Wind velocity and dispersion/advection-diffusion of artificial droplets and droplet nuclei in a domed all-weather multi-purpose stadium

Abstract

To evaluate the COVID-19 infection risk and the effectiveness of countermeasures at mass-gathering events, we measured the dispersion and advective diffusion of artificial droplets and artificial droplet nuclei at the Tokyo Dome, Japan (capacity 55,000 people). We also measured and evaluated the effectiveness of wearing masks and increasing the space between seating areas. If people were seated facing forward, artificial droplets did not reach the mouths of surrounding people, suggesting low risk of droplet transmission. For an artificially generated cough or sneeze, the volume of droplets deposited on the hair, back of the neck, and back of the human in front, and the backs of the seats in front, decreased by two to three orders of magnitude when a mask was worn, regardless of the type of mask. However, when the mask was worn with the nose out, the amount deposited on the back of the seat in front was reduced by only 17%. Even in seats with the highest particle concentration in the vicinity of the source, only 0.097%-0.24% of the generated droplet nuclei (1.0-3.0 μm) from the source were inhaled. Our results suggest that the infection risk at the Tokyo Dome via droplet and airborne transmission was low.

Keywords: Coronavirus infection 2019 (COVID-19); Cough simulator; Face mask; Infection risk; Mass-gathering events (MGEs); Transmission risk.

Fig. 1

(A) Exterior and (B) interior views of the Tokyo Dome and (C) diagram of the test site showing the potential spreading sites of droplets and droplet nuclei. The red circles show the test locations, and the arrows labeled Vx, Vy, and Vz indicate the directions of the cartesian coordinate axes for the wind directions and velocities at each sampling point.

Fig. 2

Locations of emission sources and measurement points. One square indicates one seat. Although the positions of the actual seats are slightly offset from front to rear in each seating area, they are shown here arranged in a uniform front-to-back alignment for clarity. The yellow arrow in the center of each diagram represents the source of artificial droplet/droplet nuclei or CO₂. (PC: particle counter, Ane: anemometer).

Fig. 3

Dispersal of droplets generated by coughing to the area surrounding the emission source and the effectiveness of masks (volume of deposited droplets). Bars and error bars indicate the mean and one standard deviation of nine tests in the seating area behind the back net and upper stand seats on the first base side infield. The chart shows the combined results at all seating area for air-conditioning settings before and after renovation.

Fig. 4

Dispersal of droplets from coughs to surrounding seats when staggered seating arrangements were used. Measurements were made in the seating area behind the back net.

Fig. 5

Deposition of droplets from sneezing on the upper parts of the backrests of the seats in front of and two rows in front of the source (deposition volume). Values in the figure are provided as the deposition volume per area per one sneeze on each strip. The blue arrows indicate the direction faced by spectators.

Fig. 6

Spread of emitted CO₂, shown as the difference between the concentration during emission and the background concentration at each location, and wind direction/velocity. The results shown are the average values of three measurements of 10-min average values during emission. The arrows in the side-view images indicate vertical wind direction/speeds; the arrows in the plan-view images indicate horizontal wind direction/speeds.

Fig. 7

Spread of emitted artificial droplet nuclei (particle diameter 1.0–3.0 μm), shown as the difference between the concentration during emission and the background concentration at each location, and wind direction/velocity. The results shown are the average values of three measurements of 10-min average values during emission. The arrows in the side-view images indicate vertical wind direction/speeds; the arrows in the plan-view images indicate horizontal wind direction/speeds.

Fig. 8

Horizontal (A) and vertical (B) directions and velocities of the air flow before and after renovation of the ventilation system.