Objective To investigate an outbreak characterized mainly by respiratory symptoms in a factory workshop, aiming to identify the source of infection and risk factors.

Methods Case search was conducted by formulating hierarchical case definitions. Individuals were investigated, clinical symptoms, laboratory tests, and characteristics of the three dimensions distribution of disease (time, place, and person) were analyzed. Environmental hygiene investigations and sample testing were performed on site to analyze the risk factors and source of infection of the outbreak.

Results All cases were concentrated in the operation workshop on the first floor, with an attack rate of 35.45% (39/110). The main symptoms included fever (89.74%) and cough (69.23%). Blood tests indicated that the proportions of patients with elevated white blood cells and neutrophils were 72.73% and 66.67%, respectively. The onset time was mainly concentrated on July 21 to July 22, with an average incubation period of 8.6 hours, and the epidemic curve exhibited an intermittent common-source exposure pattern. Spatial analysis showed that the "SMT" area had the highest incidence rate; the risk of illness in areas near the humidifier was 2.934 times that in non-exposed areas, and the risk for day-shift workers was 3.349 times that for night-shift workers. Legionella pneumophila was detected in 10 environmental samples, and 2 out of 3 hospitalized patients tested positive for Legionella pneumophila IgM antibodies in their serum.

Conclusion This outbreak was caused by contamination of the humidifier water tank and pipelines with Legionella bacteria, with Pontiac fever being the predominant clinical phenotype. The investigation revealed that in a confined industrial environment, humidifiers can convert bacterially contaminated water into inhalable aerosols through atomization, which may then spreadvia the air-conditioning system, leading to exposure of multiple individuals within a short period. The fundamental cause of the outbreak was the lack of standardized cleaning, disinfection, and microbial monitoring of the humidification and ventilation systems by the enterprise. It is recommended to strengthen occupational health management, establish a mechanism for regular maintenance, disinfection, and pathogen monitoring of water systems, and enhance health education for employees to prevent similar incidents from recurring.