What does Fume Category D indicate?

• A. Mild, sweet odor.
• B. Acrid, burning rubber, sulfur.
• C. Pleasant scent with no irritation.
• D. None.

Answer: (B)

Fume categories are used to gauge how potent a cabin air contamination is based on the odor and irritation it causes, so crews can match the level of risk with an appropriate response. For Category D, the odor is described as acrid and very noticeable, specifically burning rubber and sulfur. This combination signals a strong, irritating, potentially hazardous contaminant, often pointing to sources like electrical equipment issues, overheated components, or other combustion-related contamination within the air system. Because the smell is harsh and occupationally risky, it warrants immediate protective actions and escalation per the emergency procedures for smoke or fume events—typically donning oxygen, isolating or limiting exposure if feasible, and preparing for urgent decision-making about diversion or landing.

The other descriptors don’t fit a true fume event: a mild, sweet odor would usually suggest a less dangerous or benign cause; a pleasant scent with no irritation would not align with a real contamination event; and none would imply no fume at all.