Final answer:
In the NFPA 704 system, a number 4 in the left blue quadrant represents a severe health hazard. The number 4 indicates the substance poses a significant risk to health if exposed.
Step-by-step explanation:
If the NFPA 704 hazardous material identification system has a number 4 in the left blue quadrant, it represents a health hazard. The NFPA 704 Hazard Identification System, sometimes referred to as the 'hazard diamond' or 'fire diamond', is a standardized system for informing emergency personnel and workers about the potential hazards of materials. Each quadrant of the diamond corresponds to a different type of hazard: the blue (left) quadrant specifies the level of health hazard, the red (top) quadrant indicates the flammability hazard, the yellow (right) quadrant relates to the reactivity hazard, and the white (bottom) quadrant identifies special hazards. The system uses a numerical rating that ranges from 0 (indicating no hazard) to 4 (indicating a severe hazard). Therefore, a number 4 in the blue quadrant signifies that the substance poses a severe health risk to exposed individuals.