Effectiveness of canvas masks depends on the type of coverage

Months in the COVID-19 pandemic, wearing a mask while in public has become the recommended practice. However, many still question the effectiveness of this.

To combat these doubts, Padmanabha Prasanna Simha, of the Indian Space Research Organization, and Prasanna Simha Mohan Rao, of the Sri Jayadeva Institute of Cardiovascular Sciences and Research, experimented the stream fields of cough under various common mouth-covering scenarios. They present their findings in the journal Physics of liquids.

“If a person can reduce the extent of how much they contaminate the environment by limiting the spread, it is a much better situation for other healthy people who can come to places that have such contaminated areas,” Simha said.

Population densities and temperatures are intricately related, and coughs are constantly warmer than their surrounding area. In this connection, Simha and Rao used a technique called glandular imaging, which visualizes changes in density, to take pictures of voluntary coughs of five subjects. By tracking the motion of a cough over successive images, the team estimates velocity and distribution of the displaced droplets.

Not surprisingly, they found N95 masks most effective at reducing the horizontal spread of a cough. The N95 masks reduce the initial speed of a cough by a maximum of 10 and limit the spread to between 0.1 and 0.25 meters.

An uncovered cough, in contrast, can travel up to 3 meters, but even a simple disposable mask can bring this all the way to 0.5 meters.

“Even if a mask does not filter all the particles, if we can prevent clouds of such particles from traveling very far, it is better than doing nothing,” Simha said. “In situations where refined masks are not available, any mask is better than no mask for the general public to slow the spread of infection.”

However, some of the other comparisons were striking.

For example, using an elbow to cover a cough is typically considered a good alternative in a cough, which is contrary to what the couple found. Except when covered by a sleeve, a bare arm cannot form the proper seal against the nose needed to impede airflow. A cough is then able to leak through all openings and propagate in many directions.

Simha and Rao hope their findings will allay the argument that ordinary cloth masks are ineffective, but they stress that masks should continue to be used in combination with social distancing.

“Adequate distance is something that should not be ignored because masks are not foolish,” Simha said.