How Face Shields and Valve Masks Fail to Stop Infectious Droplets, as Shown by Lasers

How Face Shields and Valve Masks Fail to Stop Infectious Droplets, as Shown by Lasers
Using lasers, engineers at Florida Atlantic University were able to visualise what happens to aerosolised droplets when they're exhaled out through a face shield (left) or N95 mask with an exhaust valve (right). (Photo: Florida Atlantic University’s College of Engineering and Computer Science)

A new study using lasers suggests that face shields and masks outfitted with an exhaust valve aren’t particularly great at protecting others from tiny respiratory droplets containing contagious germs like the coronavirus that causes covid-19. These aerosols can spill through and around these types of face equipment, the study found, weakening their potential to keep users from spreading an infection to others.

Mask wearing has been embraced by public health experts as one of the most impactful ways to reduce the chances of someone giving covid-19 to other people. To a lesser extent, masks seem to also lower the risk of wearers catching the coronavirus from others. And despite a noisy contingent of sceptics, particularly in the U.S., much of the public in countries around the world have adapted to wearing masks in situations where they’re around people outside their household.

But there are many different kinds of face coverings that have become popular. Two in particular are plastic face shields and N95-respirator masks that come with exhaust valves. N95 respirators filter inhaled air from the outside, significantly reducing the potential for catching a respiratory infection, while the valves are intended to make breathing out easier. Shields are less cumbersome on the user’s breathing but have large gaps on the bottom and sides that, presumably, would let germs enter and escape fairly easily. Medical professionals typically wear face shields in addition to masks and other protective equipment, as a way to prevent sneezed or coughed droplets from a patient from landing in their eyes and other parts of their face.

In this new study, published Tuesday in the journal Physics of Fluids, both face shields and valve masks were shown to be pretty bad at stopping the flow of aerosols.

Engineers at Florida Atlantic University created a sort of light show to visualise what happens to our exhalations while using these coverings. They lit up the area around a mannequin’s mouth with lasers, outfitted the dummy with either an exhaust-valve mask or face shield, then pumped a mixture of water and glycerin through its mouth, creating a synthetic fog with a similar consistency to the aerosol droplets emitted by a person while coughing and sneezing. In the dark, the lasers were able to eerily illuminate the path of these droplets as they left the mannequin’s mouth.

The results were plain to see. The face shield did blunt the initial forward burst from the mouth, but the aerosolised droplets were then easily dispersed to the sides and even behind the shield in still high concentrations. Though the concentration of droplets dissipated as they moved further from the mannequin’s mouth, they would likely still be able to cover a lot of ground before they evaporated under the right conditions, such as indoor places with little air flow. Exhaust-valve N95 masks were even less effective at blocking the forward movement of droplets, with the valve serving as an easy escape hatch.

The team also tested several brands of surgical and N95 masks. Though these masks weren’t foolproof either at blocking aerosols, with some masks performing worse than others, they were still overall more effective in limiting aerosol concentration than either the shield or valve masks.

“Overall, the visuals presented here indicate that face shields and masks with exhale valves may not be as effective as regular face masks in restricting the spread of aerosolised droplets,” the authors wrote. “Thus, despite the increased comfort that these alternatives offer, it may be preferable to use well-constructed plain masks.”

Lab experiments aren’t perfect imitations of the real world, of course, so it’s possible that these conditions aren’t doing a great job of stimulating how coronavirus-filled aerosols would really travel from someone’s mouth while wearing a shield or exhaust-valve mask. But there has been other research suggesting that shields don’t provide as much protection in reducing infection as more conventional masks, while experts have long warned that exhaust valve masks are defeating the primary purpose of why people should widely adopt mask-wearing (since a person can be infected and contagious without feeling sick, everyone wearing a mask is preferable).

Disability advocates have noted that face shields may be the best option available for some people whose health problems make mask-wearing difficult. And shields may further cut transmission risk when combined with masks. But for the rest of us, it seems like sticking to masks without a hole cut in them is really the best and simplest way to prevent the spread of sickness.

Indeed, the authors warn that the widespread adoption of these more comfy options “could have an adverse effect on ongoing mitigation efforts against COVID-19.”

This article has been updated to note that the researchers also tested standard surgical and N95 masks, which were found to be generally much more effective in blocking aerosolised droplets.