Weak masks may leave cough droplets suspended in air

A graphic showing how cough or sneeze droplets smash into the insides of masks and penetrate single or double-layered masks.

Facemasks may be the primary weapon against Covid-19, but some types of masks are inadvertently causing cough droplets to become aerosolised and airborne, scientists have found.

The findings also imply that wearing weak masks such as handkerchiefs or even double-layered masks may be the same as not wearing a mask at all.

Researchers at the Indian Institute of Science (IISc) explained that when a person coughs, large droplets greater than 200 microns in size, hit the inner surface of a mask at high speed, penetrating the mask fabric and atomising into smaller droplets. These small droplets have a greater chance of aerosolisation or becoming suspended in the air. The study was published on Friday in Science Advances.

Using a high-speed camera, the team closely tracked individual cough-like droplets impinging on single, double and multi-layered masks, and noted the size distribution of the "daughter" droplets generated after penetration through the mask fabric. They found that in single and double-layered masks, the atomised daughter droplets were smaller than 100 microns, meaning that they had the potential to become aerosols.

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This means that a person with Covid-19, for example, can discharge a quantity of infected droplets that could infect others who have no protection short of a face shield, explained Professor Saptarshi Basu of IISc’s Department of Mechanical Engineering and senior author of the study.

The findings are unequivocal: single-layered masks could only block 30% of the initial droplet volume from escaping. Double-layered masks were better (about 91% was blocked), but more than a quarter of the daughter droplets that were generated were in the size range of aerosols. Droplet transmission and generation was either negligible or zero in triple-layered and N95 masks.

Because the aerosols can stay airborne for hours, they can infect a person through the eye or by even seeping down the face into the nasal-mouth area.

Professor Basu explained that a non-mask-wearer discharges a smaller quantity of aerosols but a greater quantity of droplets overall. While only 30 to 40% of droplets escape a single-layer mask, 70% of this is usually aerosolised.

The study was carried out in collaboration with scientists in UC San Diego and University of Toronto Engineering.