Answer to Question #1438 Submitted to "Ask the Experts"

Q

In regards to irradiation of the mail, I would appreciate a list of items that would not be damaged by high radiation exposure and those that would be damaged by 40 kGy or higher.

A

Our best indications are that the dose required to disinfect mail of anthrax spores is, as your question suggests, in the range of 40 to 50 kGy. However, note that, since the response of the spores to radiation is log/linear with dose, the actual dose required depends on the initial bioburden and the degree of disinfection required. This dose is quite high, even by medical device sterilization standards (normally about 25 kGy). There is a long list of things that might be destroyed or rendered unserviceable, even to a dangerous degree, by irradiation in the post. For example, a medical disposable device sent through the mail that had already been sterilized with radiation could receive a further high dose of radiation which might degrade the device material and cause it to fail in use with serious harm. The following is a list (not nearly a complete list, I'm sure) of some of the things that I can think of that may be adversely affected by radiation treatment:

 

1. photographic film
2. dyes (bleaching)
3. colouring on papers and cloth (bleaching and fading)
4. environmental samples
5. medical devices
6. pharmaceuticals
7. cosmetics
8. foods
9. some plastics (embrittlement or change of functional properties through scissioning or cross linking)

I'm not sure what the effects of high doses of radiation would be on magnetic storage media but it probably isn't good. Note, as well, that the temperature rise in metals for a dose of 40 kGy will be in the range of 80 degrees centigrade if an electron beam is used directly. Some plastic/metal devices or plastic materials adjacent to metal materials may be overheated or distorted. This, of course, could be mitigated, either by fractionating dose with cooling off periods in between or by using x rays. The latter approach results in a reduction in efficiency in the use of radiant energy as a result of losses due to x-ray conversion, but also has the advantage of allowing penetration of whole sacks of mail on pallets. I understand that the U.S. Post Office is using the x-ray approach now. There is another possible reason for using x rays. Direct electron beams produce static charging effects in materials. The discharging of these static charges can result in "treeing" (dialectric breakdown) of plastics and damage to semiconductors in electronics components. Of course such static discharges could also detonate any explosives sent through the mail. I'm not sure that anyone has thought of all the possibilities yet.

John Barnard, PhD
Director of Facilities
Acsion Industries Incorporated