Fit and design
Our research showed that cloth masks performed similarly to medical masks because they fit better, were designed thoughtfully and had overhead ties. Although medical masks are composed of materials with better filtration performance, they exhibit greater leakage, with completely unfiltered air passing around the mask.
The cloth masks included in past studies may have been haphazardly selected: their filtration, using the same methods that we used, was found to be between 23 and 52 per cent. A thoughtfully designed two-layer T-shirt mask with overhead ties, tested on human volunteers, filtered 50 per cent of aerosols, in keeping with our study, and again highlighting the importance of fit.
To compare like with like, when we looked at other studies, the filtration percentages that we quote above were taken from studies using a similar design: protection of a human wearer, using particles 0.02 to 3 microns, 0.02 to 1 microns and 0.02 to 0.1 microns. Other studies examining source control, using mannikins and using larger particles come to similar conclusions.
Good masks fit well, with minimal obvious leaking at the edges. Nosewires, overhead ties or earloop adjusters all contribute to fit. Two layers or more, and a middle layer of non-woven polypropylene improve overall filtration.
Masks meeting the new U.S. Centers for Disease Control and Prevention standard for community masks, ASTM-F3502, are likely to filter well, though generally not as well as respirators.
Hospitals report that the current supply of N95-type masks meets but doesn’t exceed demand, and supply of these masks is insufficient for the general population. Access and cost issues require public health and economic solutions along with consideration of environmental impact of disposable PPE.
If buying respirators for personal use, we recommend buying only what you need and practising extended reuse. If buying KN95s and equivalent certified medical masks, we advise paying close attention to the fit of the mask. We recommend double masking medical masks or using minor modifications or mask hacks that enhance fit and reduce leakage.
Carefully designed, well-fitting, multilayer reusable cloth masks still have an important ongoing role in reducing transmission of SARS-CoV-2, especially in lower risk settings.
Protection is highest and transmission is lowest when everyone wears masks, because masks both protect the wearer while also reducing the number of contaminated particles reaching the environment (source control). There are important reductions to individual risk from wearing any mask, which have been observed in the community and quantified in the lab. We can protect ourselves, others and vulnerable people. Let’s all wear the best mask available.