Final answer:
The quantity of combustible metals requiring an "M" on the 704 placard isn't standardized and varies depending on specific metal characteristics and guidelines. Entities must consult SDS and guidelines for appropriate placarding.
Step-by-step explanation:
The query about the quantity of combustible metals that mandates an "M" in the fourth space of a 704 placard refers to the National Fire Protection Association (NFPA) 704 Diamond. The NFPA 704 Diamond is a standard system for identifying the health, flammability, instability, and special hazards presented by chemicals, including combustible metals, through a coded color system. When it comes to combustible metals, an "M" is placed in the special hazards quadrant of the diamond to indicate that the material can burn in the presence of moisture or that it is a metal fire hazard.
The specific quantity that would require the "M" hazard symbol is not standardized across all materials, as the need for the symbol can vary depending on the characteristics of the specific metal and its form. It's ultimately the responsibility of the entity that stores or uses such materials to determine, based on safety data sheets (SDS) and industry guidelines, when an "M" should be included on the placard for safety communication.