A number 4 in the blue quadrant of the NFPA 704 system indicates there is ______ risk in this category from the chemical involved.

• A. Severe
• B. Moderate
• C. Minimal
• D. No

Answer: (A)

NFPA 704’s blue health quadrant uses a 0–4 scale, where 4 is the most severe health hazard. A 4 in this quadrant means there is severe risk to health from exposure to the chemical—potentially life-threatening injury or death with even brief exposure. This is why the best answer is severe. The other levels indicate progressively lower risks: lower numbers imply less severe health effects, so a 4 cannot be described as moderate, minimal, or none. In practice, a substance with a blue 4 demands maximum precautions to prevent exposure.