In chasing through the radiation exposure levels for the backscatter systems, I ran across some interesting numbers, and even more interesting ways the calculations were done.
The manufacturer for the units at a local airport claims an exposure level of 3 microREM, which is certainly small. The absorption calculation, though, assumed the dose was absorbed through the entire body, and not in the 1/10 inch of skin that actually is penetrated by the soft X-ray dose.
The actual skin dose is higher, 56 microREM, per FDA & NIST Assessment of the Rapiscan Secure 1000® Body Scanner for Conformance with Radiological Safety Standards - July 21, 2006. Low level radiation exposure studies (BEIR VII report - NAS/NRC 2006) show that each 1 μrem of effective dose received is considered to contribute 5 × 10^–10 (one chance in two billion) to an individual's risk of contracting a fatal cancer during his or her lifetime.
At 56 microREM, each exposure has about a 1 in 35,000,000 chance of producing a fatal cancer over a lifetime. The
odds of dying in a terrorist attack on an American commercial plane ran at 1 in 25,000,000 last decade, roughly similar.
If this screening method is 100% effective in blocking all terrorist attacks on American commercial flights, it would appear that people are trading death by cancer for death by terrorist. If the screening method turns out to be less effective, then people are simply adding additional risk. The overall level of risk is small compared to other causes, such as tornado, lightning strike, or the ubiquitous automobile accident.
The risk compared to the risk that is trying to be avoided shows this is not a particularly effective bet, and it is quite expensive. It also remains to be seen if a $10/hour operator can correctly operate and maintain a device whose improper adjustment and use poses a radiation risk to the operator and traveling public. Exposure to low levels of radiation appears to be linearly additive to the lifetime cancer risk, so 'dialing it up' to get a better image might have a negative impact.