How Did We Get Here: What Are Droplets and Aerosols and How Far Do They Go? A Historical Perspective on the Transmission of Respiratory Infectious Diseases

Abstract

The COVID-19 pandemic has exposed major gaps in our understanding of the transmission of viruses through the air. These gaps slowed recognition of airborne transmission of the disease, contributed to muddled public health policies, and impeded clear messaging on how best to slow transmission of COVID-19. In particular, current recommendations have been based on four tenets: 1) respiratory disease transmission routes can be viewed mostly in a binary manner of "droplets" versus "aerosols"; 2) this dichotomy depends on droplet size alone; 3) the cutoff size between these routes of transmission is 5 μm; and 4) there is a dichotomy in the distance at which transmission by each route is relevant. Yet, a relationship between these assertions is not supported by current scientific knowledge. Here, we revisit the historical foundation of these notions, and how they became entangled from the 1800s to today, with a complex interplay among various fields of science and medicine. This journey into the past highlights potential solutions for better collaboration and integration of scientific results into practice for building a more resilient society with more sound, far-sighted, and effective public health policies.

Note: Funding Statement: KR - none; ETE - National Endowment for the Humanities Grant HG-229283-15 (ended 2019) and Grant FV-256881-17 (ended 2018); LCM - National Institute of Allergy and Infectious Diseases Center of Excellence in Influenza Research and Surveillance (HHSN272201400007C) and the NSF National Nanotechnology Coordinated Infrastructure (ECCS 1542100 and ECCS 2025151); JLJ - none; LB - Smith Family Foundation, the Burroughs Wellcome Fund, the Wellcome Trust, the National Science Foundation, the Massachusetts Institute of Technology (MIT) Policy Lab, the MIT Ferry, Reed, and Edgerton Funds

Declaration of Interests: The authors declare no competing interests.

Keywords: COVID-19, aerosols, droplets, transmission, history