Observation and quantification of respiratory droplets by laser light scattering

National Institute Of Diabetes And Digestive And Kidney Diseases

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Abstract

Accurate measurements of the size and quantity of aerosols generated by various human activities in different environments are required for efficacious mitigation strategies and accurate modeling of respiratory disease transmission. Previous studies of speech droplets, using standard aerosol instrumentation, reported very few particles larger than 5 micron. This starkly contrasts with the abundance of such particles seen in both historical slide deposition measurements and more recent light scattering observations. We have reconciled this discrepancy by developing an alternate experimental approach that addresses complications arising from nucleated condensation. Measurements reveal that a large volume fraction of speech-generated aerosol has diameters in the 5-20 micron range, making them sufficiently small to remain airborne for minutes, not hours. This coarse aerosol is too large to penetrate the lower respiratory tract directly, and its relevance to disease transmission is consistent with the vast majority of SARS-CoV-2 infections initiating in the upper respiratory tract. Our measurements suggest that in the absence of symptoms such as coughing or sneezing, the importance of speech-generated aerosol in the transmission of respiratory diseases is far greater than generally recognized. The airborne lifetime of particles is experimentally confirmed to closely follow Stokes' Law, and to scale with the inverse square of particle diameter. In the presence of thermal convection, the airborne lifetime also scales inversely with ceiling height of indoor spaces. Both of these dependencies indicate that it is very challenging to reduce the density of speech-generated aerosol by increased ventilation, whereas such ventilation is highly effective at reducing breath aerosol density that poses severe risks in healthcare facilities occupied by COVID-19 patients with viral pneumonia.