Why masks with exhalation valves do not slow COVID-19 spread.
Amid the COVID-19 pandemic, if you step out wearing a valve mask you bought to tackle pollution, you should consider changing it. Scientists have used high-speed videos of airflow to show why masks with exhalation valves do not slow the spread of COVID-19.
The researchers from the National Institute of Standards and Technology (NIST) in the US created the videos which show airflow patterns through masks with and without exhalation valves.
NIST research engineer Matthew Staymates, the corresponding author of the study published in the journal Physics of Fluids stated that while comparing the video's side by side, the difference is striking.
"These videos show how the valves allow air to leave the mask without filtering it, which defeats the purpose of the mask," Staymates said.
Exhalation valves, which make masks easier to breathe through and more comfortable, are appropriate when the mask is meant to protect the wearer, the researchers said.
For instance, valved masks can protect workers from dust at a construction site or hospital workers from infected patients, they said.
Staymates created two videos using different flow visualisation techniques. The first video was created using what is known as a schlieren imaging system, which causes differences in air density to show up on camera as patterns of shadow and light.
With this system, exhaled breath becomes visible because it is warmer, and therefore less dense, than the surrounding air.
He built an apparatus that emits air at the same velocity and tempo as a resting adult, then connected that device to a mannequin.
As a stand-in for exhaled droplets, the air carries water droplets in a range of sizes typical of the droplets that people emit in their breath when exhaling, speaking, and coughing.
A high-intensity LED light behind the mannequin illuminates the airborne droplets, causing them to scatter the light and show up brightly on camera.
In contrast to the schlieren video, this video shows the movement of droplets in the air. The droplets escape unfiltered through the valve of an N95 mask.
The use of a mannequin and a mechanical breathing apparatus allowed Staymates to observe airflow patterns while holding steady the breathing rate, air pressure, and other variables.
The Centre for Disease Control (CDC) recommends cloth masks, as layers of cotton can prevent the potentially infectious respiratory droplets from entering or escaping. According to the CDC, these valves reduce heat and moisture inside masks, making them more comfortable to wear for long periods. But this design doesn't prevent the spread of infectious diseases. The main reason for wearing masks in the COVID-19 era is to prevent respiratory droplets from traveling into the air when a person coughs, sneezes, or talks, the CDC says. But when a mask has a valve, respiratory droplets from the wearer are expelled into the air and could reach other people.
With 44,879 new coronavirus infections and 547 deaths in the past 24 hours, India's overall tally stood at 87,28,795 on Friday, as the toll touched 1,28,688. Currently, there are 4,84,547 active cases, whereas 81,15,580 patients have been cured and discharged so far. The recovery rate stands at 92.97 per cent, and the fatality rate is 1.47 per cent.
(With inputs from Agencies)
