing. Not even remotely. In fact, your analogy is a bit silly.


Medical radiation machines have malfunctioned and killed people before: http://scienceblogs.com/insolence/2010/01/unforgivable_medic...


A laser scalpel (designed to deliver large radiation doses) can also kill someone if used improperly. Does this mean I should be afraid of a malfunctioning flashlight burning my head off?

Medical radiation machines deliver 45-80 Gy. The backscattered x-ray machine delivers 1e-7 Gy, if I've got my math right. That's roughly the difference between a punch and a nuclear bomb.


Please actually read about the Therac. It was not used improperly! Software errors combined with sloppy design caused the machine to malfunction under normal use conditions, delivering the wrong dose and/or wrong type of radiation, with victims perceiving flashes of light, feeling heat from the radiation, and dying.


Let me repeat: Therac is a radiation cannon, designed with the explicit purpose of killing cells. It's billions of times more powerful than an x-ray backscattering scanner.

The fact that a cell killing radiation cannon malfunctioned and killed someone does not imply that an illumination device billions of times weaker could do the same thing. Similarly, a laser scalpel will kill you if you shine it on the wrong place. A flashlight won't.


Many consumer radio-emitting devices can be configured to emit far greater power than their default. For example, hacked router firmwares enable one to boost the transmit power of their wireless router's radio to the point of burning itself out. As pointed out by another comment, the Therac gives an example of what can happen when software controls malfunction. Since these scanners aren't medical devices subject to FDA regulation, it's conceivable that hard-wired emission level safety systems were omitted in favor of a completely software-based system.

The burden of proof of safety lies with the TSA and scanner manufacturers.


It is not a theoretical concern: http://en.wikipedia.org/wiki/Therac-25


Therac was designed to send a concentrated beam of radiation into somebody. That was the machine's purpose.

Are the scanners designed or even capable of doing the same?


You're playing the role of skeptic, but you're not going far enough. How would I actually answer that question? You could have said the same thing about the Therac; of course it can't blast that much radiation, there's no good reason for it. Yet, here we are.

What evidence do you have that it is actually not possible for the machines to dangerously malfunction, and how much confidence can actually be placed in it? Those telling us it is safe don't have entirely clean hands on the topic, they've lied about other things like picture retention policies and capabilities before.

You're making an appeal to incredulity. I would not actually be that incredulous to discover that the machines do indeed have some sort of flaw that would permit them to blast out more radiation than intended. After all, it's happened before. That said, I don't necessarily consider it likely, either, what I find far more problematic is that this is merely one consistent piece in a larger puzzle of unconcern about the real safety of the public.

For the same reasons we ought to be able to examine our voting machines, we ought to be able to examine these machines.


The Therac was designed to blast dangerous amounts of radiation. It had two modes of operation - "deadly radiation" and "VERY deadly radiation". A software bug caused it to go into "VERY deadly radiation" mode when it should have been only in "deadly radiation" mode.

A smaller radiation source, like that of an x-ray backscattering scanner, does not need to have a "deadly radiation" mode. And it's highly unlikely that a hardware failure can accidentally turn "10x weaker than chest x-ray" mode into "deadly radiation mode". The more likely failure mode will be something along the lines of "6x weaker than chest x-ray mode".

It's just highly unlikely that any device will accidentally emit millions to billions of times more energy than it was designed to.


Millions to billions, no, but would anybody notice if it never properly turned off? That would be in the thousands.

Again, not really worried about this, I'm more concerned by the blithe acceptance of the idea that nothing can possible go wrong.


Backscatter X-ray scanners use X-ray photons. Millimeter-wave scanners use far-infrared/microwave photons. To the best of my knowledge, nobody's every shown that microwaves or infrared light cause cancer: the photons just don't have enough energy to cause that kind of cell damage.

(Yes, this is quibbling about what was probably just an error in definition; I think your comments are right as applied to backscatter X-ray scanners. But if we keep our language straight, our arguments against these blasted things will be that much harder to dismiss.)


Yes, of course you are right, I was just looking at it from a perspective of the x-ray scanners. I have no idea about the microwave based scanners.


Here's the key idea: your cross-country flight exposes you to 2-5 mrem. MWBS is about 0.01 mrem. It's trivial.


the UCSF letter explains why this is not a valid comparison. the ambient radiation absorbed in flight and the radiation of the mwbs are not in the same part of the spectrum so they are not comparable using simple agregate statistics. the stopping power of your (or any) body is function of the energy of the incident radiation (or particle) and this not considered in the mrem comparison.


If you demand that human trials be done for every possible permutation of conceivable risk---throwing out as incomparable all data that is taken in even remotely different circumstances--then you can claim health risks about every single thing. And it's pretty much impossible to satisfy you because of the cost of human trials.

The concerns raised in the UCSF letter are strained beyond belief. Further, a number of points (like the breast-cancer bullet point) are clearly there to influence emotions rather than critical reasoning.

I'll just explain this one. As the UCSF letter states, the radiation is being deposited on a volume smaller than usual by a factor possibly as large as "one to two orders of magnitude". That means that at absolute worst this is like radiation dose of 1 mrem (still less than the flight tiny compared to your yearly dose), and that's assuming that all of the risk of cosmic radiation comes from skin cancer (which is wildly wrong).


i agree that demanding perfect testing is ridiculous in most scenarios, especially when talking about human trials. however, that's not what i'm suggesting should happen. i also don't deny that the UCSF letter has a sensationalist or alarmist tone. i'm not touting their concerns, just suggesting it as a place that explains the difference between cosmic xrays and the xrays in these scanners. i guess my point is that it's very easy to mislead people that have no appreciation for these very real differences. glossing over details and nuance that have not yet been proven insignificant, and doing so with the goal of making concerned people stop asking questions about their own safety- that's not cool. when it's our own government it's even less cool.

there are so many issues swirling around this subject- privacy, security, safety, fear, "terror"... i guess i'm just wanting the science of the safety to be completely objective, open, reviewed, failsafes and failure modes known... all that jazz. as a member of the public it seems like these things were just thrown out there. the tsa is covering their butt by doing whatever is possible, a few scanner manufacturers are having a good year, and we're just supposed to leave it like that, keep calm, carry on... i can't say that makes me feel great.


My general position is to avoid things that could conceivably cause cancer until hard evidence shows they don't rather than the reverse. X-rays could conceivably cause cancer.


You get a pretty decent radiation dose from flying around high in the atmosphere too. From what I have read the crossover point where you absorb more x-ray radiation in the flight than the scanners is about an hour and a half.

See here for a bunch of things to read (http://en.wikipedia.org/wiki/Backscatter_X-ray#Health_effect...). According to this page, 42 minutes of ordinary living is sufficient to give you a similar dose of ionizing radiation compared to the backscatter machines. It's certainly possible that the FDA is lying to us, but it seems more likely that these machines are indeed harmless. Sounds to me like the people saying "I avoid things until I'm sure they don't cause cancer" are throwing science to the wind and embracing a line of thought they wouldn't if they didn't already have some other reason for opposition.


Radiation is cumulative. Saying that you will absorb more radiation in flight is not a justification for accepting radiation from another source. You will still absorb more total radiation. It's a lot easier to avoid the scanner, than it is to find an alternate mode of transportation across long distances.


In one situation, large quantities of X cause no visible concern to people. In another situation, much smaller quantities of X cause serious outcry, even when no harmful effects from those smaller quantities have been demonstrated over twenty years of study.

I suppose it's possible for someone to be genuinely concerned over these very small quantities of radiation. But it strikes me that for the average person what underlies the panic is

1. a counter-factual belief that more health-harm is done by a new technology than actually is,

2. a diversionary appeal to concern for safety when health risks are not really a significant negative of a new technology.

This happens in all kinds of areas. Artificial sweeteners, GMO foods, irradiated meat products, and vaccines have all fought or are fighting this battle. The only reason this type of thinking is given a pass here or anywhere reasonable people congregate is because the community is so opposed to the technology for other reasons.


According to what I've read, the energy of the radiation prevents it from penetrating deeply (indeed, the Compton effect backscatter at the surface is the whole point of it).

This might sound like a good thing, but it also means that the radiation isn't "diluted" over the full mass of your body. The damaging effects will be localized to -- and concentrated within -- the skin and shallower tissues. So those organs affected are getting a dose that's disproportionately much higher than the guidelines for full-body dosage considers.


This argument betrays a lack of understanding of radiation physics.

Think of each cell in your body as a dice. When a cell is hit by radiation, the dice is rolled - you get cancer if any dice comes up 6 (these are billion-sided dice). The radiation dose is the number of dice rolls you need to make.

Now, under which circumstance is a roll of 6 more likely? If you roll 100 dice 1 time each (traditional x-ray), or 1 dice 100 times (backscattered xray)?


Smaller doses over time are safer than a large dose. Time to fix things up. Concentrating the radiation has a negative effect.

Edit: Maybe this is incorrect when it comes to cancer risk, but I had the impression it still mattered, with multiple impacts in the same area of DNA causing things to be more likely to permanently break. Unless that is extremely unlikely to happen at all at this level of dose.

Still, these machines are probably not particularly harmful.


When radiation causes a significant amount of cell death, yes. I.e., if you are going to have 2-3% of your cells killed (by heat, radiation, alchohol, whatever), it's better if it happens slowly over time than all at once.

Backscattered X-rays are not remotely near that point.

edit: Just want to respond to this: I had the impression it still mattered, with multiple impacts in the same area of DNA causing things to be more likely to permanently break.

This is called the multiphoton effect. It's real, but it's more important for radiation by low frequency radiation (e.g., cell phone microwaves). You can observe this in gases with high powered lasers or microwave resonating cavities. Weird things happen in this regime, such as stabilization (more radiation -> less ionization).

I suppose you could have molecular effects from multiple photons (e.g., one photon damages the DNA, a second damages the repair mechanism), but this is vanishingly unlikely at the doses we are discussing.


No, it's all about bit errors and error correction. Highly concentrated radiation is more likely to cause multiple errors to a particular DNA strand, which can defeat the error correction and cause a mutation.

The same radiation distributed through the body will cause fewer errors per strand and be corrected.


It's been a long time since I studied dosimetry, but I was under the impression this was only significant at high doses. Do you have evidence otherwise?


The key issue is the actual wavelength being used. I have yet to see this number published. If you knew the wavelength of the X-ray then you could attain a good estimate of the radiation dose. It is well established that X-rays cause protein damage (in vitro) at 1-1.5 Angstroms which is certainly smaller than what is being used in the is case. The wavelength at which radiation damage does not occur would be interesting to determine.


The US Federal Code of Regulations has a section defining acceptable radiation doses for radiation workers[1]. This section places an upper limit on the shallow dose to the skin of 50 rems. To get an equivalent shallow skin dose from a backscatter machine, you would need to go through it over five and a half million times. The limits for adult members of the public are 10% of the limits for radiation workers, which would cut it down to only about 500,000 times through.

To be fair, I haven't found any studies either way on the safety or danger of the 10 C.F.R. 20 limits, probably because very few people get close to hitting the dose limits. One study I found[2] that surveyed radiation workers in three different studies concluded that the relative risk for a sustained protracted dose of 10 Rems compared to no dose is 0.99 for cancer excluding leukemia, and 1.22 for leukemias. This means, as a radiation worker, you're somewhat less likely to get a non-leukemia cancer, and about 20% more likely to get leukemia. This is for a dose that is over a million times larger than what a backscatter machine will give you. Furthermore, these risk rates are lower than what would be expected from a linear estimate based on cancer rates in atomic bomb survivors, suggesting that smaller radiation doses are safer than larger ones, and the dose these machines give you is about 200-500 times smaller than the does you're going to get on the airplane (if you're flying cross-country).

I just wanted to put in perspective that the dosage these machines put out is extremely small when compared to the dosages you might normally receive, let alone the dosage which radiation workers commonly receive. Their health effects are minimal, and if they actually contributed to the safety of passengers, I'd say they'd absolutely be worth it. The real travesty here is that they violate privacy for no real gain, not that they are in any way a significant health risk.

[1]: http://www.nrc.gov/reading-rm/doc-collections/cfr/part020/

[2]: http://www.jstor.org/pss/3579020 (JSTOR link. If you're not affiliated with an academic institution, you'll only be able to read the first page)


> Just curious, does anyone have some hard evidence that the MWBS is a cancer risk?

The eventual outcome of a policy of assuming things are not cancer risks until proven otherwise is cancer. It is much more sensible to assume a cancer risk until proven otherwise.


Don't forget cell phones, they cause cancer too.


You're not forced to use a cell phone in order to fly. The radiation from the phone is not concentrated and directed at your entire body. It's a different frequency and intensity. A lot more time has been spent studying the dangers of RF from consumer phones than the dangers of these new machines based on medical devices that require lead clothing or lead walls for safe use.

You're also exposed to a variety of radiation when you fly in the high atmosphere or eat a banana. It doesn't mean additional radiation from insufficiently tested machines is a good idea.


I could be wrong here, but I believe hfinney was referring to the footnote about how people claim "X causes cancer" because they're against X, not because of evidence. Opponents of cell phones/power lines have frequently claimed they cause cancer, thus making it another valid example for the footnote, because their claims are untrue.


Right, although the NY Times on Saturday claimed the jury is still out on cell phones:

http://www.nytimes.com/2010/11/14/business/14digi.html

OTOH apparently there is evidence that cell phones actually prevent cancer, so go figure.




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