Irradiated mail is mail that has been deliberately exposed to radiation, typically in an effort to disinfect it. The most notable instance of mail irradiation occurred in response to the 2001 anthrax attacks; the level of radiation chosen to kill anthrax spores was so high that it often changed the physical appearance of the mail, in some cases spectacularly so.

The United States Postal Service began to irradiate mail in November 2001, in response to the discovery of large-scale contamination at several of its facilities that handled the letters that were sent in the attacks.

A facility in Bridgeport, New Jersey, operated by Sterigenics International, uses a Rhodotron continous wave electron beam accelerator built by IBA Industrial, to irradiate the mail. A few facilities were planning to use cobalt-60 sources, though it is unclear whether this was ever done.

Effect on mail

The USPS warned that a number of products could be adversely affected, such as seeds, photographic film, biological samples, food, medicines, and electronic equipment. In addition, a number of papers and plastics have been observed to react badly to the irradiation; paper may become grayed and fragile, and some plastics have bubbled or melted.

Irradiation's effects on paper caused some alarm in the philatelic world, which sends large numbers of rare postage stamps and covers through the mail. A number of auction houses stopped sending material through the mail, and Linn's Stamp News regularly featured reports on stamps and covers that had been ruined by irradiation.

Although at one time the USPS expected to irradiate all mail, it later scaled back to just treating mail sent to government offices.