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 in the US 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 . 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
2001 anthrax attacks
2001 anthrax attacks
United States Postal Service Irradiated mail