Transmission of Infectious Bioaerosols : Sources, transport and prevention strategies for airborne viruses and bacteria

Abstract: Infectious diseases that can be transmitted via air often spread rapidly, sometimes causing large epidemic and pandemic outbreaks. As an increasing number of people live in crowded urban environments, and with frequent and long-distance traveling across the world, infectious diseases can spread even faster. Yet, our knowledge of how much airborne transmission (here defined as aerosol particles <100 µm that contain infectious agents) contributes to the spreading of diseases is scarce and frequently debated. The aim of this thesis was to increase knowledge about the sources and airborne transport of infectious bioaerosols in order to prevent diseases from spreading via air. To identify possible sources of infectious bioaerosols, we collected air samples in hospitals for detection of bacteria (in operating rooms) and norovirus (in hospital wards) and correlated the results with possible source events. To study bacterial viability and viral infectivity after airborne transport, we developed an experimental setup in the laboratory where aerosolized model organisms were examined. The setup was also used to evaluate the particle collection efficiency of a novel bioaerosol sampler. In addition, three types of high-airflow ventilation systems for operating rooms were compared for their ability to maintain clean air during ongoing surgery.The median bacterial concentrations measured in operating rooms ranged from 0 to 22 CFU m-3 (colony forming units) depending on the sampling point and ventilation type. However, no correlations were found between bacterial concentrations and the number of door openings or the number of people present in the room. Based on the comparison of three types of ventilation, we concluded that the two ventilation techniques with the incoming airflow above the operating table, directed downwards, resulted in lower bacterial concentrations close to the wound than the ventilation based on turbulent mixing.We detected norovirus RNA in air samples collected in hospitals during outbreaks of the winter vomiting disease. Our results showed a significantly higher risk of finding norovirus RNA in the air within a short time (3 h) after a patient vomited. From size-separated sampling, norovirus was detected in aerosol particles >4.5 µm and <0.94 µm, indicating that norovirus has the potential to remain airborne for hours and spread in indoor environments. To evaluate the infectivity of airborne norovirus, murine norovirus was used as a model organism in a laboratory study. The infectivity of murine norovirus relative to the virus genome copy number was reduced by two orders of magnitude when aerosolized by either twin-fluid nebulization or bubble bursting. We proposed that aerosol droplet drying from a low-solute solution caused the loss of viral infectivity. A similar experimental setup, was used to study the viability of Pseudomonas syringae in air with varying levels of relative humidity. The bacterial survival was higher when aerosolized into air with low relative humidity, corresponding to rapid drying. For detection of bioaerosol sources in the field, we evaluated the particle collection efficiency of a novel electrostatic bioaerosol sampler. Owing to the small liquid collection volume of ~0.3 mL, the new bioaerosol sampler had higher sample concentrations than a commonly used impinger when collecting microspheres of sizes >1 µm.Airborne transmission of infectious diseases has long been neglected; however, as new infectious diseases emerge, knowledge that can be generalized across organism types is highly valuable. With this research, I highlight its importance, in particular for nosocomial infections, by showing that sufficient concentrations of bacteria and viruses are present in hospital air that can trigger new infections, and that bacteria and viruses aerosolized under controlled laboratory conditions remain viable and infectious. Finally, I also show that by choosing appropriate preventive measures, such as room ventilation, airborne microbial concentrations can be significantly reduced, limiting transmission of airborne disease.