A Healthy Dose of Skepticism: Are TSA Strip-Search Scans Safe?

When the TSA first announced its rollout of body-scanners, my first thought was: What about privacy? My second: Are they safe?

While privacy is obviously and inarguably a massive issue, the facts aren’t quite as evident on the question of safety. Back in January, Lindsay Beyerstein made the point that one of the two types of scanner – the “backscatter” technology – uses x-rays, yet the machines are not subject to the sort of rigorous testing expected of medical equipment. Even CT scanning equipment, which is operated by trained medical personnel, has resulted in a distressing number of overdoses from wrongly calibrated machines, some of which were uncovered only after the FDA and the New York Times launched an investigation starting in late 2009.

TSA employees have no medical training – none! – and we have no reason to believe that they are prepared to recognized machine malfunctions. Indeed, they are so unprepared that one TSA employee told a pregnant traveler that the machines emit less radiation than do sonograms – a stunningly ignorant statement, given that ultrasound doesn’t rely on x-rays at all. (The pregnant woman was subsequently bullied into the body-scanners by repeatedly refusing her a pat-down.)

Lindsay also raised the question of cumulative radiation. How much is safe? I would argue that no matter how small the dose of x-rays, it is only justified if it will save lives, and if safer techniques would not produce the same results. The Israelis manage to make it work without routine body scans, primarily through painstaking intelligence on potential terrorists – no x-rays needed. (Link via the excellent overview of issues at Sister Sage.)

My husband has had cancer twice. It was treated with radiation the first time around. He’s had lots of CTs – too many – and caught some of the fall-out from Chernobyl back in 1986. He does not need any extra x-rays. My kids don’t need any extra x-rays. Nor do you or I, dear reader.

If I were a TSA employee on the front lines, I’d also want to know how much of an occupational risk I was running. On the one hand, the intensity of the radiation is purported orders of magnitude less than used in medical x-rays. On the other hand, what’s to keep the x-rays contained? The name “backscatter” is not reassuring on this score. TSA operators are spending entire workdays in close proximity to these devices.

The other type of strip-scanner does not use ionizing radiation. It relies on millimeter waves. Some questions have been raised as to the safety of terahertz waves, which may have the potential to essentially “unzip” DNA, but terahertz waves are not identical to millimeter waves, just adjacent to them in the spectrum. (Two abstracts on terahertz waves are here and here. My main takeaway is that their safety is not yet well researched.)

On its website, the TSA simply asserts that millimeter-wave technology is safe; it does not supply any data or link to any studies. I just ran a PubMed search on “millimeter waves” and “safety,” which turned up only six hits, only one of which seemed relevant. A review article in Health Physics from 2000 raised the question of whether occupational exposure (that is, of the sort some TSA employees experience) could result in hazards such as burns or cancer; I can’t access the full text, so I don’t know what they concluded.

It is striking, in any event, that PubMed yielded so little information on the safety of millimeter-wave scans. Business Week reports that their health effects are “largely unknown,” and that the president of the National Council on Radiation Protection and Measurements favors conducting a study that would assess their safety. Much of the information on the web conflates millimeter waves with the terahertz spectrum and thus appears less than trustworthy.

In short, the TSA may be correct that the low intensity of the energy from both types of scanners makes them unlikely to create a real health threat. If I were a frequent flyer or a flight crew member, I would still wonder why there’s so little hard information on their safety.

More importantly, I worry about a the lack of medical/technical oversight. Largely uneducuated low-level employees are operating these scanners. If a scanner were wrongly calibrated and delivered much higher doses, who would know?

At the end of the day, I still think the best health-related objection to the strip-scanners comes from Revere of the now-dormant but wonderful blog, Effect Measure. Revere applied his skills as an epidemiologist. He noted that any machine purporting to catch every would-be terrorist will have a substantial number of “false positives” – people who are flagged though they’re innocent. Precisely that is now occurring, as evidenced by the story of passenger Christine Holland (who subjected to a grope-search after the scanner suggested she was carrying contraband). Revere calculated how many false alarms would be raised by a machine with only a 1 in 100,000 false positive rate:

According to the Department of Transportation, during the last year there were about 710 million enplanements (US carriers, October 2008 – September 2009; excludes all-cargo services, includes domestic and international). That would produce 7100 false alarms, about 20 a day. How many passengers carrying explosives would the technology pick up? Well, we’ve had exactly 2 since 2001 (Richard Reid the shoe bomber and the current underpants bomber), or .25/710,000,000 enplanements (it’s actually less because enplanements have decreased substantially since 2001). So the probability of an alarm being correct is about 1 in 30,000 or .000033.

(Read the whole thing here.)

I swear Revere argued at some point that screeners will eventually become inured to false positives and thus won’t be alert if a real threat were to appear. I can’t find where he said that, but it’s a key point, so I’ll make it anyway. Add to this the tremendous waste of resources that goes into checking for liquids and gels, printer cartridges, baby formula, and other innocuous items. Now add the diversion of TSA energies toward thoroughly frisking and groping everyone from Jeffrey Goldberg to little kids.

In other words, the biggest health risk from the scanners is that we’re actually less safe from terrorists than we were before. Anyone else feeling queasy yet?

[Variation on my usual "I'm not a lawyer" disclaimer: I'm also not an M.D., a physicist, or an epidemiologist.]

Update 11/14/10, 9:40 p.m.: According to Agence France Press, serious scientists have raised concerns about the x-ray machines. Michael Love, a scientist who runs an x-ray lab at the Johns Hopkins medical school, stated that “statistically someone is going to get skin cancer from these X-rays.” In April, scientists at UCSF wrote the White House Office of Science and Technology, saying, ”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.”

Update 11/15/10, 11:10 p.m.: Here’s the full text of the letter (.pdf) from the UCSF scientists. The potential health risks it outlines are compelling enough that I’m not about to let my kids go through a backscatter machine. I’m also floored by how little study has been done on their safety.