When it comes to aerosols — liquid droplets small enough to remain suspended in the air — face shields are almost completely ineffective at trapping them, show new simulations from Fugaku, the world’s fastest supercomputer.

As evidence mounts that the novel coronavirus SARS-CoV-2 can also spread through aerosols, researchers are revisiting recommendations about face masks and face shields. As we already know, these methods offer imperfect protection, but they are still vital in helping us contain the spread of the virus. However, in light of this emerging research, face shields might not be as good as thought.

According to Riken, Japan’s government-backed research institute, about half of the droplets measuring 50 micrometers and over escaped the face shield, while almost 100% of airborne droplets of less than 5 micrometres in size escaped.

The conclusion relies on a fluid dynamics simulation carried out using the world’s fastest supercomputer, Fugaku. We still don’t know exactly how the coronavirus propagates through droplets of different sizes, but if such a large proportion of droplets are able to escape face shields, this would suggest that at least on their own, they are by no means an effective means of protection.

This does not mean that face shields are necessarily useless — they still limit the exposure to some of the aerosols, which means that they can somewhat augment the protection offered by face masks. However, wearing a face shield without any other means of protection does not offer protection, and they are by no means a replacement for face masks, says Makoto Tsubokura, team leader at Riken’s centre for computational science. Tsubokura adds that as an exception, people who cannot wear masks (such as those with respiratory problems), can consider wearing only face shields.

Face shiedls have become popular especially in the hospitality industry, as they offered a less cumbersome way for businesses to protect their employees. But in light of recent findings, that’s not good practice.

This is not the first time that Fugaku, which can perform over 415 quadrillion computations a second, offered valuable input on the pandemic. The 130bn yen ($1.23 billion) supercomputer recently found that face masks made from non-woven fabric are more effective at blocking the spread of the virus than those from cotton or polyester — although cotton masks were also found to be effective.

“What is most dangerous is not wearing a mask” just because the weather is hot, said Makoto Tsubokura, a team leader at Riken’s Center for Computational Science. “It’s important to wear a mask, even a less-effective cloth one.”

The supercomputer is not fully operational and won’t be until next year, when researchers want to use it to identify treatments from about 2,000 existing drugs.