Indoor Airborne Pathogen Transmission and Control: A Role of Environmental Factors and Material Properties

The transmission of infectious airborne pathogens has severe implications on human health and economic stability. This is evidenced by the re-emergence of global pandemics with high mortality rates, various respiratory diseases, crop damage, incurred nationwide financial costs, etc. These causative pathogens, encompassing viruses, fungal spores, etc., attach to aerosol and droplet particles, enabling their aerial dispersal and leading to a cascade of detrimental events. Developing interventions to control pathogenic transmission requires an in-depth understanding of the various modes of disease transmission. However, existential gaps within the respective disease transmission modalities remained. For instance, while viruses are predominantly transmitted via aerosols, droplets, and fomites (i.e., materials), fomite-mediated viral transmission is not well understood. Additionally, airborne fungal spore irradiation-based control strategies necessitated increased efficacy with higher temporal resolutions. In this study, the dichotomy of aerosols (< 5 μm; 5 – 100 μm) vs. droplets (> 100 μm) in relation to indoor viral persistence on fomites was investigated. In light of the recent COVID-19 pandemic, this study was conducted with a SARS-CoV-2 surrogate, Phi6. Preceding the investigation, empirical methodologies were developed to understand the bio-colloidal behavior of Phi6 and effectively sample the viral aerosols and droplets. A full-scale aerobiological chamber with well-controlled aerosol generation was also utilized to adequately mimic the built environment. Levitated viral aerosols were then pre-exposed to the ambient environmental stressor of humidity, followed by their deposition on test materials. The resultant infectivity was in juxtaposition with that from conventional viral droplet inoculation on similar test materials. Specifically, we found that there was a 1 – 4 log10 decrease in infectivity on the porous materials relative to the nonporous materials when the test materials were inoculated with conventional viral droplets. However, with viral aerosols, infectivity was similar across both the porous and nonporous materials, revealing the dominant role of local humidity over material porosity. These findings unraveled new mechanistic insights into the role of local humidity in determining fomite-mediated viral infection risk. Considering the heterogeneity of the indoor microbiome, we further explored the transmission of airborne fungal spores, particularly under reflectivity-modulated ultraviolet irradiation. We traversed aerosolized spores of Botrytis cinerea, a necrotrophic fungus, via a single pass across a flow-through UV-C reactor with tunable reactor wall material reflectivity. We observed that airborne B. cinerea spores are resistant to high-powered UV-C irradiation induced with specular reflectance. This was demonstrated by the minimal 30% spore inactivation despite the high UV-C irradiation power output of 200 W, as the reactor wall material only provided 85% specular reflectance. However, when the reactor wall material was replaced with one that provided 95% diffuse reflectance, 2 log10 spore inactivation was observed despite the similar irradiation power output. The results emphasize the significance of reflectivity in assessing the viability and transmissibility of airborne fungal spores in the presence of ultraviolet irradiation. The findings from this study provide new knowledge on the influence of material properties such as porosity and reflectivity on airborne viral and fungal transmission, respectively. These insights will help guide control strategies for mitigating infectious disease transmission within the built environment.