The Role of Aerosol Science in Understanding and Minimizing the Risk of Airborne Infection Transmission

LIDIA MORAWSKA, Queensland University of Technology, Australia

     Abstract Number: 666
     Working Group: Plenary Lecture Invited by Conference Chair

Abstract
The term “prevention of infection transmission” evokes in most people an image of a white-clad medical professional, shielded from head to toe in personal protective equipment, while disinfecting, testing, or performing medical procedures on infectious patients. But apart from physicists and other scientists working in this field, few people realise that in fact physics, and more specifically aerosol physics, plays a major role in infection transmission. The interdisciplinary nature of the process of infection transmission makes it an immensely complex and interesting area to study, but also opens the door to misunderstanding and misinterpretation.

Understanding the numerous physical mechanisms involved in infection transmission is critically important in lowering the risk of infection transmission; this is where aerosol science comes to the fore. Yet the broader role of aerosol science is to interact and communicate with other scientific fields, particularly the medical community, to facilitate an understanding of the physics of the process in the “languages” of these disciplines. When the physics is understood, appropriate risk mitigation measures can be implemented according to the roles and responsibilities of these disciplines.

The problems start with the definitions of aerosol science terms. According to aerosol science, an aerosol is defined as “an assembly of liquid or solid particles suspended in a gaseous medium long enough to enable observation or measurement” (Kulkarni et al. 2011) . In contrast, medical science defines an aerosol as a small particle, while a droplet is a particle larger than 5 µm. Discussion about the terminology is still raging and dividing expert communities, so to help unite the fields, we propose to use the term particles, rather than aerosols or droplets, [Morawska and Buonanno, 2021]. But addressing the terminology is just the beginning. The next step is to develop a quantitative understanding of particle generation, particle emission, particle evaporation, particle flow dynamics, and particle disposition.

Particle generation occurs in the respiratory tract during human respiratory activities, which include breathing, speaking, singing, or coughing. After the particles are emitted, complex physico-chemical reactions occur as a result of particle evaporation in the air and flow dynamics drive the process of particle transport between the infected and a susceptible person. The final step is the disposition of the particles in the respiratory tract of the susceptible person, at which point the biological process of infection starts.

How well do we understand these processes? In our recent review on this topic [Morawska and Buonanno, 2021], we concluded that although the generation of particles in the respiratory tract is understood qualitatively, there is little quantitative knowledge about the characteristics of particles emitted during respiratory activities, their fate after emission, and their deposition during inhalation. More studies are clearly needed to address these knowledge gaps.

However, we should not discount the importance of what we actually do know, and how we can and should use this knowledge in lowering the risk of infection. I previously advocated for the use of our knowledge base to prevent infection after the SARS1 epidemic (Morawska, 2006). Since then, many studies using novel technologies have increased our understanding of the size distribution of particles from our respiratory processes and demonstrated that particles emitted from our respiratory activities can be suspended in the air for a long time and travel long distances in the indoor environment. A fraction of the virus contained by the particles remains infectious during this process (Oswin et al., 2022).

The hard lessons of the COVID pandemic have led to the realisation that as a society we have not effectively used the knowledge we already have. One of the key directions to follow to improve our future performance is to develop and legislate indoor air quality standards that include control of infectious respiratory particles emitted by building occupants. Our paramount argument continues to be that clean indoor air is a basic human right (Morawska, 2022).

References
[1] Kulkarni, P., Baron, P.A. and Willeke, K. eds. Aerosol measurement: principles, techniques, and applications. John Wiley & Sons, 2011.
[2] Morawska, L. Indoor Air 16(5): 335-347, 2006.
[3] Morawska, L and Buonanno, G. Nature Rev Phys, 3:300-301, 2021.
[4] Morawska, L., Marks, G.B., Monty, J. Medical Journal of Australia, 217(11): 578-581, 2022.
[5] Oswin, H.P. et al. PNAS, 119(27): p.e2200109119, 2022.