2020.12.08

Prediction and Countermeasures for Infection by Virus Contaminated Droplet in Indoor Environment #3

Research and Development by

Tsubokura Makoto, RIKEN Center for Computational Science / Kobe University

Corresponding Research Area

Indoor airflow simulation and visualization of droplets, which are necessary for improving the COVID-19 Infection Prevention Guidelines for each industrial sector


Droplet simulations (restaurants and bars) (Preliminary findings: Impact of distance on spread of droplets)

■Droplet/aerosol infection risk evaluation and countermeasures for restaurants. The project investigated whether the distance from the other party was able to reduce spread of droplets.

Fig. Number of droplets arriving at a person sitting directly in front

0.8 m directly in front

1.2 m directly in front

Courtesy of Riken, Kobe University,
in cooperation with Toyohashi University of Technology, Kyoto Institute of Technology, Kajima

Distance significantly changes the number of incoming droplets. If an infected person sitting immediately in front coughs, only about 5% of droplets arrive at a distance of 1.2 m, where as roughly 40% arrive at a distance of 0.8 m. It is therefore crucial to maintain sufficient distance.

Droplet simulations (restaurants and bars) (Preliminary findings: Impact of direction of speech and position of seat)

■Droplet/aerosol infection risk evaluation and countermeasures for restaurants. There are clear differences in infection risk according to the direction of speech and position of seats.

Number of droplets arriving at a person sitting directly in front of a carrier
  • Almost no droplets arrive at people who are not sitting directly in front
Number of droplets arriving at a person sitting diagonal from the carrier
  • Almost no droplets arrive at people not sitting diagonally across
Number of droplets arriving at a person sitting beside the carrier
  • Almost no droplets arrive at people not sitting beside

Courtesy of Riken, Kobe University, in cooperation with Toyohashi University of Technology, Kyoto Institute of Technology, Kajima

Droplets have a strong tendency to move in a straight line, and almost none arrive at people other than the person being addressed. The infection risk is highest if the carrier talks to the person sitting beside them. The lowest number of droplets arrives at the person sitting diagonally across from the carrier, approximately 1/4 the number arriving at a person directly in front.

Droplet simulations (restaurants and bars) (Preliminary findings: Effectiveness of mouthguards)

■Droplet/aerosol infection risk evaluation and countermeasures for conversations at restaurants and bars. The project investigated the droplet spread suppression impact of mouthguards and partitions, and the aerosol infection risk reduction impact of interior ventilation.

Face-to-face conversation

Face-to-face conversation with mouthguard

Side-to-side conversation

Side-to-side conversation with mouthguard

Courtesy of Kobe University, Riken,
in cooperation with Toyohashi University of Technology, Kyoto Institute of Technology

When dining and unable to wear masks, the use of mouthguards is effective in restraining the spread of droplets. However, mouthguards are not effective in restraining aerosols and must therefore be used in conjunction with sufficient ventilation.

Droplet simulation (singing)

Music Classroom is not supposed.

■Droplet/aerosol infection risk evaluation and countermeasures for singing. The project investigated appropriate distances and postures to study the droplet spread suppression impacts of masks and mouthguards in particular.

No measures taken

Impact of distancing

  • Greater distancing reduces the risk of direct droplets

Impact of mouthguards

  • Mouthguards reduce droplets in the forward direction
  • Aerosols spread above due to droplets rising as a result of body temperature

Courtesy of Kobe University, Riken, in cooperation with Kajima, Toyohashi University of Technology, Kyoto Institute of Technology

The flow of droplets to the front can be restrained if everyone wears a mouthguard. It is necessary to reduce risk by combining this with measures to deal with the aerosols that leak out.

Droplet simulations (taxi) (Preliminary findings)

■Droplet/aerosol infection risk evaluation and countermeasures for taxis. The project will evaluate the ventilation performance of air-conditioning and open windows, and will investigate the droplet spread suppression impact of partitions and other physical barriers.

Courtesy of Riken, Kobe University, in cooperation with Toyohashi University of Technology, Kyoto Institute of Technology, Toyota Motor

Representation of ventilation inside the vehicle with the AC on and external air circulated. Half of the internal air is ventilated in about 1 minute, and even with 4 passengers, vehicles maintain ventilation on par with offices etc. It is recommended that ACs be run in external air circulation mode and that windows be opened as appropriate to further reduce aerosol infection risk. (Going forward, the project will investigate the impact of ventilation with open windows and the impact of partitions.)

Droplet simulation (impact of humidity on spread of droplets)

■Measures to combat aerosols during dry, winter seasons. The project investigated the ability of humidification to reduce droplet transmission.

Fig. Number of droplets arriving at a person 1.8 m in front
Fig. Number of droplets falling to the surface

Humidity 30%

Humidity 90%

Courtesy of Kobe University, Riken,
in cooperation with Kajima, Toyohashi University of Technology, Kyoto Institute of Technology

When it is dry, the volume of droplets falling to the surface declines, but the volume of droplets aerosolizing and remaining in the air increases. This effect is particularly pronounced when humidity is below 30%. During the winter, humidifiers and similar equipment will be needed to control humidity, in combination with better ventilation.