Posted in General Science on Nov 29, 2010
The widespread deployment of Rapiscan’s body scanners in major airports around the US has raised concerns about the ionizing radiation doses to traveling public. Rapiscan doesn’t publicly provide dose calculations, but the company does claim that the X-ray screening machines are in compliance with the 2002 ANSI standard (American National Standards Institute), they also says that “Emission Per Scan: Less than 10 microRem” which would translate to <0.1 µSv. But as it turns out, Rapiscan may have significantly under-calculated per scan exposure.
The 2002 ANSI standard calls for effective radiation dose of 0.1 µSv or less per scan per person (interestingly, in 2009 ANSI relaxed that standard and increased the limit to 0.25 µSv). How much is 0.1 or even 0.25 µSv? Well, “Sv” stands for Sievert – a derived SI unit that measures biological absorption of ionizing radiation. The National Council on Radiation Protection and Measurements recommends the following annual radiation dose limit for an individual from all radiation sources other than natural background and the individual’s medical care is:
- For members of the public who are exposed continuously or frequently, the recommended annual effective dose limit is 1 mSv (1000 µSv)
- On an infrequent basis, a member of the public may receive more than 1 mSv (1000 µSv). In such a case, the annual effective dose may exceed 1 mSv up to a value of 5 mSv (5000 µSv)
* – the term “infrequent,” in the context used here, should refer to a justified exposure that is not likely to occur often in an individual’s lifetime, with each occurrence justified independently of any other.
As you can see, the Rapiscan Secure 1000‘s 0.1 µSv per person per scan radiation exposure is 10,000x lower than the maximum allowed annual limit.
However, Rez et al. in their recent article “The Dose From Compton Backscatter Screening”  estimate that the effective doses maybe be as high as 0.8 – 0.9 µSv which would be make it four times higher the allowed maximum 0.1 µSv per scan (with the assumption of two scans per passenger). The authors do point out that the increased doses to the passengers remains well below the exposure levels that are known to cause harm, but warn:
The major public health effect of concern at low doses of ionising radiation is cancer. There is clear evidence of cancer induction at effective dose above ~200 mSv. Below an effective dose of ~100 mSv radiogenic cancer mortality risk estimates for all cancers is highly uncertain. It is not possible to determine reliably whether a radiogenic risk is present in an X-ray screened population because of the high spontaneous incidence of cancer and the multifactorial nature of disease causation
Based on these findings I will probably have to permanently opt-out or go through L3′s millimeter wave RF machines. Since I was about 9 years old , I’ve been exposed to gamma rays from Cesium-137 (137Cs) at 22 Ci/km2 ( curies per square kilometer) . And don’t forget about radioiodine (Iodine-131 131I), xenon and a bunch of other short half-life isotopes. Can you guess where I grew up ? :)
1. Rez, P., Metzger, R. L. & Mossman, K. L. The Dose From Compton Backscatter Screening. Radiat Prot Dosimetry 1-7 (2010).