An outbreak of methyl mercaptan intoxication caused by an inappropriate arrangement of exhaust ventilation system in a laboratory

Objectives and Backgrounds: This study is to determine the cause and etiological agent of an outbreak of chest tightness, upper respiratory symptoms and dizziness among a group of office workers. The incident occurred on the morning of July 8th, 1998, when a total of 13 office workers smelt a foul odor and developed symptoms of throat irritation (77%), chest tightness (62%), nose irritation (46%), fatigue, dizziness and drowsiness (69%). Some of them suffered from diarrhea (46%), nausea (31%), muscle weakness (38%) and a staggering gait (15%). The odor stayed in the office for about 2 days before it faded. Clinical Examination and Field Investigation: We followed up these workers for three months with detailed clinical examinations and laboratory tests. No significant clinical sequelae or laboratory abnormalities were found. A field investigation found that this outbreak correlated well with an experiment, which discharged methyl mercaptan on the 5th floor of the same building. The outlet of the laboratory discharge directly faced (less than 1 meter away) the inlet for the general ventilation pipelines for the building. As the density of methyl mercaptan is heavier than that of air, it sank to the first floor and the incident occurred. After the duct and outlet for the laboratory exhaust were extended to the roof of the building, the outbreak stopped. Conclusions: The outbreak was due to methyl mercaptan and the poor design of the local exhaust ventilation system.