Physics suggests that homemade masks are useful in a pandemic, as long as they’re the right sort and fitted right.

In recent simulations, a team from Florida Atlantic University found that loosely folded facemasks and bandana-style coverings had minimal capability to stop the smallest aerosolised respiratory droplets, but well-fitted homemade masks with multiple layers of quilting fabric were effective, as were off-the-shelf cone style masks.

Both were able to curtail the speed and range of the respiratory jets significantly, albeit with some leakage through the mask material and from small gaps along the edges.

Droplets travelled more than 2.4 metres without a mask and more than a metre with a bandana, falling to 38 centimetres for a folded cotton handkerchief, 20 centimetres for a cone-style mask and just six centimetres for a mask using two-layers of cotton quilting fabric consisting of 27 threads per centimetre.

To calculate these figures, Siddhartha Verma and colleagues employed flow visualisation in a laboratory setting using a laser light sheet and a mixture of distilled water and glycerin to generate the synthetic fog that made up the content of a cough-jet.

They visualised droplets expelled from a mannequin’s mouth while simulating coughing and sneezing, mapping out the paths the droplets took.

“The main challenge is to represent a cough and sneeze faithfully,” Verma says. “The setup we have used a simplified cough, which, in reality, is complex and dynamic.”

The researchers note the need for further quantitative analysis but say straightforward visualisation provides an initial indication of the effectiveness of protective equipment

“While there are a few prior studies on the effectiveness of medical-grade equipment, we don’t have a lot of information about the cloth-based coverings that are most accessible to us at present,” says Verma.

“Our hope is that the visualisations presented in the paper help convey the rationale behind the recommendations for social distancing and using face masks.”

The pathogen responsible for COVID-19 is found primarily in respiratory droplets that are expelled by infected individuals during coughing, sneezing, or even talking and breathing.

Respiratory droplets also are the primary means of transmission for other viral and bacterial illnesses, including the common cold, influenza, tuberculosis, SARS (Severe Acute Respiratory Syndrome) and MERS (Middle East Respiratory Syndrome).

The findings are published in the journal Physics of Fluids.