The main function of masks is to prevent infectious particles exhaled from the source from reaching a host by preventing their spread (source control) or their inhalation or deposition on the facial mucous membranes. They contribute to the blocking of particles in the air by different mechanisms which can vary according to the characteristics of the particles (eg, size, shape, charge):
interception, inertia against surfaces, electrostatic attraction, diffusion, etc. 11, 12. Researchers are using different approaches to assess the protective effect and source control ability of masks, ranging from experimental studies quantifying particle blockage by determining the filtration efficiency (EF) of materials, to studies in humans that measure differences in clinical outcomes (eg, level of infection) based on mask-wearing modalities.
Experimental studies. often carried out in the laboratory, provide information on the physical properties of the materials from which masks are made. They can assess the protective effect or control at the source by quantifying the movement of particles of different sizes either from the external environment, through the material of the mask and to the breathing zone of the wearer, or the inverse, during different actions of the respiratory system (breathing, coughing, etc.
Often, these studies are done using aerosols or artificial droplets, created for example by nebulization of a NaCl solution. Sometimes they use mannequin heads to represent the emitter or the exposed subject. Some also include humans to measure the spread of particles during various actions of the respiratory system.
Observational studies examine. the effect of mask-wearing by comparing the clinical outcomes of wearers and non-wearers under different conditions or in particular settings. They can also assess the effect of mask-wearing modalities such as duration of wearing and type of mask, without introducing an intervention, and be retrospective in nature 28-33 . Some are called ecological , that is to say that they compare observations at the scale of a locality or a population, for example to estimate the effect of the obligation to wear a mask on the number of cases in a given geographical area or sector of employment 34-38 .
Comparative trials compare. the clinical outcomes of a control group (eg, non-mask wearers) with those of an intervention group (eg, respirator wearers). Among these studies, some assess the reduction in the incidence of clinical respiratory illnesses, flu-like illnesses and laboratory-confirmed respiratory illnesses in healthcare workers randomly assigned to groups that differ on mask wearing 39-47 .
The reviews provide an overall portrait of the scientific literature based on the quality of the studies and the results presented. Systematic reviews vary in scope and inclusion and exclusion criteria: some are limited to randomized controlled trials or selected groups (e.g., health system versus general population ), while others include a wider variety of studies. Therefore, conclusions may vary depending on the research question and the articles selected 48-54 . Finally, the reviews are sometimes accompanied by a meta-analysis, which combines the results of several studies.
Different types of studies such as those mentioned above have been used to determine the effectiveness of masks in reducing the transmission of respiratory viruses, specifically to get an idea of the types of materials of construction and the characteristics of masks that best block the movement of particles, how they are worn, and their ability to lower infection rates in various settings. The following section reviews this approach.
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