This study gives information about surgical mask containment of particles coughed out by an infected person and is relevant to personnel who have to work amongst potentially infectious members of the public who may or may not be wearing masks.
In this study a dummy of a 70kg man enclosing a mechanical lung model for human respiration was sat at a 45°C angle in a negative pressure room. Smoke was added to the ‘lungs’, and oxygen flows adjusted to simulate a normal, healthy cough. The dummy was bare-faced, or fitted with a surgical mask, or a N95 respirator mask (see images). A laser light sheet and video were used to capture images of smoke dispersal during 20 coughs for each of surgical mask, N95 mask or no-mask conditions. Multiple frames from each of the cough videos were subjected to image analysis using purpose-built software to construct contours of smoke particle concentration (roughly equivalent to viral particles) around the dummy.
RESULTS: Without a mask the dummy’s cough formed a turbulent jet, some of which was downwards but most projected forwards to a maximum of 75cm at 7.39m/s. The surgical mask did not prevent the forwards projection of smoke particles but did reduce the distance to a maximum of 34cm, as well as 31cm out to each side. The N95 mask reduced forwards projection to 18cm and sideways projection to 17cm each side.
Small particles, which may include infectious viral particles, can escape from behind masks, but masks still reduce transmission risk. In a negative pressure room the unmasked cough plume reached 75cm, whereas a similar study (Tang et al., 2012) found a normal healthy cough plume in normal atmospheric pressure reached 64cm. Recommendations for 1.5 – 2m distancing allow for additional dispersal and reduction of infectious dose, and further reduce risk of COVID19 infection for personnel.
REFERENCE: David S. Hui, Benny K. Chow, Leo Chu, Susanna S. Ng, Nelson Lee, Tony Gin, Matthew T. V. Chan (2012) PLoS ONE 7(12): e50845.