Droplet Concentration Produced during Expiratory Activities and Evaluation of Relative Infection Risk

Abstract

The outbreaks caused by COVID-19 have prompted researchers to quantitatively assess the risk of infection. Since airborne transmission is caused by inhalation of droplet from infected persons, it is important to understand the droplet concentration and size distribution of aerosols. In this study, we examined the size distribution of droplets produced by various expiratory activities, compared the results with previous studies, and tested the consistency of a simple measurement method. We realized the measurement by conducting the experiment in a clean room with low background concentration, using an optical particle counter and a device that can constantly ventilate the generated droplets. Quanta emission rate is a method of evaluation the risk of infection. Among the variables in the equation to determine it, we measure droplet concentration and inhalation rate, which we can measure, and from the product we get the relative risk of infection for each of the various expiratory activities. In the expiratory activities, in addition to the same cases as in the previous study, we conducted conversations and vocalizations while wearing a mask. In this study, we mainly analysed particles smaller than 1μm, based on the theory that viruses are highly proliferative and pose a high risk of infection. The concentration of droplets generated by exhalation activity is dominated by particles smaller than 1 μm in number concentration, but only a small percentage in mass concentration. In addition, the risk of infection increased in proportion to the volume of voice, and loud vocalizations showed a prominent risk of infection. Furthermore, it was confirmed that the risk of infection was reduced by wearing a mask, and the degree of reduction depended on the method of wearing the mask.