Microorganisms can be spread in the air by small droplets referred to as aerosols. Although we commonly think about the impacts of pollution aerosols on human health, air quality and climate change, biological aerosols are much less well studied and understood. Indeed, aerosols have been suggested as providing unique environments for prebiotic transformations that could have led to life. The COVID-19 pandemic has highlighted the gaps in our understanding of airborne viruses and their transmission between humans via aerosols. As well as transporting viruses, aerosols can transport living bacteria that include human, animal and plant pathogens.

The transmission of microbes between humans can occur by direct (person-to-person), indirect (e.g. through contaminated surfaces) and airborne routes. Respirable aerosol droplets (<10 μm diameter, about a tenth of the diameter of a human hair) and larger droplets (up to millimetre size) are exhaled when we breathe or speak, and are ejected by coughing or sneezing. Large droplets settle very quickly over short distances, contaminating surfaces (referred to as fomites), whereas particles smaller than ~100 μm can remain airborne for hours. Some diseases, such as tuberculosis and measles, show preferential airborne transmission, whilst others, such as influenza, are opportunistic. Identifying when these different modes of transmission are dominant and ways to mitigate them must be priorities for protecting public health, informing the implementation of non-pharmaceutical interventions such as physical distancing and the use of face masks. Despite the recognised importance of aerosols and droplets in the transmission of microbes, robust innovative approaches to study the factors controlling the survival of pathogens when in aerosols, droplets and as fomites are crucial to move our understanding forward. This requires the close collaboration of researchers with expertise at the forefront of the physical (particularly aerosol microphysics) and life sciences (virology, microbiology and respiratory diseases).

In this project, we bring together a multidisciplinary team with expertise in aerosol chemistry and physics, microbiology and virology, and infectious and respiratory diseases in the Schools of Chemistry, Cellular and Molecular Medicine, and the Bristol Medical and Veterinary Schools at the University of Bristol. We are exploring how the properties of aerosol droplets are transformed on exhalation, measuring how rapidly particles lose water and change size when they leave the humid lung and how the concentration of salts increases. In conjunction with comparative studies of the airborne survival (viability and infectivity) of bacteria and viruses, and how survival depends on the environment (relative humidity and temperature), we are working to establish the physical, chemical and biological factors that influence aerosol transmission. We are comparing the survival of microbes when deposited in droplets on surfaces (creating fomites) with their airborne survival and investigating how respiratory aerosol droplets interact with light and the consequences for photochemically driven changes in microbe survival.