Tuesday, June 19, 2012

The LA Times and the scanners-are-safe interpretation

As soon as I read the LA Times' take on a new scientific article on naked scanner radiation study, I knew they had misrepresented the findings. I have now read the original research article in full, and my hunch holds.

The article, by Hoppe and Schmidt in the Biomedical Engineering Department at Marquette University, seems solid to me. The introduction nicely summarizes what studies have been done at this point. Two that were done in conjunction with the TSA (by the FDA and by Johns Hopkins Applied Physics Lab -- which is functionally a government research lab doing a lot of work requiring security clearances) had access to some form of naked scanner technology, although not necessarily the exact models used in airports. They both were assigned the task of determining whether the scanners met radiation guidelines specified by an ANSI, which is suspect. So this is where Hoppe and Schmidt take off. They note the shortcomings of these earlier studies, and also reference the "scanners aren't safe" articles from 2010.

Nonetheless, as the scanners have not been made available to anyone outside of the TSA for testing, Hoppe and Schmidt were left to use TSA-provided data for their analysis. They created a computational model for measuring radiation doses in various organs of the body. They replicated the radiation from the x-ray scanner by correlating it with the TSA-provided data first, then running the simulation on their human organ models. It is true, as the LA Times reports, that they find that the effective dose of radiation that passengers receive is below the ANSI standard, as has been claimed by the TSA.

They do not claim the scanners are, therefore, safe. Indeed, they make no judgement on this at all. They note the caveats that they did not have access to an actual machine several times throughout the paper, which means that they can not independently verify the radiation levels coming from the scanner. They also had to make some assumptions and simplifications with their computational model. Although I can't find fault with their methods, it is not the same as testing the real thing, or even a physical model, with an actual scanner, as the authors are well-aware.

The other really significant conclusion from this paper is that their calculations show that the radiation penetrates below the skin. This is independent confirmation of the Kaufmann and Carlson paper which also showed, by analyzing publicly available scanner images, that radiation penetrates the skin. Hoppe and Schmidt go further by estimating the dose of radiation that each organ in the body absorbs. Thyroid, testes, and eyes receive higher doses, children have higher doses - particularly in their bones - than adults, and all of the organs examined get some radiation exposure.

The authors also stress that their goal was to estimate the order of magnitude of the effective dose. In layman's terms, they want to know whether the exposure is about the same as the ANSI limit, 10 times higher or lower, 100 times higher or lower, etc... In doing estimates, scientists often rely on this factor of 10 to see if things are "close." Their results are similar to the two TSA studies - no more than twice as high. And they are less than the ANSI standard.

I am unaware of an ANSI standard for organ dose, but it may not be equivalent for a so-called "safe" effective dose for the whole body to also be safe for reproductive organs or a child's quickly growing bone. Indeed, in referencing the line that this is less than the radiation received during two minutes of air travel, the authors state, "However, it may be difficult to draw conclusions from this comparison, as the type and distribution (i.e., organ dose) differ between the two types of exposure." They do make a much better comparison - to a mammogram - and their estimates show exposure to be at least 10 times (and up to 100,000) lower, depending on the organ. My own editorial comment here is that mammograms are a medical procedure, and are not tossed around willy-nilly. The recent changes over mammogram recommendations highlight this crucial difference.

Furthermore, what strikes me, is a factor of two can be irrelevant for certain measurements. And, for a child, the effective dose calculated by Hoppe and Schmidt is 50% of the ANSI recommendations. What if the estimates here are off by a factor of two from the actual exposures? Then it could no longer be said that the scanners are irradiating at a level below the ANSI standard. Also, the younger the child, the higher the dose. If the 6-year-old phantom in the study is receiving half the recommended maximum dose, what about a toddler or infant? What is the effective dose to the fetus in a pregnant woman? These are not shortcomings of the present study, they are just questions that remain unanswered and will remain unanswered until the scanners are made publicly available for testing or are withdrawn from use.

I would like to let the authors' conclusion speak for itself, which the LA Times failed to do (emphasis mine):
Overall, this paper presents radiation dose estimates from backscatter security scanners for a range of subjects, which may be used for risk estimates and for understanding the impact of backscatter scanners on the average annual effective dose. The estimated doses comply with standards set by ANSI/HPS in N43.14-2009 for radiation safety for personnel security screening systems using x-ray or gamma radiation assuming an exposure of approximately 4.6 μR at 30 cm, as previously measured according to the ANSI standard. A maximum total effective dose of 0.0529 μSv due to a frontal scan and a maximum total effective dose of 0.0730 μSv due to a full screening are below the 0.1 and 0.25 μSv limits. However, as with all imaging modalities using ionizing radiation, the risk must be weighed against the benefit, both of which must be quantified for backscatter security scans.