What we know and don’t know about the role of droplets and aerosol in transmission of SARS-CoV-2

Friday, October 23, 2020

Dr. Ad Bax

Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases

National Institutes of Health

Time: 4:45 PM

Location: Hand 1100


Transmission of respiratory diseases by droplets has been studied for over a century and definitive knowledge was summarized by Wells in 1955 [1] but seems to have long been forgotten. Of the various types of respiratory droplets, which include breathing, speaking, coughing, and sneezing, the latter two are most widely recognized as they relate directly to symptoms. By default, speech and/or breathing droplets are strongly implicated in the now well-established importance of SARS-CoV-2 transmission by disease carriers without symptoms. Laser light scattering shows that breathing droplets are highly abundant but mostly sub-micron in size. The physical mechanism by which the various types and quantities of droplets are generated can yield a better understanding of their infectivity in relation to the disease state of the emitter. Rapid evaporation of their aqueous fraction transforms smaller droplets (< ~50 microns) into aerosol prior to reaching ground, and their distance traveled is then dominated by air convection. Their rapid volume shrinkage by up to 100-fold presents a major challenge in limiting their spread, but also an opportunity in blocking them prior to evaporation. Simple light scattering measurements are illustrated that can visualize and quantify droplet nuclei spanning more than 4 orders of magnitude in volume.

[1] W.F. Wells, Airborne Contagion and Air Hygiene: An Ecological Study of Droplet Infections. Harvard University Press, Cambridge, 1955.


Adriaan (Ad) Bax was born in 1956, in The Netherlands and became a US citizen in 1999.  His Ph.D. thesis was reprinted in book format and for many years served as a popular text, introducing students to the application of two-dimensional NMR in chemistry. Bax joined NIH in 1983, where he has been working on the development and application of a wide variety of advanced multi-dimensional NMR techniques to problems of biochemical and biomedical interest.  His group spearheaded the introduction of triple resonance NMR spectroscopy of 13C/15N-enriched proteins, developed the now standard joint analysis of 15N R1, R2, and NOE for characterizing protein backbone dynamics, and introduced the first methods for weakly aligning proteins in a magnetic field by the use of liquid crystals.  Bax’s work has been recognized by numerous awards, including 2018 Welch Award in Chemistry. In 2002, he was elected to both the National Academy of Arts and Sciences and the National Academy of Sciences.

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