Explained: For arresting cough particles, mask shape and material matter

While evidence on such aspects is still emerging, researchers from Florida Atlantic University recently demonstrated a way to check mask performance. Drawing from the principles of fluid mechanics, they used a mannequin to simulate coughs, and tracked the movement of the ejected articles by using a laser. Their results are published in the journal Physics of Fluids.

A homemade mask arrests forward motion of droplets almost completely, with a little escape from gaps along top edge. In a commercial cone-style mask, leakage from these gaps are noticeable. (Siddhartha Verma, Manhar Dhanak & John Frankenfield/Physics of Fluids)

The broad findings

* Face masks built with a loosely folded handkerchief, as well as bandana-style coverings, showed little capability to stop the smallest aerosolised respiratory droplets.

* Well-fitted homemade masks with multiple layers of fabric, and commercial cone-style masks, curtailed the speed and range of the respiratory jets significantly.

* Even in these homemade masks and cone-shaped masks, a little leakage happened through the material, and from small gaps along the edges.

* In uncovered coughs, droplets were able to travel far beyond 6 feet (the recommended distancing guideline).

The set-up

The researchers used a hollow mannequin head, which was padded on the inside to approximate the internal shape and volume of the nasal and oral cavities, and mannequin at a height of 5 ft 8 in to match that of an average human male.

To emulate a cough or a sneeze, a pump delivered the pressure impulse. The air capacity of the pump was 500 ml, comparable to the volume expelled during a cough.

A recreational fog/smoke machine generated tracer particles from a mixture of distilled water and glycerine. The droplets were less than 10 microns in diameter, comparable to the smallest droplets expelled in a cough. A 5 mW green laser pointer helped capture the visuals.

The takeaways

When without a mask, the mannequins were projecting droplets much farther than 6 feet. This distance was reduced by 1/8 when the mannequin wore a loosely folded handkerchief, and by 1/2 when it wore a bandanna-style covering. Well-fitted homemade masks with multiple layers of quilting fabric and off-the-shelf cone-style masks proved the most effective, some leakage notwithstanding (see diagrams and table).

The bandanna-style mask used was of elastic T-shirt material, the folded handkerchief was of cotton, the stitched mask was of quilting cotton, and the cone-style mask was of random assorted material.