Streaking images of coughs with varying degrees of facial coverage. Photo credit: Padmanabha Prasanna Simha, Indian Space Research Organization
Months into the COVID-19 Pandemic, wearing a mask in public has become the recommended practice. However, many still question its effectiveness.
To dispel these doubts, Padmanabha Prasanna Simha from the Indian Space Research Organization and Prasanna Simha Mohan Rao from the Sri Jayadeva Institute for Cardiovascular Science and Research experimentally visualized the flow fields of coughs under various common mouth-covering scenarios. They present their results in the magazine Physics of liquids, from AIP Publishing.
“If one person can reduce the level of their environmental contamination by mitigating the spread, it will be a far better situation for other healthy people to enter places with such contaminated areas,” Simha said.
Density and temperature are closely related, and coughing tends to be warmer than the surrounding area. In this context, Simha and Rao used a technique called Schlieren imaging, which visualizes changes in density, to capture images of voluntary coughs from five test subjects. By tracking the movement of a cough over successive images, the team estimated the speed and spread of the droplets emitted.
Unsurprisingly, N95 masks were most effective at reducing the horizontal spread of a cough. The N95 masks reduced the initial speed of a cough by a factor of 10 and limited its spread to 0.1 to 0.25 meters.
In contrast, an uncovered cough can go up to 3 meters, but even a simple disposable mask can do this up to 0.5 meters.
“Even if a mask doesn’t filter out all of the particles, it’s better than doing nothing if we can prevent clouds of such particles from traveling very far,” Simha said. “In situations where sophisticated masks are not available, any mask is better than no mask at all for the general public to help slow the spread of infection.”
However, some of the other comparisons were noticeable.
For example, using an elbow to cover a cough is usually seen as a good alternative to emergency, which is at odds with what the couple found. If a bare arm is not covered by a sleeve, it cannot form the proper seal against the nose necessary to obstruct air flow. A cough can then come out through any openings and spread in many directions.
Simha and Rao hope their results dispel the argument that regular cloth masks are ineffective, but emphasize that masks must continue to be used in conjunction with social distancing.
“Adequate distancing is something that cannot be ignored as masks are not foolproof,” Simha said.
Reference: “Universal Trends in Human Coughing Airflows at Long Distances” by Padmanabha Prasanna Simha and Prasanna Simha Mohan Rao, August 25, 2020, Physics of liquids.
DOI: 10.1063 / 5.0021666
