These placards are compliant with NFPA 704.
This standard presents a simple, readily recognized, and easily understood system of markings (commonly referred to as the "NFPA hazard diamond") that provides an immediate general sense of the hazards of a material and the severity of these hazards as they relate to emergency response.
provides criteria for assessing the health, flammability, instability, and related hazards that are presented by short-term, acute exposure to a material under conditions of fire, spill, or similar emergencies. A number rating system of 0-4 is provided to rate each of the four hazards and is placed on a placard to provide emergency responders with the information they need to determine the immediate actions to be taken in an emergency. Tables in the standard provide the criteria for the ratings and placard specifications such as letter size and arrangement of numbers and colors are provided in the standard.
This is the placard only. If you're looking for the placard kits (which include the symbols as well as the numbers), they can be found here.