In response to the anthrax bioterrorism of October 2001, the Office of Science and Technology Policy determined that targeted irradiation of mail during processing by the U. S. Postal Service (USPS) is the most effective method to neutralize such biohazards. Irradiation is a proven sterilization technology and has been used to sanitize medical equipment for decades.
The Department of Defense Armed Forces Radiobiology Research Institute established the minimum irradiation dose required to ensure the complete eradication of anthrax spores. Preferring to err on the side of caution, the USPS typically uses doses greater than the established minimum for the sanitation of mail.
Currently, all mail directed to the White House, Congress, and the Library of Congress is irradiated. For mail directed to government agencies in ZIP Codes 20200 through 20599, all letters, flats, Express, and Priority mail with stamps for postage are irradiated.
In addition to the desired neutralization of biohazards, the irradiation process can produce undesirable effects that result from both the irradiation itself and the heat generated on its interaction with materials.
Many of the immediately visible irradiation effects are due to the high temperatures experienced, which can reach 130 °C [266 °F] under the current dosage protocols: desiccation and embrittlement; browning or other discoloration; melting, fusing, or blocking.
Chemical reactions induced by irradiation, and the resulting damage, may be manifested immediately or may develop over time: depolymerization and loss of strength; embrittlement, acidification, discoloration; and accelerated rates of subsequent aging deterioration in susceptible materials.
Ionizing radiation can immediately disrupt photographic, magnetic, and electronic media. The energy of the irradiation systems in use for the sterilization of mail is NOT sufficient, though, to induce radioactivity in the treated materials, even if they are metal.
The irradiation of U. S. government mail and its deleterious effects are of concern to the present users and future custodians of our nation’s archival heritage. The National Archives and Records Administration will soon issue a NARA Bulletin to provide federal agencies and records officers with guidance on managing and maintaining the irradiated mail that must be incorporated into agency files.
For further reading, the Smithsonian Center for Materials Research and Education (SCMRE) has posted two online articles that provide both a theoretical background to and practical observations of the effects of irradiation on museum and research specimens, artifacts, and records:
“The effects on research specimens and museum collection items from electron beam irradiation of mail by the U. S. Postal Service.” Ann N’Gadi, Technical Information Officer, SCMRE, November 5, 2001.
“Recent Examination of Some Irradiated Mail.” David von Endt, David Erhardt, Abdel-Salam El-Esseily, Walter Hopwood, Marion F. Mecklenberg, and Charles S. Tumosa, SCMRE, February 2002.