The International Commission on Radiological Protection lays down three universally-accepted principles governing the exposure of humans to controllable ionising radiation. The exposure must be justified, optimised, as low as reasonably achievable, and subject (except for patients) to statutory limits.

The statutory dose limit for a member of the public, for exposure to radiation arising from any “work” process (i.e. any intentional use of radiation other than for diagnosis or treatment of that person) is 1 millisievert per year*. Conventionally, we design work processes so that no third party can receive more than 0.3 mSv/yr (the “design dose constraint”) from any one source. At 0.25 uSv per scan, you would have to undergo over 1000 CONPASS scans per year to exceed the design constraint, and 4000 to exceed the statutory limit. It is impossible for aircrew to exceed the constraint, and most unlikely that even security staff could exceed the statutory limit for “other persons”. The limit for employees is 20 mSv/yr, well beyond the likely exposure of even the most incompetent and self-harming security staff.

Optimisation requires determining what outcome is required from a radiation exposure, and choosing the best available technique for obtaining that outcome. In the case of air passenger scanning for explosives, the optimum is clearly 100% detection of anomalies in any luggage or on the person. Unlike medical radiography, where a missed fracture or tumor is likely to be detected later if the patient complains about continuing pain, and 90% “first-time hits” may be acceptable, a missed bomb certainly means 200 deaths. False positives are inconsequential if the subject can be rescanned at low risk or hand-searched. False negatives are intolerable. Precise first-time diagnosis is not required — the suspicion of an anomaly is sufficient to trigger a secondary investigation — but detection is essential.

“ALARA” is secondary to optimisation in the matter of detection rate, but must be applied to the exposure of staff and other persons, including those waiting to be scanned. This is a matter of optimal design and operation of each specific installation, and can be addressed for each device installed.

Justification is in this case closely bound up with optimisation. If radiological technique A delivers more or less dose than B, we need to consider the benefit/risk ratio of both techniques. Conventional medical radiology approaches are inadequate: a false negative passenger scan will result in 200 immediate and certain fatalities, whereas a failure to diagnose, say, early tuberculosis, may lead to a few extra, treatable infections.

Thus if A and B involve similar doses, well below any reasonable constraint, but B has a measurably lower detection rate for any or all anomalies, the radiation dose from B is not justified since it confers no benefit. In particular we can see that surface analysis such as low-energy backscatter, can be bypassed by a passenger secreting contraband in the anal or vaginal passage, and the radiation dose, however small, is not justified, so the technique must not be used.

Alan M Calverd MA PhD CertRPA
Quality Associates
35 Badgers
Bishops Stortford
Herts CM23 4ET
England


*The terminology varies for different legislatures, but the number is universally recognised