Answer to Question #4009 Submitted to "Ask the Experts"

Customs officials are routinely x raying baggage and the ship while we are alongside the wharf in Miami. This x raying is carried out by a large mobile unit which drives along the dock doing its business. Since these units can x ray inside containers made of metal, it is assumed that this machine can also x ray through the ship and into cabins which are situated on the same level. According to specification from a manufacturer of a similar machine, stowaways can be subjected to ~5 mR, which would be one pass I assume. However, this machine used is x raying multiple passes along the length of the ship. Should I as the environmental officer and part of the crew be concerned by this?

In general, these cargo screening systems are not designed specifically as a system that is intended to expose people to radiation for screening purposes. I say that because the American National Standards Institute, in its standard N43.17-2002 "Radiation Safety for Personnel Security Screening Systems Using X-Rays," states that no individual should receive a dose in excess of 25 mR in a 12-month period from personnel x-ray screening devices.

I was unable to verify that someone might receive as much as 5 mR from one screening pass of this cargo system but have no reason to doubt the information as true. That amount of radiation exposure is minimal. I think, though, that we have to keep a few things in mind.

One is whether the individuals being exposed need to be. Can they do something so they will not be exposed at all or as much when the ship is being scanned?

Another is the relative benefit. I suspect these pieces of equipment are operated with the benefit being the overall security of the country—a huge benefit. So while it may not benefit any one individual (like in the case of a person having a medical x ray to find out whether a bone is broken), the insignificant additional risk from the radiation exposure is far outweighed by societal benefit.

The third item to keep in mind is that most states have regulations stating that members of the general public should receive no more than 100 mrem (mrem, for our purposes is the same as mR) in a year from man-made sources of radiation. Your state's regulations may also say this, which is something you may want to check.

You didn't mention how many passes there might be or how often someone might be exposed but, I hope, with the two numbers I've given—the 25 mR and the 100 mrem—you can make some decisions regarding whether as the environmental officer, you need to do any follow-up.

Kelly Classic
Certified Medical Health Physicist