Face shields and masks with valves do not stop the spread of Covid-19, warns new research.
Cloth and surgical masks are better at stopping the spread of coronavirus than plastic face shields or masks with breathing valves, according to the study.
The findings suggest it may be better to use high quality cloth or surgical masks that are plain in design to help fight the spread.
As countries around the world experience a steep surge in Covid-19 infections, face masks have become increasingly accepted as an effective means for combating the spread of the disease along with social distancing and frequent hand-washing.
More and more people are using clear plastic face shields and masks with exhalation valves because they can be more comfortable.
But a new study reveals they are not as effective as regular cloth or surgical masks.
Scientists found face shields block the initial forward motion of a simulated jet of a cough or a sneeze but the expelled droplets can then move around the visor with ease and spread out over a large area.
The findings suggest masks with an “exhalation port” allow a large number of droplets to escape through the air valve unfiltered which make it ineffective at stopping the spread of Covid-19 if the person wearing the mask is infected.
Study author Dr Siddhartha Verma, of the American Institute of Physics, said: “As students return to schools and universities, some have wondered if it is better to use face shields as they are more comfortable and easier to wear for longer periods of time.
“But what if these shields are not as effective? You would be essentially putting everyone in a tight space with droplets accumulating over time, which could potentially lead to infections.”
In the study, researchers used a hollow manikin head and simulated a cough or sneeze with a pressure impulse from a manual pump.
Tracers made from droplets of distilled water and glycerin were expelled through the mouth opening, and laser sheets visualised the spatial and temporal development of the ejected flow.
Dr Verma said: “We focused on the smaller droplets since they can stay suspended for very long times and might contain enough virus particles to transmit COVID-19.”
The research suggests that to minimise the community spread of COVID-19, it may be preferable to use high-quality cloth or surgical masks that are of a plain design instead of face shields and masks equipped with exhale valves.
Dr Verma added: “Even the very best masks have some degree of leakage. It’s still important to maintain physical distance while wearing them to mitigate transmission.”
The findings were published in the journal Physics of Fluids.