Joint size and velocity statistics of droplets exhaled while speaking, coughing, and breathing

Human respiratory activities induce the exhalation of a cloud of air laden with droplets. These droplets can harbor pathogens and play a role in the transmission of infectious diseases. Experimentally determining the size and velocity of these droplets, along with the properties of the exhaled air cloud, is crucial for predicting their behavior postemission and developing effective strategies to mitigate infection transmission. Despite the efforts of the scientific community, there is still a lack of comprehensive characterization of exhaled droplet size distribution, with different studies yielding significantly varied results. Additionally, while several studies have measured airflow rate and velocity, droplet velocity is rarely measured and typically only within a limited size range. In this study, we use an enhanced version of the interferometric laser imaging for droplet sizing (ILIDS) technique to simultaneously measure the size and velocity of droplets exhaled by 23 volunteers during speaking, coughing, and breathing. This method allows us to detect droplet diameters as small as 2μm, extending the size range reported in the literature for the coupled measure of droplet size and velocity. The ILIDS technique addresses several issues encountered in previous studies by (i) directly measuring droplets larger than 20μm, (ii) conducting measurements close to the mouth to reduce droplet evaporation and dilution effects, and (iii) distinguishing between ambient air dust and exhaled droplets, eliminating the need for air filtration. Additionally, this study assesses the impact of protective masks on droplet size and velocity distributions, as well as the variability of the results among different volunteers and the same volunteers repeating the tests multiple times.