Publication 52, Hazardous, Restricted, and Perishable Mail > 3 Hazardous Materials > 33 Hazard Classes > 338 Packaging Requirements for Hazardous Materials
Hazardous materials in Classes 3, 4, 5, Division 6.1, Class 8 and some Class 9 substances are assigned to a DOT packing group based on the degree of danger presented by the material during transport. The packing group assigned to a hazardous material is listed in column 5 of the Hazardous Material Table in 49 CFR 172.101 and in column (d) of Appendix A. Packaging Group designators are always written in Roman numerals, for example PG II. Packing Group I indicates a great danger, Packing Group II a medium danger, and Packing Group III a minor danger.
For mailing purposes, the packaging of a mailable hazardous material must follow the appropriate Packaging Instruction in Appendix C. The correct Packaging Instruction to use for a specific hazardous material can be determined using Appendix A.
Mailers may not package or combine hazardous materials assigned to different hazard classes within a single mailpiece unless permitted by Postal Service standard.