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. A flammable solid that can qualify as an ORM–D material is 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 are eligible to be reclassified as mailable limited quantity or ORM-D 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.
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 “ORM–D” immediately following or below the proper shipping name (i.e., Consumer Commodity). Optionally, mailpieces may be marked with an approved DOT limited quantity square-on-point marking (see Exhibit 325.2a). The top and bottom portions of the square-on-point and the border forming the square-on-point must be black, and the center must be white or of a suitable contrasting background. Surface shipments containing qualifying ORM-D material bearing the square-on-point limited quantity marking are not required to be marked with the shipping name and identification number. A shipper’s declaration for dangerous goods is not required for mailable Class 4 materials sent via surface transportation. When DOT square-on-point markings are used, markings must be durable, legible, and readily visible, and must be applied on at least one side or one end of the outer packaging. The border forming the square-on-point must be at least 2 mm in width, and the minimum dimension of each side must be 100 mm, unless the package size requires a reduced size marking of no less than 50 mm on each side.
- 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.