Experimental studies normally take place in a laboratory and are designed to simulate and test certain real-world conditions in a safe way.
Several experimental studies have simulated the effectiveness of various mask types to a) stop an infectious person spreading respiratory virus (‘source control’) and b) prevent a healthy person from getting a respiratory virus. A recent study found surgical masks could effectively reduce the emission of coronaviruses and influenza viruses from infected patients, potentially limiting onward transmission. Another study found that N95 respirators, surgical masks, and fabric masks (made from tea towels) all reduced exposure during various activities and that the effectiveness of masks did not decrease over a three-hour period. However, in simulated human breath experiments, fabric masks had minimal effect on limiting emission of respiratory particles. A study comparing surgical masks, homemade cotton t-shirt masks, and no masks found that both surgical masks and fabric masks reduced the number of microorganisms released during coughing. While both offered some protection, the surgical masks were more effective in filtering microorganisms, potentially because the homemade masks did not achieve as close of a fit as surgical masks.
Past studies of fabric mask filtration efficiency have found different fabrics are capable of reducing particle emission between 10-97% depending on the size of the particle and the fabric and design (number of layers) of the masks. A recent study examined the filtration efficiency of several common types of fabric, used in single or multiple layers, and used in combination with other fabric types (‘hybrids’), to prevent the emission of particles in two size ranges (<0.3 microns and 3-6 microns). Filtration efficiency varied between 9 - 99% depending on mask design and fabric and particle size range. Hybrid masks that contained high-thread count cotton combined with silk, chiffon or flannel, were among the most efficient at filtering out particles of both size ranges. Masks with multiple layers of chiffon, high-thread count cotton, and silk were also effective in filtering particles of both sizes.
When interpreting lab-based studies, remember that experimental conditions may not always replicate real-world conditions. For example, filtration efficiency is not typically assessed for individual pathogens like SARS-CoV-2. Instead, reported filtration efficiency is often based on the ability of a fabric mask to filter out different sized particles (or pathogen surrogates in some studies). This is partly why there is little data on the efficacy of fabric masks for SARS-CoV-2 specifically. Further, the generation of aerosols in the laboratory (often produced via equipment like a nebulizer) may not reflect the pattern or frequency of aerosol production during normal human speaking, singing, or coughing.
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Editor's note
Author: Jackie Knee
Review: Peter Winch, Julie Watson
Last update: 06.18.2020