Exploring the Factors that Determine the Survival of Viruses in Aerosols and Droplets

The transmission of microbes which cause disease between humans, such as bacteria and viruses, can occur by direct (person-to-person), indirect (contact with contaminated surfaces) and airborne routes. Small aerosol particles of a respirable size (<10 micrometres diameter) and larger droplets (up to 100 micrometres, about twice the diameter of a human hair) can remain airborne for minutes to hours and are emitted by coughing, sneezing, speaking and breathing. Larger droplets settle very quickly over short distances contaminating surfaces (referred to as fomites). Although the transmission of some diseases, such as tuberculosis and measles, show preferential airborne transmission, other diseases, such as influenza and norovirus, are opportunistic. Identifying both the circumstances when these different modes of transmission are dominant and clarifying ways to mitigate them must be priorities. Such knowledge informs the implementation of non-pharmaceutical interventions, such as physical distancing and the use of appropriate personal protective equipment such as face masks. Further, the clinical context sees the wide use of many aerosol generating procedures that are poorly understood but could carry pathogens through the air, for example in dental procedures and in anaesthesia. Despite the recognised importance of aerosols and droplets in the transmission of microbes, the evidence-base on which decisions are made to mitigate microbe transmission often remain epidemiological. Robust innovative instruments for studying the factors that control the survival of pathogens in aerosols, droplets and fomites are crucial to move our understanding forward.

Using a novel technology platform developed at the University of Bristol, we will deliver a comprehensive framework for understanding the factors that control the survival of the virus SARS-CoV-2, the cause of COVID-19 in aerosols, droplets and fomites. We have developed a bespoke system to study how well microbes survive in aerosols and droplets containing microbes referred to as CELEBS. We will study the interactions between the SARS-CoV-2 virus (in respiratory droplets) and its immediate environment in a recently commissioned state of the art facility. By levitating a known number of aerosol droplets of identical size with a known viral load for a specified period of time, the survival of the virus will be measured and the impact of inactivation measures studied.

More specifically, we will fully explore the survival of the SARS-CoV-2 virus in droplets of varying size exposed to varying environmental conditions (relative humidity and temperature) and in realistic simulated fomites deposited on surfaces. We will also explore the influence of light and atmospheric oxidants (open air factor) and the rate of dynamic changes in particle size and moisture content upon droplet/aerosol generation (i.e. desiccation/drying kinetics on exhalation). The survival of different variants of the SARS-CoV-2 virus will be examined. This project will build on preliminary work that has confirmed and validated our experimental approach. This more comprehensive work will provide clarity to inform non-pharmaceutical interventions such as social distancing, recommended indoor operating temperatures and humidities (e.g. hospitals, care homes, transport) and other methods of inactivation (e.g. light and oxidants). Extending beyond these immediate studies, the technique will be flexible, opening up opportunities to study a wider range of important airborne pathogens.

Using a unique technology developed by our team of aerosol scientists and virologists, we will provide a comprehensive survey of the factors that determine the survival of the SARS-CoV-2 virus in surrogate respiratory droplets 5-100 micrometres in diameter, referred to as aerosol droplets below. When exhaled from the moist respiratory tract, aerosol droplets respond rapidly (<10s) to their environment, losing moisture, undergoing evaporative cooling and increasing in solute concentration. Larger particles impact on surfaces forming fomites while smaller particles remain airborne for minutes to hours and are exposed to sunlight and atmospheric oxidants. Previous work has established that SARS-CoV-2 can remain infectious while airborne for hours. Uniquely, using our approach, we will:

- Develop our understanding of a biphasic decay in infectivity seen in preliminary measurements, correlating the rapid loss that can occur within 2 minutes with the microphysical processes transforming the aerosol with variation in relative humidity (RH) and temperature.

- Establish the dependence of decay rates in infectivity on aerosol size and composition, comparing surrogate respiratory fluids with cell culture media, and comparing with fomites generated from realistic droplet sizes with appropriate viral loads undergoing aerosol processing before deposition.

- Deliver a platform that can routinely assess and compare the airborne and fomite survival of emerging variants.

- Undertake a robust analysis of the dependence of survival on exposure to UV light and oxidants by studying decays in infectivity with particle size, composition, RH and temperature.

Original elements include the extremely small samples of pathogens required, exact definition of droplet size and composition in measurements, and precise studies of synergistic interactions with surfaces, light and oxidants. Longer term the approach can be deployed to study a range of respiratory and airborne pathogens