By John C. Zink, Ph.D., P.E.,
I was surprised by the lack of public controversy over the U.S. Postal Service’s (USPS) irradiation of the mail. Although I have not made an effort to determine the anti-nuclear organizations’ responses to this use of radiation, it is noteworthy that the customary scare words which accompany anything nuclear were nowhere to be seen in the press. Ours is an era when food irradiation to eliminate disease-causing bacteria must be euphemistically called “electronic sterilization,” and nuclear magnetic resonance imaging in medicine has dropped the “nuclear” from its now politically-correct name. Yet, there was no attempt to sugarcoat the fact that the mail is being irradiated to kill any potential anthrax contamination.
Although heat, especially in the form of steam, can kill anthrax, USPS decided its customers would object to receiving soggy mail with smeared ink and steamed-open envelopes. Irradiation by gamma rays, x-rays or beta-particles (electrons) is highly effective in killing anthrax in either the spore or vegetative states, but would have no adverse effects on most mail items. Clearly, USPS chose the better option in selecting irradiation.
The web site of the Health Physics Society (HPS) contains some interesting information regarding anthrax elimination using radiation. According to HPS, researchers have determined that the radiation dose required to kill anthrax depends somewhat on the particular strain and form of the bacteria. In general, however, radiation doses from 40 to 50 Kilogray are effective in killing anthrax. This is about twice the typical dose used to sterilize medical devices. (For those of us more accustomed to the old system of radiation units, 1 Gray = 100 rads, so 50 Kilogray is equivalent to 5 million rads – a big radiation dose in any units.)
In addition to considering the bacteria’s characteristics, USPS had to take into account the great variation in the pieces of mail it handles. For example, commercial electron beam equipment, which is typically used for sterilizing medical devices, can be effective in treating normal envelopes and some thin packages. Thicker packages require more-penetrating radiation such as gamma rays. Some industrial x-ray machines could substitute for gamma ray sources, but commercial medical x-ray machines are not powerful enough to deliver the required dosage.
Besides the problem of how to deliver the radiation in the right form and in the right quantity, there is the issue of when an item has been treated adequately. It is not practical to insist that all anthrax organisms be killed, but it is necessary that their number be reduced below a hazardous level. Companies that use radiation routinely to sterilize medical devices must comply with ISO 11137: Sterilization of Healthcare Products – Requirements for Validation and Routine Control – Radiation Sterilization. This standard covers all means of delivering the radiation: electron beam, x-ray machine or gamma-ray source. But since there is no such standard for treating purposely tainted mail, USPS has had to develop new criteria for controlling the radiation dose delivered and for determining its effectiveness.
It is well known, of course, that radiation will partially expose or “fog” photographic film. So, the anthrax treatment will certainly harm any film going through the mail. It is also possible that high radiation doses may harm such things as cosmetics, dyes and pharmaceuticals. Some polymers experience damage by cross-linking under radiation, so the radiation is potentially damaging to plastic items, making them brittle. Of most concern, however, is the fact that electron beams can produce static charging effects in many materials. When these static charges eventually discharge, they can create “treeing” in plastics, which causes dielectric breakdown in these materials. The static charges can also damage semiconductors in electronic components. Shippers that use USPS to deliver merchandise need to be aware of these potential risks.
Last October and November USPS contracted with San Diego-based Titan Corp., and a Belgium-based company, Ion Beam Applications (IBA), to provide electron beam systems and services to treat mail for potential anthrax contamination. IBA has given USPS sole access to its Bridgeport, N.J., facility, which can treat high volumes of mail using electron-beam and x-ray technology. According to Nuclear News, an IBA official said, “It must be recognized … that a definitive long-term solution to contaminated mail will require readdressing the logistics of mail, identification of fragile contents, and installation of specific sterilization equipment in the mail sorting centers.”
If the public continues to accept radiation sterilization of its mail, as it seems to have done so far, will it then begin to see radiation as just another technology in service to mankind? Will the anthrax crisis thus contain a silver lining for nuclear technology? It is too soon to tell, of course. The patriotism that may be restraining some of the anti-nuclear groups will wear off eventually, leading them to renew their attacks. But it may have become more difficult for them to demonize all things nuclear.