Plastic visors and masks with valves could increase coronavirus transmission rates, according to new research.
The study by scientists at Florida Atlantic University has visualised how aerosol droplets spread around visors and pass through mask valves, showing these coverings don't perform as well as cloth ones and medical masks.
The researchers warn that widespread public use of alternatives to medical and cloth masks "could potentially have an adverse effect" on attempts to constrain the pandemic.
As countries have exited the COVID-19 lockdown, members of the public have been instructed to wear masks in order to further prevent them from spreading the virus to other people.
But people who are wearing alternatives to cloth or medical coverings could still be ejecting virus particles into the air when they cough or sneeze, the new study warns.
It found that even though face shields could block the initial forward movement of an aerosol jet caused by a cough or a sneeze, the droplets can still spread around the visor quite easily.
At the same time, masks with exhalation valves appeared to let "a large number of droplets" escape from inside the mask.
The scientists placed a plastic face visor and an N95-rate face mask with a valve on a mannequin for the experiment.
Using lasers they were able to track aerosol droplets made of distilled water and glycerine which was sprayed through an artificial cough-jet, following how they moved despite the facial coverings.
When hairdressers and barbers reopened in July, the UK government advised employees to wear a face shield while treating customers.
However, after top scientists said face visors were "unlikely" to offer any protection against coronavirus, the government reviewed its guidance saying that people working in beauty salons should wear a face visor as well as a medical mask.
According to the World Health Organisation, face shields should only be advised "in the context of non-medical mask shortages" and for people who would find wearing a medical mask difficult.
However, the WHO stresses that face visors are "inferior to masks" when it comes to preventing droplet transmissions.