Final answer:
The blue section of the NFPA 704 marking system indicates the level of health hazard associated with a substance.
Step-by-step explanation:
In the NFPA 704 marking system, the blue section indicates the level of health hazard associated with a particular substance. This means it provides information about how the substance can affect a person's health if they are exposed to it. The blue section is one of the four sections in the NFPA 704 diamond symbol, which also includes sections for flammability, reactivity, and special hazards.