United States Postal Service irradiated mail
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.
Source: Wikipedia — United States Postal Service irradiated mail (CC BY-SA 4.0)