“Any mask is better than no mask” to slow spread of COVID-19
“Fine droplets can pass through layers of masks and are carried away by the exhaled airflow unlike larger droplets that settle down due to gravity,” wrote the researchers.
“Hence, it is important to quantitatively assess the maximum distance of travel of typical human coughs with and without different types of masks.”
Study lead author Padmanabha Prasanna Simha is an expert at the Indian Space Research Organization.
“If a person can reduce the extent of how much they contaminate the environment by mitigating the spread, it’s a far better situation for other healthy individuals who may enter places that have such contaminated areas,” said Simha.
Density and temperature are closely related, and coughs tend to be warmer than their surrounding area. Based on this knowledge, the researchers used a technique called schlieren imaging to visualize changes in density as various study participants coughed.
By analyzing the motion of the coughs over consecutive images, the team was able to estimate the velocity and spread of the projected droplets.
As expected, the study showed that N95 masks were the most effective at reducing the horizontal spread of a cough. This mask is known to filter at least 95 percent of airborne particles.
The N95 mask reduced a cough’s initial velocity by up to a factor of 10 and limited its spread to between 0.1 and 0.25 meters.
While an uncovered cough can travel up to 3 meters, the researchers found that a simple disposable mask can bring this velocity down to 0.5 meters.
“Even if a mask does not filter out all the particles, if we can prevent clouds of such particles from traveling very far, it’s better than not doing anything,” said Simha. “In situations where sophisticated masks are not available, any mask is better than no mask at all for the general public in slowing the spread of infection.”
On the other hand, some of the findings were concerning. For example, it is often advised to use an inner elbow to cover up a cough, but the testing showed that a bare arm cannot form the necessary seal against the nose without a long sleeve. As a result, droplets could leak through any openings and spread in many directions.
The researchers hope the findings will disprove the argument that regular cloth masks are ineffective, but they emphasize that masks must continue to be used cooperatively with social distancing.
“Adequate distancing is something that must not be ignored, since masks are not foolproof,” said Simha.
The study is published in the journal Physics of Fluids.