We developed a cough machine that can generate human-like cough to evaluate the infection risk in indoor environments. The cough expired volume, propagation distance and velocity of cough, total mass of droplets and droplet size were compared with experimental results of subjects. Using the cough machine, the droplet deposition on horizontal surfaces and the position of facial mucous membranes were measured. It was shown that the number of droplet deposited on the facial mucous membranes decreased drastically over 90 cm. On horizontal surfaces, the droplet deposited most at 50-60 cm and over 90% of the droplet deposited within 90 cm.
Short-distance airborne infections are often thought to occur by large droplets or direct contact. Recent studies have investigated the phenomenon of short-range airborne infection. In this study, the effect of relative humidity and short-distance exposure to cough droplet nuclei were evaluated. The evaluation model for infection risk was designed based on experimental data and previous studies and included sampling efficiency and virus survival rate. As observed in the comparison between short-range exposure and long-range exposure, the short-range exposure to the droplet nuclei by one cough was equivalent to a long-range exposure of several seconds to, depending on the number of ventilations, even for a momentary high-concentration exposure. This suggests the possibility of an infection risk corresponding to the receiving condition. Since short-range exposure and long-range exposure have different factors affecting the risk of infection, it is suggested that effective infection control should be selected for each condition. The risk of infection due to long-range exposure is considered to be sufficiently reduced to that of a single cough if the building meets the necessary number of ventilations. It was suggested that the influence of relative humidity on infection risks is smaller than other factors such as ventilation.
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