This study experimentally examines and compares the dynamics and short‐term events of airborne cross‐infection in a full‐scale room ventilated by stratum, mixing and displacement air distributions. Two breathing thermal manikins were employed to simulate a standing infected person and a standing exposed person. Four influential factors were examined, including separation distance between manikins, air change per hour, positioning of the two manikinsand air distribution. Tracer gas technique was used to simulate the exhaled droplet nuclei from the infected person and fast tracer gas concentration meters (FCM41) were used to monitor the concentrations. Real‐time and average exposure indices were proposed to evaluate the dynamics of airborne exposure. The time‐averaged exposure index depends on the duration of exposure time and can be considerably different during short‐term events and under steady‐state conditions. The exposure risk during short‐term events may not always decrease with increasing separation distance. It changes over time and may not always increase with time. These findings imply that the control measures formulated on the basis of steady‐state conditions are not necessarily appropriate for short‐term events.

This study examined the characteristics of the exhaled airflow pattern and breathing cycle period of human subjects and evaluated the influence of pulmonary ventilation rate and breathing cycle period on the risk of cross‐infection. Measurements with five human subjects and a breathing thermal manikin were performed, and the peak exhaled airflow velocity from the mouth and the breathing cycle period were measured. Experiments on cross‐infection between two breathing thermal manikins were then conducted in a full‐scale test room, in which the pulmonary ventilation rate and breathing cycle period were varied systematically. Both peak flow velocity and breathing cycle length varied considerably between different subjects. The breathing cycle period in a standing posture was 18.9% lower than in a sitting posture. The influence of pulmonary ventilation rate and breathing cycle period extended up to a separation distance of 1.0 m between the two manikins. Increasing the pulmonary ventilation rate of the exposed person greatly increased the risk of cross‐infection. Decreasing the breathing cycle period from the widely used “6 second” value led to a considerable increase in the risk of cross‐infection. Standing posture resulted in a higher risk of cross‐infection than sitting posture.

Contact behavior in a public space influences the risk of contact infection because public spaces have many environmental surfaces contaminated with pathogens. It is useful for risk reduction to examine the factor of infection risk among behaviors. In the present study, a video monitoring survey was conducted in a simulated cabin of a commuter train, we had built, to investigate the relationship between face-touching frequencies and individual attributes. As a result, the average face-touching frequency was 17.8 times per hour. Of all face touches, mucosal contact was 42.2%. Focusing on the sex, the face-touching frequency was significantly higher for the males than for the females. Focusing on the skin condition, the face-touching frequency of those who did not wear makeup was significantly higher than that of those who did. The significant sex differences may depend on the makeup. Focusing on pollution awareness, higher pollution awareness related to lower frequency. Thus, by improving pollution awareness of the environmental surfaces in public spaces, it is possible to reduce effectively face-touching frequency and, hence, infection risk.

No abstract

Past studies on airborne spread of expiratory droplet nuclei between occupants were focused on long-term exposure under steady-state conditions. However, exposure during short-term events can be widely found in practice, e.g. medical examination or short meeting. Airborne transmission during short-term events under stratum ventilation was examined experimentally in this study. Two breathing thermal manikins were employed to simulate a standing infected person and a standing exposed person. The manikins were placed face-to-face and face-to-back to reproduce the exposure conditions with the highest and the lowest risk, respectively. Tracer gas was dosed into the air exhaled by the “infected” manikin to simulate the droplet nuclei. A newly developed average exposure index was used to evaluate the exposure risk. The time-averaged exposure index increases over time, but the increasing rate depends strongly on the duration of exposure time, e.g., the exposure index increases much faster during the first 5 minutes than during the period after 5 minutes. The exposure index during short-term events does not always decrease with the increase of separation distance. These findings imply that the control measures formulated based on steady-state conditions are not necessarily effective to short-term events.

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.