Which index is derived from measurements of smoke obscuration versus time in NFPA 255 testing?

• A. Flame Spread Index
• B. Smoke Developed Index
• C. Life Safety Evaluation
• D. Incapacitation

Answer: (B)

NFPA 255 uses how much smoke a material produces as it burns to classify its fire characteristics. The measurements of smoke obscuration over time are processed to yield the Smoke Developed Index, which quantifies the amount of smoke generated relative to a standard reference. This index is a key measure of how smoky a material is during a fire, affecting visibility and inhalation risk. Flame spread is determined from how far the flame front travels along the material, not from smoke measurements, so it isn’t derived from the obscuration data. The other two options aren’t indices used in this context. So, the index based on smoke obscuration versus time is the Smoke Developed Index.