The dispersion of respiratory droplets when walking behind a person going downstairs carries an increased risk for infection compared with those generated when walking behind a person going upstairs, according to results of a study published in AIP Advances.

Investigators used a laboratory water tunnel experiment to study the dispersion of respiratory droplets exhaled from a person going up- or downstairs. To mimic a person going up- or downstairs, a human-shaped manikin with a height of 9 cm was placed 27 cm downstream from the inlet of the test section. The investigators used inclination angles of 60°, 90°, and 120° to simulate a person going downstairs, walking on flat ground, and going upstairs, respectively. The median diameter of particles used in the experiment to mimic virus-containing respiratory droplets from sneezing and coughing was 0.007 mm; the mean ejection speed was 33 cm/s. Investigators carried out flow visualization of the particle dispersion using fluorescent particles ejected from the mouth of the manikin, as well as flow field measurement behind the manikin using particle image velocimetry (PIV).

A PIV analysis showed the head as the main point of flow separation for a person walking upstairs, and the hips as the main point for a person walking downstairs. Of note, a “downwash” mean velocity field was observed behind the manikin when the flow separated at the head, and an “upwash” mean velocity field was observed when the flow separated at the hips. In addition, in the inclination angle has a distinct effect on the airflow behind a walking person.


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Flow visualization with fluorescent particles showed different dispersions for upstairs vs downstairs movement. For a person walking upstairs, most particles distributed below the shoulder and moved downward. In addition, the particles were spread uniformly in space, and the local concentration was quickly decreased. For a person walking downstairs, the particles were distributed over the head and concentrated in a narrow band slightly above the head that was elongated downstream. The local concentration of particles remained at a high level for a long distance, supporting the theory that a person walking behind a person going downstairs is at an increased risk for infection.

This study was potentially limited by the use of experimental equipment.

According to the investigators, “these results suggest that we should cough with the head down toward the ground to ensure that most of the droplets enter the wake region.” They concluded that this practice may be the “simplest and most efficient way to [decrease] droplet transmission when there is no tissue to cover the mouth and nose.”

Reference

Wang H, Li Z, Liu Y, Zhou Z. Experimental study of the dispersion of cough-generated droplets from a person going up- or downstairs. AIP Advances. Published online January 4, 2022. doi:10.1063/5.0073880