Hazard Class 4 consists of three divisions:
- Division 4.1, Flammable Solids. Any solid material other than one classed as an explosive that, under conditions normally incident to transportation, is likely to cause fires through friction or retained heat from manufacturing or processing, or that can be ignited readily and, when ignited, burns so vigorously and persistently as to create a serious transportation hazard.
- Division 4.2, Spontaneously Combustible. A liquid or solid pyrophoric material that even in small amounts and without an external ignition source can ignite within 5 minutes after coming in contact with air, or a self–heating material that when in contact with air and without an energy supply is liable to self heat.
- Division 4.3, Dangerous When Wet. A material that, by contact with water, is likely to become spontaneously flammable or to give off flammable or toxic gas at a rate greater than 1 liter per kilogram of the material per hour.
Examples of flammable solids include certain metallic hydrides, metallic sodium and potassium, oily fabrics, processed metals, matches, and nitrocellulose products.
The following conditions apply:
- International Mail. All flammable solids are prohibited.
- Domestic Mail via Air Transportation. All flammable solids are prohibited.
- Domestic Mail via Surface Transportation. Flammable solids that can qualify as a Limited Quantity surface material are permitted.
When flammable solids that are nonmailable under 344 are found in the mailstream, the procedures in POM 139.117 must be followed if the materials present an immediate threat to persons or property.
Strike–anywhere matches are nonmailable in international mail and domestic mail. Safety matches (book, card, or strike–on–box) are nonmailable in international and domestic mail via air transportation.
The following are mailable:
- Flammable solids that qualify as Limited Quantity surface materials are permitted in the domestic mail via surface transportation.
- Safety matches (book, card, or strike–on–box) are permitted only in domestic mail via surface transportation.
All labels and text markings must be placed on the address side of the mailpiece unless specified in 221.1 and 325.1. The following conditions apply:
- Mailable Flammable Solids. The conditions in Packaging Instruction 4A in Appendix C must be followed. Mailpieces containing mailable Class 4 materials must be plainly and durably marked on the address side with an approved DOT Limited Quantity ground marking (see 325.4). Shipments bearing the Limited Quantity surface marking are not required to be marked with the proper shipping name and identification number. A shipper’s declaration for dangerous goods is not required for mailable Class 4 materials sent via surface transportation.
- Safety Matches. The conditions in Packaging Instruction 4B in Appendix C must be followed. Mailpieces must be plainly and durable marked on the address side with “Surface Only” or “Surface Mail Only” and, as applicable, “Book Matches,” “Strike–on–Card Matches,” or “Card Matches.” A shipper’s declaration for dangerous goods is not required.