| Summary: | Due to the COVID-19 pandemic, research on the quantitative evaluation of the risk of infection has become necessary. Because airborne transmission occurs by the inhalation of droplets from infected people, understanding the mechanism of droplet generation is important. In this study, the size distribution of droplets produced by various expiratory activities was investigated, the results were compared with those of previous studies, and the applicability of the simple measurement method was confirmed. The experiment was performed using an optical particle counter and a device that could continuously ventilate the generated droplets in a clean room with a low background concentration. Among the variables in the equation for calculating the quanta emission rate to evaluate the risk of infection, the droplet concentration and inhalation rates that could be measured were determined, and the relative risk of infection for each of the various expiratory activities was quantitatively evaluated. In addition to the cases identical to those in previous studies, conversations and vocalizations were conducted while wearing a mask. Particles smaller than 1 μm were analyzed based on the theory that viruses have a high proliferation rate and a high risk of infection. The concentration of droplets produced by the expiratory activity was dominated by particles with a number concentration of <1 μm; however, the mass concentration was only observed at a low rate. The risk of infection increased in proportion to the volume of voice, and was markedly higher in the case of loud voices. In addition, the risk of infection decreased when wearing a mask, and the extent of reduction varied depending on the method of wearing the mask.
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