| Summary: | The transmission of some infectious diseases requires that pathogens can survive (i.e., remain infectious) in the environment, outside the host. The viability of pathogens that are immersed in aerosols and droplets is affected by factors such as relative humidity (RH) and the chemical composition of the liquid media, but the effects of these stressors on the viability of viruses have not been extensively studied. The overall objective of this work was to investigate the effects of RH and media composition on the viability of viruses in suspended aerosols and stationary droplets. We used a custom rotating drum to study the viability of airborne 2009 pandemic influenza A(H1N1) virus across a wide range of RHs. Viruses in culture medium supplemented with material from the apical surface of differentiated primary human airway epithelial cells remained equally infectious for 1 hour at all RH levels tested. We further investigated the viability of two model viruses, MS2 and Φ6, in suspended aerosols and stationary droplets consisting of culture media. Contrary to the results for influenza virus, we observed a U-shaped viability pattern against RH, where viruses retained their viability at low and extreme high RHs, but decayed significantly at intermediate to high RHs. By characterizing the droplet evaporation kinetics, we demonstrated that RH mediated the evaporation rate of droplets, induced changes in solute concentrations, and modulated the cumulative dose of solutes to which viruses were exposed as droplets evaporated. We proposed that the decay of viruses in droplets follows disinfection kinetics. Lastly, we manipulated the chemical composition of media to explore the stability of viruses as a function of pH and salt, protein, and surfactant concentrations. Results suggested that the effects of salt and surfactant were RH and strain-dependent. Acidic and basic media effectively inactivated enveloped virus. Protein had protective effect on both non-enveloped and enveloped viruses. Results from this work has advanced the understanding of virus viability in the environment and has significant implications for understanding infectious disease transmission. === Doctor of Philosophy === Pathogenic organisms, including bacteria, viruses, fungi, protozoa, and helminths, cause infections that are responsible for substantial morbidity and/or mortality. For example, it is estimated that influenza has caused 9 million to 45 million illnesses and 12,000 to 61,000 deaths annually since 2010 in the United States. The spread of certain diseases relies on people touching the pathogenic organism on surfaces or inhaling it from the air. Successful transmission requires that the pathogen survive, or maintain its infectivity, while it is in the environment. The survival of pathogens can be affected by temperature, humidity, composition of the respiratory fluid carrying them, and other factors. However, there is limited research investigating the effects of these factors on the survival of viruses in the environment. In this work, we studied the effect of relative humidity (RH) on the survival of viruses, including influenza virus and two other types of viruses, in inhalable aerosols and larger droplets. We found that influenza viruses survive well in aerosols across a wide range of RH levels for at least 1 h. Conversely, the two model viruses survived best at both low and very high RHs, such as found indoors in the wintertime or in tropical regions, respectively, but had a pronounced decay at intermediate RHs. By measuring how fast droplets evaporated, we found that RH affected their chemistry and determined the total amount of stress that viruses were exposed to. This explained why a "U-shaped" survival pattern was observed against RH. We also investigated the survival of viruses in droplets containing different components. Results indicated that the effects of salt, surfactant, protein, and droplet pH depended on RH and the type of virus. The outcomes of this work are meaningful in predicting the survival of viruses in aerosols and droplets of various compositions in the environment and could provide insight on developing strategies to minimize the spread of infectious diseases.
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