What signifies the most serious health hazard in the NFPA 704 labeling system?

In the NFPA 704 labeling system, which is used to identify the hazards associated with materials, the health hazard is indicated by a blue color-coded section of the diamond. The severity of health hazards is rated on a scale from 0 to 4, where each number corresponds to a specific level of danger.

A rating of 4 signifies the most serious health hazard. This level indicates that the substance poses a significant risk of fatality or serious injury upon short exposure, even a few minutes. It is essential to indicate this level prominently so that first responders and emergency personnel can quickly assess the necessary precautions and protective measures required when dealing with the material.

In contrast, lower ratings, such as those in the 0-1 range, indicate minimal or no significant hazards, and ratings of 2-3 represent increasing levels of risk but do not reach the critical severity associated with a 4 rating. Therefore, the designation of 4 as the highest risk reflects the urgent need for extreme caution and appropriate safety protocols when encountering such materials.