Filtration & fluid control The best mask for the job
The team selected 32 different types of face coverings designed for use in hospitals. They included cloth masks, surgical masks, face shields and respirators. Some of these met quality standards set by the EU and China, while others did not. In the first experiment, the masks were attached to a tube inside an airtight tank, which was pumped with an aerosol containing the chemical di-ethyl-hexyl-sebacate (DEHS). These particles were 0.5µm – thought to be representative of SARS-CoV-2 particle clusters. The particles that made their way through into the tube were counted, which gave a measurement of how the material prevented outside particles from filtering through. This experiment confirmed the high filtration efficacy of KN95 and FFP2 respirators as 94% and 98%, respectively. However, as measured in the second part of the study, they also showed a drop in pressure (32.3PA/cm² for KN95 and 26.8PA/cm² for FFP2) consistent with inhalation and exhalation resistance. In the third part of the experiment, the same set-up was used to explore the ‘as-worn’ filtration efficacy, but the masks were affixed to a life-sized dummy head with a skin-like coating and an artificial trachea, rather than an air-collecting tube, to mimic the conditions of wearing the mask. KN95 respirators performed worse than expected, filtering just 41% of particles, with the FFP2 type filtering 65%. The former produced similar results as surgical facemasks, whose as-worn filtering efficacy was 47% compared with a material filtration efficacy of 70%, and a comparatively low pressure drop of 12.9PA/ cm². The worst-performing coverings overall were cloth masks, indicated by a material and as-worn filtration efficacy of 28% and just 11% respectively. However, the pressure drop was as low as 6.9PA/cm², meaning breathability was markedly higher than other masks. The study’s authors concluded that the best benefit of a face covering is achieved by both good filtration and breathability, as indicated by a low pressure drop. “If you have a high filtration efficacy, in most cases you also have a large pressure drop because of the breathing resistance of the mask material,” says Gunther. These masks, therefore, may be less comfortable to wear for long periods, because it’s more difficult to breathe. So, the best face covering depends on the circumstances.
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“[Surgical] facemasks have good filtration that might be good enough for low and medium-risk situations, if there is no specific exposure risk and if you additionally keep a distance,” Gunther adds. “For high-risk situations, if you are working on Covid-19 patients, FFP2 respirators are the ones that should be used,” he says. They should be limited to short periods of wear due to the low breathability, which would create greatly increased breathing exertion over time and lead to exhaustion.
Home-made masks Gunther’s research found the lowest protection against penetration from outside aerosol particles
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