Alice Yan, in Shanghai
3 Sep, 2020
Face shields are becoming a popular choice for people looking for protection against Covid-19. Photo: Dickson LeeFace shields and masks with exhalation valves have become a popular alternative for people looking for protection against Covid-19, the disease caused by the new coronavirus, but American research has cast doubt on their effectiveness.
Researchers from Florida Atlantic University examined the performance of face shields and exhalation valves in impeding the spread of aerosol-sized droplets – one of the main forms of Covid-19 transmission – with disturbing results.While clear plastic face shields blocked the initial forward motion of the jet of droplets, once expelled they were able to move around the visor relatively easily and spread out over a large area, according to the study published on September 1 in peer-reviewed scientific journal Physics of Fluids.
Visualisations for a mask equipped with an exhalation port indicated a large number of droplets passed through the valve unfiltered.
Scientists said they did the research to help the public understand the effectiveness of face shields and masks equipped with exhalation valves, increasingly popular substitutes for regular cloth or surgical masks because people find them more comfortable.
Face shields reduce humidity and fogging when worn with glasses and are easier to breathe in. They also protect the eyes from splashes and sprays of infected droplets, are easily cleaned and disinfected, and allow visual communication for the hearing-impaired.
Unfortunately, smaller aerosolised droplets can penetrate under the bottom of the shield and from the sides of the visor. The researchers found that, over an exposure lasting between one minute and half an hour, the shield was only 23 per cent effective in reducing the inhalation of droplets.
Source : SCMP