[Environmental factors relating to a mass outbreak of tuberculosis in a junior high school]

To clarify environmental factors relating to a mass outbreak of tuberculosis.

A 15-year old girl, a third-grade student of a junior high school (the index case) was found to have smear-positive cavitary pulmonary tuberculosis. Among 718 subjects who underwent contacts investigation, the rates of infection and cases among different exposure groups were compared. The ventilation rate within the room of the junior high school was analyzed using sulfur hexafluoride (SF6) as the tracer gas.

Up to 56 months after the detection of the index case, a total of 34 tuberculosis patients were newly diagnosed, and 155 persons were subjected to chemoprophylaxis. The rates of infection were 90.0% among homeroom classmates and 60.8% among other classmates, respectively. Out of the subjects who had only indirect contact with the index case, 11 patients were diagnosed. Most of the windows of the building were of the fixed sash type, permitting only low ventilation ranging from about 1.6 to 1.8 room air change per hour. When sliding doors of the room were opened, the SF6 concentration in the room was rapidly mixed up with that in the passage.

Low ventilation of the room and overcrowding contributed to the high infection rate among homeroom classmates. Infectious droplet nuclei spread to the passage at recess. The homeroom of the index case was located in front of the building's entrance. The index case used some common rooms of the building. In addition to these environmental factors, other factors, especially the high infectiousness of the index case also contributed to the mass outbreak.