MASKS with valves and clear plastic shields do not stop the spread of coronavirus, experts have claimed.
Cloth and surgical masks that are plain in design are more effective at reducing the risk of Covid-19, new research has found.
Read our coronavirus live blog for the latest news & updates
Face masks with valves are useless at stopping the spread of Covid-19, experts warn[/caption]
But with masks now compulsory in places, such as shops, takeaways and on public transport, people are looking for ways to feel more comfortable when their faces are covered.
Clear plastic shields and masks with exhalation valves have proven popular, but researchers wanted to understand how effective they are.
Scientists in the US found masks with an “exhalation port” allow a large number of droplets to escape through the air valve unfiltered.
They warn that this makes them useless at stopping the spread of Covid-19 if the person wearing the mask is infected.
Researchers used a hollow manikin head and simulated a cough or sneeze with a pressure impulse from a manual pump[/caption]
They also tested the efficacy of clear plastic face shields and found them to be less effective than surgical or cloth masks[/caption]
The team also found plastic face shields can 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.
Dr Siddhartha Verma, from the American Institute of Physics and who led the study, 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.