Brown burn spots and discoloration of this cover are the effects of the irradiation process implemented by the US Postal Service in October 2001. This irradiated cover is addressed to Ms. Esther Washington, from the Singapore Philatelic Society, and dated December 1, 2001. It is marked "MAIL SANITIZED."
There is correspondence inside the cover, but the cover has never been opened.
The practice of irradiating mail was initiated for all Washington, DC, Federal offices, following incidences of anthrax-contaminated mail being received by the US federal government, American Media Inc., and the National Broadcast Company (NBC). The anthrax attacks killed five people and made 17 others ill.
Mail addressed to the National Postal Museum is decontaminated at the irradiation facility run by Titan Industries in Lima, Ohio. Although the irradiation process is effective against eliminating biological threats, it has drawbacks. The process is expensive and slows delivery. Additionally, it can damage mail pieces, such as discoloring paper and burning film and photographs.
Nakashima, Ellen. “Postal service may end irradiation” Washington Post. December 4 2001. p. A23.
Cole, Leonard A., Anthrax Letters, A Medical Detective Story. Washington DC: Joseph Henry press, 2001.