A study by a Japanese supercomputer has reportedly suggested face shields are largely ineffective at blocking smaller respiratory droplets.
However, modelling in Japan has reportedly cast doubt over how effective face shields are - especially when it comes to stopping the smallest droplets from getting out into the air.
Riken, a Japanese research institute, claimed around half of droplets measuring 50 micrometres in size escaped, the newspaper reported.
Fugaku has been looking at Covid-19 during the pandemic, and has been simulating how the virus spreads.
Makoto Tsubokura from Riken's centre for computational science reportedly warned against using face shields as a replacement for face masks if the switch is not necessary.
“Judging from the results of the simulation, unfortunately the effectiveness of face guards in preventing droplets from spreading from an infected person’s mouth is limited compared with masks,” he told The Guardian.
“This is especially true for small droplets of less than 20 micrometres,” he said, adding: “At the same time, it somehow works for the droplets larger than 50 micrometres.”
Coronavirus mainly spreads through droplets coming from the mouth or nose, according to the World Health Organisation.
Countries have told people to wear masks in certain situations, such as on public transport or in shops, as part of efforts to control the pandemic.
Boris Johnson, the British prime minister, expanded rules over face masks in England on Tuesday, telling people they must use coverings when in taxis, and in hospitality venues when they are not eating or drinking.