The letter "A" indicates that a material is only regulated if shipped by which mode?

The letter "A" is used in the context of hazardous materials to specify that a material is only regulated when shipped by aircraft. This classification is essential for ensuring the safety and compliance of hazardous materials in transport, as the regulations for air shipping are often stricter due to the unique risks associated with aviation.

Air transport is subject to specific guidelines established by organizations such as the International Air Transport Association (IATA) and the Federal Aviation Administration (FAA), focusing on the potential hazards these materials may pose in flight, including flammability, toxicity, and environmental dangers. Thus, the designation of "A" serves to inform shippers and carriers that the material must adhere to these stringent regulations exclusively when being transported by air.

The other modes of transport—road, rail, and water—may have different regulatory requirements and do not specifically require the same level of scrutiny for the materials that are labeled with the letter "A."