With the big holidays just around the corner, thousands of folks are about to get their first taste of the TSA's new virtual strip search machines - X-ray body scanners. Privacy issues may be the main concern for most people, but the safety of these things has some people worried.

Back in April, a group of UCSF professors with a range of expertise in x-rays and biology wrote a letter to White House advisor John Holdren (PDF) raising some potential safety concerns about the TSA's X-ray scanners. The gist of the letter was not to make accusations, but to raise key questions about safety that should be answered before these things went into wide use. The questions certainly had me a little worried, in part because of the government's history of safety screw-ups. As the UCSF professors wrote:
We would like to put our current concerns into perspective. As longstanding UCSF scientists and physicians, we have witnessed critical errors in decisions that have seriously affected the health of thousands of people in the United States. These unfortunate errors were made because of the failure to recognize potential adverse outcomes of decisions made at the federal level. Crises create a sense of urgency that frequently leads to hasty decisions where unintended consequences are not recognized. Examples include the failure of the CDC to recognize the risk of blood transfusions in the early stages of the AIDS epidemic, approval of drugs and devices by the FDA without sufficient review, and improper standards set by the EPA, to name a few. Similarly, there has not been sufficient review of the intermediate and long-term effects of radiation exposure associated with airport scanners. There is good reason to believe that these scanners will increase the risk of cancer to children and other vulnerable populations. We are unanimous in believing that the potential health consequences need to be rigorously studied before these scanners are adopted. Modifications that reduce radiation exposure need to be explored as soon as possible.

Back in October, the FDA responded to the concerns raised in the letter. Their response was essentially this:

1. X-ray body scanners for security purposes are not new, and the FDA has been on this issue for nearly two decades. Manufacturers of these devices are regulated by the FDA. A variety of independent, outside committees have examined the safety issues.

2. The specific safety concerns raised in the UCSF letter have been addressed:

- The X-ray output of these machines and the effective dose to the skin and other organs has been measured/estimated/modeled, and they fall within long-established safety guidelines.

- The machines have safety features to prevent a stalled scan from zapping one small part of your body with a massive dose of radiation.

- TSA screeners can't control the speed of the scan, and therefore can't cause the scanner to go over your groin more slowly, for example. Your future children are likely safe.

Here's the FDA's conclusion:
This technology has been available for nearly two decades and we have based our evaluation on scientific evidence and on the recommendations of recognized experts. Public meetings were held to discuss these products with FDA’s advisory panel (TEPRSSC), and the American national radiation safety standard was available for public comment both before its initial publication and before its recently published revision. There are numerous publications regarding the biological effects of radiation and the appropriate protection limits for the general public that apply to these products. As a result of these evidence-based, responsible actions, we are confident that full-body x-ray security products and practices do not pose a significant risk to the public health.

Strangely, ARS Technica claims that this response is merely a brush-off:
But, in the FDA's response, the agency gave the issues little more than a data-driven brush off. They cite five studies in response to the professors' request for independent verification of the safety of these X-rays; however, three are more than a decade old, and none of them deal specifically with the low-energy X-rays the professors are concerned about. The letter also doesn't mention the FDA's own classification of X-rays as carcinogens in 2005.

I can't agree. I find the FDA convincing in this case. First, the FDA does not just cite older studies; they also cite later reports from the assessment committees mentioned in the FDA's letter. But they are justified in citing the older studies. Here's why: radiation biology (not to mention the physics of x-rays) is an old and well understood science. Since the beginning of the Manhattan Project (at least), the biological effects of radiation have been a subject of intense scrutiny. We know how to measure exposure, what the likely effects of various doses are, etc. You don't need new studies to reinvent the wheel.

Second, the FDA provided a detailed response to all of the hardware/software/device concerns raised in the letter, and basically stated that it's been regulating machines like these for decades. Maybe you don't believe the FDA and think the standards it sets are incorrect/unenforced/whatever, but that's a different argument.

Third, why should the FDA have to mention in the letter that X-rays are classified as a carcinogen? Isn't that why it's regulating these devices in the first place?

The FDA's response is thorough and convincing, with the appropriate references to where you can go for more details. Next time you get screened at the airport, worry about the privacy aspect of this virtual strip search, but not about the safety. Read the feed: