The occupants of a building wing complain of nausea, dizziness, or drowsiness.

Symptoms of nausea, dizziness, and drowsiness are typical of poor indoor air quality (IAQ), most often due to insufficient outside air ventilation leading to buildup of carbon dioxide or other contaminants.

C. Insufficient outside air (Correct).

A. Mercury spill: Causes neurological issues but less common in general building complaints.

B. Humidity levels: Can cause discomfort but not acute dizziness/nausea.

D. Medical gas leak: Hazardous, but would usually trigger alarms and involve smaller affected areas (not whole wing complaints).