Face shield can’t be a substitute for mask

Pushpa Girimaji

How effective are the plastic face shields available in the market in preventing the spread of coronavirus? Some of my colleagues at work are wearing these instead of the mask on the pretext that the mask is suffocating, particularly when worn for long periods.

The mask is certainly uncomfortable, but experts do not recommend face shields as a substitute for masks. They can only be used as an additional precautionary measure along with the mask.

I must mention that earlier, when Covid-19 was believed to spread mainly through large respiratory droplets propelled from the nose or mouth of an infected person, good quality plastic face shields were thought to be quite effective in protecting the wearer as well others. Some scientists even argued that they were better than the mask in blocking respiratory droplets. Besides, they also covered the eyes of the wearer, thereby reducing their exposure to the virus and also prevented the wearer from touching the face with contaminated hands.

However, with growing evidence of airborne transmission of the disease, it is feared that the gaps between the visor and the skin — near the chin and the ears — can allow aerosols or microscopic respiratory droplets carrying the virus to escape from an asymptomatic infected wearer and infect others nearby. The gaps could also expose the wearer to the virus.

So much so that today some of the countries that recommended or allowed face shields as an alternative to masks are now changing their stand. In June, for example, Singapore said its Covid-19 task force had concluded that shields were not as effective as masks. Similarly, in July, Swiss health officials warned citizens against using face shields and said it gave the wearer a false sense of security.

According to media reports, the warning came after an outbreak of Covid-19 in a hotel in the Graubunden region, wherein it was found that hotel employees who wore face shields were infected while those who wore masks were spared. The Centres for Disease Control and Prevention, US, also says there isn’t enough evidence on the level of protection a face shield offers to people nearby from the spray of respiratory droplets from the wearer.

A recent work on the topic in Physics of Fluids, a journal by the American Institute of Physics on September 1, discourages the use of face shields (and also masks with exhalation valves) and says that their widespread public adoption could have an adverse impact on Covid-19 mitigation efforts.

As of now, or till research to the contrary comes up, face masks score over shields, but along with social distancing and hand hygiene. All the three are extremely important precautions that one should take.

Can face masks prevent the spread of coronavirus?

When it comes to face masks, their effectiveness depends on their quality and how they are worn. Even the best of masks will fail if they are loose, leaving large gaps between the face and the mask or covering only the mouth and not the nose.

As far as quality is concerned, several studies have recommended three-layered masks made of different fabrics with varying mechanical and electrostatic filtering properties, for maximum efficiency. Accordingly, the World Health Organisation recommends an inner hydrophilic material such as knitted cotton, a middle polypropylene layer for good filtration and an outer hydrophobic material such as polyester or a blend of polyester and cotton to repel droplets and moisture.

I must also mention that despite the health ministry’s warning, several people use masks with exhale valves. This may protect the wearer, but can affect others. In fact, the abovementioned research paper in Physics of Fluids also looked at masks with exhale valves and said they “significantly reduced the effectiveness of a mask as a means of source control as a large number of droplets (expelled by the wearer) pass through the valve unfiltered”.

So wear a good quality mask, without valves, properly. And for extra protection, you can wear the shield over it.