A lab experiment which used mechanical mannequins to test the efficacy of face masks in reducing the spread of large droplets when coughing and speaking has found that they can block 99.9 percent of drops when worn properly. The findings were published in the journal Royal Society Open Science.
The respiratory coronavirus SARS-CoV-2 which triggered the COVID-19 pandemic spreads through water droplets and with new strains on the loose which have an increased transmissibility capacity it’s more important than ever to try and prevent disease spread. Face coverings have been promoted as one way of achieving this, as while they can’t stop the pathogen entering the mask, they can keep the water droplets of the wearer in. By doing this, it reduces the chance of drops containing the virus making it into the nose, mouth or eyes of a person who hasn’t yet had the disease.
To find out how effective face masks are at blocking these droplets, the researchers first used an anatomically realistic mannequin head to eject fluorescent droplets of water. They also looked at human volunteers and how the water droplets behaved when they spoke and coughed both with and without a mask on. The masks included a surgical mask and single-layer cotton face covering.
Using laser sheet illumination and UV-light they reviewed how many droplets were spread in the masked and mask-less conditions. Both the manikin and human experiments revealed that wearing a face covering decreased the number of projected droplets by 1000-fold. Furthermore, their calculations predicted that social distancing was less relevant than mask wearing in droplet spread, as someone standing 2 meters away from a coughing person without a mask is exposed to over 10,000 times the number of respiratory droplets compared to someone standing at just half a meter away but wearing a basic single-layer mask.
“Our results indicate that face coverings show consistent efficacy at blocking respiratory droplets and thus provide an opportunity to moderate social distancing policies,” wrote the study authors in the paper. While a promising result with regards to larger droplets which are more likely to carry a higher viral load, they accept that the study is limited in its application to smaller droplets and aerosols. “However, the methodologies we employed mostly detect larger (non-aerosol) sized droplets,” they wrote. “If the aerosol transmission is later determined to be a significant driver of infection, then our findings may overestimate the effectiveness of face coverings.”
It’s possible then that mask wearing needs to be combined with physical distancing measures to effectively reduce the various droplets and aerosols the upper respiratory tract is capable of releasing.