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Publication 52 - Hazardous, Restricted, and Perishable Mail >
2 General Guidelines > 22 Marking Requirements > 224 Tag 44 for Hazardous Materials
Category B infectious substances are the only type of hazardous materials permitted to be placed in a sack or tub. All mailpieces must be properly marked with the proper shipping name “Biological Substance, Category B” and “UN3373” inside a diamond marking as outlined in 346.32a. A sack or tub containing Category B infectious substances must have attached Tag 44, Sack Contents Warning. The tag must be removed when the Category B infectious substances are no longer in the sack or tub. See Chapter 7 for more information.