Two hairdressers in Missouri kept cutting hair while infected with the coronavirus. They wore a mix of cloth and surgical masks, and so did their 139 clients. Not one client caught it.
The case is cited by an Australian research group whose new video study, published in the journal Thorax in July 2020, ranks which masks stop the most droplets. The group filmed volunteers talking, coughing and sneezing with no mask, with two kinds of cloth mask, and with a surgical mask.
What the cameras caught
The researchers used an LED lighting system and a high-speed camera to make droplets and aerosols visible. The cloth masks were made from DIY templates posted online, one with a single layer and one with two. The surgical mask had three layers.
Speech alone put out substantial droplets. Coughing produced more, and sneezing more again.
The three-layer surgical mask was significantly better than the single-layer cloth mask at cutting emissions from all three activities. The double-layer cloth mask came second. A single layer still reduced the spread, just not as well as either of the others.
“A surgical mask was the most effective at blocking droplets and aerosols from talking, coughing and sneezing,” the authors wrote. If you cannot find one, they said, a cloth mask is the next best thing, and the more layers the better.
The team was careful about what the footage proves. Infection risk will depend on how many people with no symptoms or mild ones are around, and the videos cannot say how the results translate. They do show that a single layer is a weaker barrier than a double one.
Sewing a better mask
With mask use mandated in Greater Melbourne and the Mitchell Shire, the authors worried that surgical masks could run short. They did not test more than two layers. Generally, though, more is better: a 12-layer cloth mask is about as protective as a surgical mask and reduces infection risk by 67%. They conceded that stitching together 12 layers of fabric is hard.
Their shorter list of tips for home sewers:
- Use at least three layers.
- Make the outer layer from water-resistant fabric.
- Choose a high thread count and a tight weave, such as good quality bed sheet, rather than a loose weave you can see light through.
- Consider hybrids such as cotton-silk, cotton-chiffon or cotton-flannel, which filter better and are more comfortable.
- Check that the mask fits and seals well around the face.
Back to Missouri
One of the two hairdressers did infect her own family, because neither she nor they wore masks at home. The authors called the clients’ escape “reassuring evidence that infection risk is reduced when everyone wears masks.”
They do not yet know which matters more, masks that stop infected people spreading the virus or masks that shield healthy people from infected aerosols. Probably both count equally, they said. Throwing a scarf over your face, their research and other work suggests, is not as protective as a well-designed cloth mask with several layers.
