Doubts cast over use of face shields, masks with valves against SARS-CoV-2

By Angela Betsaida B. Laguipo, BSNSep 2 2020

Many countries across the globe are experiencing a sharp surge in COVID-19 cases, with the total global case toll reaching more than 25.86 million. Face masks have become one of the most effective means to combat the virus spread, particularly if used in combination with regular hand hygiene and physical distancing.

However, there is an increasing trend of substituting regular cloth masks or surgical masks with masks equipped with exhalation valves and clear face shields. Now, a new study shows that face shields and masks with valves are ineffective against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection.

In the study published in the journal Physics of Fluids, a team of researchers from Florida Atlantic University’s College of Engineering and Computer Science aimed to determine the effectiveness of face shields alone, as well as face masks with exhalation valves.

Although face shields block the initial forward motion of the jet, the expelled droplets move around the visor with relative ease and spread out over a large area depending on light ambient disturbances.

Testing face shields and valve masks

To arrive at the study findings, the researchers utilized qualitative visualizations to test how face shields and masks with valves perform in preventing the spread of aerosols, which are tiny droplets suspended in the air when an infected person sneezes, coughs, speaks, or breathes.

In the test, the researchers found that though face shields block the initial forward motion of the jet, the expelled droplets can move around the visor and spread over a large area. On the other hand, the visualizations for masks with exhalation valves showed that a considerable number of droplets pass through the valve unfiltered, hence, reducing the effectiveness of the mask.

To see the performance of the face shield, the team used a horizontal laser sheet in addition to a vertical laser sheet, showing how the droplets across the horizontal plane. The team noticed that the face shields prevented the forward spread of the droplets, but the droplets also spread in a reverse direction.

“Our observations suggest that to minimize the community spread of COVID-19, it may be preferable to use a high-quality cloth or surgical masks that are of a plain design, instead of face shields and masks equipped with exhale valves,” the researchers concluded in the study.

Improved comfort

Amid the coronavirus pandemic, wearing of masks across the globe has become the new normal. In some countries, people are not allowed to leave their houses or stay in public places without a mask on. Some public vehicles also ban passengers who are not wearing their masks.

The U.S. Centers for Disease Control and Prevention (CDC) recommends using handsewn cloth masks, which are effective in blocking infectious droplets. Surgical masks are also recommended for higher efficacy and protection, but the health agency says that these masks are recommended for health care workers. It is important to keep the stocks of N95 and surgical masks stable, especially for health workers.

Some people may find it hard to wear masks all the time since it becomes harder to breathe. Since then, many companies have produced masks with exhalation valves. Though these valves are effective for polluted areas, they may not be recommended to prevent infection. Exhale valves allow for improved breathability and reduce fogging when wearing glasses.

Meanwhile, face shields provide advantages, such as they are easier to disinfect and clean, and provides visual communication of facial expression for those who are hearing-impaired, the researchers said.

Though previous studies claim that using face shields alone, without a mask can prevent infection. Shields prevent the ejecta from the mouth and nose from spreading as they hit the visor. Yet, this is true for large respiratory droplets, but may not be as effective on smaller aerosol-sized droplets.

The CDC also released a guideline, discouraging the use of face shields only as a means to prevent the spread of the coronavirus. The health agency also said that masks with valves should not be used when a sterile environment is required.

The best ways to control the coronavirus spread is to wear masks in public places, those which are recommended by health agencies, observe physical distancing, and regularly wash the hands. In some countries, they require residents to use their masks, with face shields, to offer more effective protection.