Hazardous materials

Means a substance or material which has been determined by the U.S, Secretary of Transportation to be capable of posing an unreasonable risk to health, safety, and property when transported in commerce and which has been so designated. Title 49, Code of Federal Regulations (U,S,) Transportation-Parts 100-199, govern the transportation of hazardous materials. Hazardous materials may be transported domestically, but they may be classified as Dangerous Goods when transported internationally by air.
(Air Cargo from A to Z, Air Transport Association of America)