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Comment by DanielStraight | Hacker News Reader

So apparently the TSA says: "Each full body scan produces less than 10 microrem of emission, the equivalent to the exposure each person receives in about 2 minutes of airplane flight at altitude."

And the author says the dose is really 14.76 microrem, but doesn't dispute the equivalence to flight time. So really it's equivalent to 3 minutes of flight, not just 2.

50% of trivial is still trivial.

Oh, and the author says that the margin of error on these measurements is large. So perhaps the TSA didn't lie at all, they just measured 10 microrem. Even if the margin of error is 1000% the result is still trivial.

So... basically this is non-news about a technology which isn't even in use anymore. The TSA now uses millimeter wave technology as the author is aware ("these backscatter x-rays are decommissioned and sitting in a warehouse").

There are a lot of good reasons to be concerned about the TSA's use of imaging technology. This is not one of them.

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The article has a reply to you built-in :)

"While these scientists still conclude that this is a low dose of radiation, it shows that, if correct, we were lied to, again, and anyone who walked through those scanners was given nearly 50% more radiation than they signed up for."

The point is not that 14µrem is dangerous to an individual on occasion, it's that the government irradiated us at a rate different than what they promised.

It will also have societal effects. I've seen calculations that estimate that something like 6 people will die from cancer as a result of the radiation from 700M TSA screenings given per year. That estimate should now be adjusted, and we should again say, "How many lives does the TSA save that justifies this?"

He answered your point in his comment. The TSA said these measurements had a fair amount of error associated with them, and since their still trivial amounts it's unfair to say the TSA lied when the number is within what they said it would be.

The TSA has never admitted error in their numbers. On the contrary, they have guaranteed the public that they will get less than 10µrem/screening.

That was my mistake. The author of the piece said that the scientists responded saying their measurements have a large margin of error (Unfortunately, he doesn't tell us exactly what they said).

You're getting upset over nothing. Really, you are. There is a LOT to get upset at TSA et al. over, but all this hurt the anti-TSA case.

It's 10 vs 14. Neither is significant, neither matters. All measurements have errors, if a source is claiming otherwise you best assume they are not educated very well on science - and have not done a good job communicating with the scientists making the measurements. So as far as anybody is concerned, the difference between 10 and 14 is so insignificant it may as well be less than 10.

I don't know about the people you're responding too, but for me it's less about broken promises and more about incompetency. I would be almost as angry if the number were 6, when they promised that they had done proper testing of the equipment.

These machines give you a dose of $\mu + \epsilon$ rems, where $\epsilon$ is random with zero mean. TSA's claim amounts to $\mu = 10$ and (presumably) $\epsilon$ normally distributed with $\sigma$ small. Now it turns out that $\mu = 14$. I agree with you that, in isolation, this is inconsequential. Taking a step back though, you have to wonder: if they are incapable of measuring $\mu$ accurately what else have they gotten wrong? In particular, suppose that one time in a million scans this machines messes up and administers a dose of $10000\mu$ (about 1mSv). A few unlucky people a year would be on the receiving end of that -- would you like to be one? This may seem like tinfoil-hat nonsense to you but if you read about the shady way these machines were sold to the government and the farcical "testing" that went on before they were deployed it doesn't seem completely implausible. At least, I'd like to see a lot more openness about how these things work, plus more third-party verification, before really trusting anything coming from the TSA.

But nobody was saying they're wrong except for this author. I question why you're so quick to trust this random author. The author himself explicitly states he emailed the scientists who did this study, and all he says about it is a quick note at the end of this article where they said the amount of error in their results means the TSA could still be telling the truth. IE. The scientists who did this study are saying that their findings aren't enough to prove anything about the TSA, but the author is saying they are. The author also didn't reproduce anything of what the scientists actually said to him. IMO, I don't blame you for not trusting the TSA, but there's nothing about this article that makes it particularly trustworthy either.

I haven't seen the particular estimate but it's very difficult to extrapolate repeated low doses into a high dose.

Lets take the case of water toxicity, if you're a little dehydrated and you drink 5 litres of water in one sitting you'll die. However, if you drink a few litres of water everyday over the entire day you'll have no issue.

See this letter by UCSF biophysicists & oncologists written to the Whitehouse with concern for these scanners.[1]

Their point is that the 'overall' dose is, not only unknown, but measured in a non-meaningful way. "Garbage-in, Garbage out." A full body scan produces 10 microrem of emission, concentrated to the first few microns of skin - (which happen to be where gonads, breast tissue, skin & eyes are...). This is very much unlike riding in an airplane where the exposure is truly whole-body. The "expose per cell" in these two cases is wildly different.

[1] http://www.npr.org/assets/news/2010/05/17/concern.pdf

Does this actually matter? If each cell has x% probability of becoming cancerous when hit with a single ray of radiation, then shouldn't the probability of getting cancer is the same either way?

The answer is mostly, we don't know. However, we do know that most of your cells in your body are not really capable of becoming cancerous. In general (again, generalities here - we don't have enough data), only dividing tissue is really capable of becoming cancerous. But most of these dividing tissues are those mentioned above and likely to be some of the cells exposed to such a machine (immune system (not near the surface), skin, testes, breast tissue, digestive system). The tissues at the surface of your body are disproportionately susceptible to cancer.

Cancer very much is a stochastic event, and so probabilities are all we can talk about. But reducing the number of encountered objects by billions-fold while keeping the rate of exposure the same can wildly skew data that you had relied upon to determine a 'safe' dose.

There's still a difference between the kind of radiation exposure that occurs during a flight compared to during a screening. My understanding is that radiation exposure during a flight is due to cosmic rays which are equally likely to cause damage throughout the body. The X-rays used by the TSA are at lower energy and are deposited almost exclusively in the skin. Since the volume of skin is much less than the total volume of your body, the radiation dose that your skin is receiving from the TSA is much larger than the dose during a flight.

While I'm very glad they're no longer in use in airports, I never saw a satisfactory answer to this. Perhaps I misunderstand the physics here, but wouldn't 10µrem across 100% of tissue be equivalent to, for instance, 1,000µrem across 1% of tissue?

Yes, that's correct. However, the difference in damage that that will do is still not clear. It's possible that skin tissue is more susceptible to damage than other kinds of tissue. (Or perhaps it's less susceptible.) Moreover, low-energy X-rays may do different kinds of damage than relatively high-energy X-rays or cosmic rays. Just looking at rem ignores a lot of variables about the long-term biological effects of the ionizing radiation.

I'm just concerned a wildly mis-calibrated machine might output 10 millirem of radiation and nobody would notice.

Considering how oblivious some companies are to the difference between $0.02 and 0.02 cents, milli and micro might be way beyond the ability of some TSA technicians to understand.

...because then the dose would be equivalent to about a day of natural background radiation.

I think "nobody would notice" is a pretty accurate description of what would happen in that case.

That was my concern, too. There's a precedent for a radiation-emitting machine failing in a very bad way: https://en.wikipedia.org/wiki/Therac-25

Of course these new machines cannot fail the exact same way as the Therac 25. But could they fail in a new, unpredicted way? I dunno. I am not familiar enough with how the backscatter machines are implemented to be able to predict how they could fail. But I maintain neither are most of the people who defend these machines as safe.

A well-founded concern. When the CDC inspected luggage scanners in 2003-2004, more than 2% were found in violation of federal radiation standards.

http://usatoday30.usatoday.com/yourlife/health/healthcare/go...

It's very safe to say it's beyond their ability to understand, considering that the TSA operators were never required to take the same health physics courses that literally everyone else whose job involves exposing the public to X-rays must undergo.

Actually, they're not sitting in a warehouse; they've been repurposed for use in prisons: http://www.latimes.com/business/la-fi-fullbody-scanners-airp...

In fairness, they did do such a good job of giving the airport that prison-like feel...

Perhaps the reasons given in the article were not compelling on their own, but the author's point was that the TSA overstated their case.

Researchers have pointed out that international pilots and stewardesses may already receive more cosmic radiation than may be safe due to high altitudes and long duration. This is why they're allowed to skip the scan. Why shouldn't frequent international business travelers receive the same treatment?

Also, all machines have a percentage of possibility of breaking down or misbehaving. Do we want to subject someone to the possibility of being irradiated by a damaged scanner when the odds are greater than harm from terrorism?

What's your opinion on why the X-ray machines have been decommissioned?

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