Final answer:
The number 4 on the NFPA 704 hazard diamond indicates the greatest hazard across the categories of flammability, health, reactivity, and special hazards.
Step-by-step explanation:
On the NFPA 704 symbol, the number that indicates the greatest hazard is 4. This symbol, also known as the hazard diamond, is used to provide safety information about certain substances. It assesses hazards in terms of flammability (red diamond), health (blue diamond), reactivity (yellow diamond), and other special hazards (white diamond). Each of these criteria is rated on a scale from 0 (representing no hazard) to 4 (indicating extreme hazard).
The NFPA 704 label is often found on chemical storage containers and in facilities where chemicals are used, providing critical information to emergency responders and employees about the potential risks posed by the materials. On an NFPA 704 symbol, the greatest hazard is indicated by a number 4. The NFPA 704 Hazard Identification System rates hazards on a scale from 0 to 4, with 0 being no hazard and 4 being extremely hazardous. The higher the number, the greater the hazard.