Valve masks don't slow the spread of the Covid-19 virus - and here's the science to back it up

  • Face masks are strongly recommended to prevent the transmission of the coronavirus 
  • However, a recent study showed that masks with exhalation valves do not slow the spread of the virus
  • The researcher hopes the evidence will encourage people to wear face masks without valves

The Covid-19 pandemic has brought in its wake the compulsory use of face masks as they help to limit the spread of SARS-CoV-2.

Masks with exhalation valves, however, do not slow the spread of the virus, a researcher from the National Institute of Standards and Technology (NIST) has found.

The videos, which show airflow patterns through masks with and without exhalation valves, were created by NIST research engineer Matthew Staymates and were published, along with an accompanying research article, in the journal Physics of Fluids.

"When you compare the videos side by side, the difference is striking," Staymates said in a news release by NIST.

"These videos show how the valves allow air to leave the mask without filtering it, which defeats the purpose of the mask."

What is the purpose of masks with exhalation valves?

Exhalation valves make breathing more comfortable for the wearer. This type of mask is commonly worn by construction workers exposed to huge amounts of dust, or by hospital workers.

However, in the context of the pandemic, the Centers for Disease Control and Prevention (CDC) advises people to wear face masks that protect people other than the wearer. This is especially important for those who don’t display symptoms, as studies have consistently shown how asymptomatic cases are driving the spread of the virus.

"I don't wear a mask to protect myself. I wear it to protect my neighbour, because I might be asymptomatic and spread the virus without even knowing it," Staymates said. "But if I'm wearing a mask with a valve on it, I'm not helping."

Staymates, who is an expert in flow visualisation techniques (which involves capturing the movement of air on camera), recently turned his expertise toward face masks to help develop novel ways to measure and improve their performance.

How the study was done

Staymates created two videos using different flow visualisation techniques. The first video used a schlieren imaging system, which causes differences in air density to show up on camera as patterns of shadow and light.

This video only shows the movement of the air itself, not the movement of exhaled droplets in the air.

On the left in this video, Staymates can be seen wearing an N95 respirator mask with a valve. This mask allows exhaled air to stream into the environment unfiltered.

On the right, there is no valve, and the air passes through the mask, which filters out most of the droplets.

The second video

Staymates created the second video using a light-scattering technique.

For this video, he built an apparatus that emits air at the same velocity and tempo as a resting adult. He then connected the device to a mannequin.

In contrast to the schlieren video, the video shows the movement of droplets in the air. On the left, droplets can be seen to escape unfiltered through the valve of an N95 mask.

In the middle, there is no valve and no breath is visible because the mask has filtered out the droplets. On the right, no mask is worn.

Staymates explained that using a mannequin and a mechanical breathing apparatus allowed him to observe airflow patterns while holding steady the breathing rate, air pressure and other variables.

More than one type of valve mask on the market

Staymates cautioned that different types of valved masks are available and may exhibit different results, and that his study involved only one type of this mask.

He also explained that masks that do not include valves, but that are not tight-fitting will allow some air to escape around the mask rather than filtering through it, which can also compromise the performance of the mask.

However, the main effect of valves is visible in the videos of this study, and Staymates hopes it will help people to understand why face masks meant to slow the spread of the new coronavirus should not have valves.

Image: Getty/Svetlana-Cherruty

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