Legionella pneumophila pollution and influencing factors in cooling water of public transport buildings in a city of China

Background Legionella pneumophila is found extensively in natural water bodies and artificial water environments, and can cause serious Legionnaires' disease. There are limited studies on the influencing factors and control strategies of Legionella pneumophila.

Objective This study investigates the status of Legionella pneumophila contamination and potential influencing factors in cooling water of central air conditioning system in public transport buildings, aiming to provide scientific evidence for risk control of Legionella pneumophila in cooling towers.

Methods A total of 174 water samples were collected from 121 cooling towers in public transport buildings in the selected city from September to October in 2018 and from August to September in 2019 based on convenient sampling. Legionella pneumophila colony and serotyping were detected in the water samples. Nine water quality indicators were analyzed simultaneously, including the total number of colonies, color, turbidity, conductivity, total dissolved solids, pH, iron, water temperature, and free residual chlorine. The information about cooling tower surrounding environment and operation management was collected by on-site inspection. Chi-square test and multiple logistic regression were used to analyze the influences of water quality, disinfection methods, and management companies on the positive rate of Legionella pneumophila.

Results Of the 174 cooling water samples, 83 samples were Legionella pneumophila positive, with a total positive rate of 47.70%; 7 serotypes were detected, of which the dominant Lp1 accounted for 59.04% of the positive samples. The Chi-square test results showed that under different conditions of water temperature, free residual chlorine, total dissolved solids, and iron, the differences in the positive rate of Legionella pneumophila were statistically significant (P < 0.05); there was also a significant difference in the positive rate of Legionella pneumophila in cooling towers under the management of different operating companies (P < 0.05). Further multiple logistic regression analysis results showed that water temperature (OR=1.250, 95% CI: 1.111-1.407) was the major risk factor for being Legionella pneumophila positive. Compared with company A, cooling towers managed by companies B, C, and D were more likely to be detected Legionella pneumophila positive (OR=3.704, 95%CI: 1.264-10.855; OR=3.088, 95%CI: 1.057-9.019; OR=2.708, 95%CI: 1.190-6.164).

Conclusion Legionella pneumophila contamination exists in the cooling towers of public transport buildings in the selected city. Water temperature control and effective management measures are key to the reduction of the growth and spread of Legionella pneumophila. Taking comprehensive measures at both technical and management levels is the best strategy for the risk control of Legionella pneumophila in central air conditioning cooling water system of public buildings.