Transmission of aerosol particles is one of the main way for transmission of respiratory infectious agents. It is defined as the passage of pathogenic microorganisms (bacteria or viruses) from a source to a person from infectious aerosols released during exhalatory events generating aerosols, such as breathing, coughing, talking, singing, and sneezing1. For example, a single sneeze can release up to 40,000 aerosol particles2. From a physical point of view, the term aerosol” corresponds to a heterogeneous mixture of airborne particles, solid or liquid, suspended in a gas and having a relatively low settling velocity3 (i.e. typically airborne particles with an aerodynamic diameter lower than 100 µm). However, in the medical literature for decades, a distinction is frequently found, appearing arbitrary (and misleading to an aerosol scientist), between “airborne” particles smaller than 5 µm in diameter and “droplets” larger than 5 µm in diameter3. This confusion, emanating from traditional medical language, has sometimes created scientific unfounded terminology distinctions between the so-called “airborne” and “droplet” transmission. Indeed, if people can inhale aerosol particles (of variable size in space and time because they are always dynamic and transitory phenomena), mainly constituted by droplets containing pathogens (from body secretions and excreta), we always breathe liquid airborne particles whatever their size4.
Therefore, physical wording, the transmission of respiratory pathogens is done in both cases (“airborne” and “droplets” transmission) by varying sizes of aerosol particles5. In other words, whether the pathogen transmission is called “airborne” or “droplet”, it can only be by aerosol in all cases. However, it is true that the mode of transmission and control measures may vary according to the physical characteristics of the aerosol particles (including their aerodynamic diameter changing in space and time). On the one hand, if an infectious pathogen is spread mainly by rapidly settling respiratory aerosol particles called “droplets”, the primary transmission control measures consist in reducing direct contact, physical distancing, or the use of face masks. On the other hand, the case of an infectious pathogen whose transmission is mainly called “airborne” requires precautionary measures such as ventilation of the room, air filtration, or attention to the quality and fit of the face mask whenever indoors.
Moreover, it is well recognized that the prevention of infection by airborne pathogens (e.g., influenza, tuberculosis, measles, or coronavirus) can be facilitated through use of mouth-nose cover3. Therefore, the use of a face mask is currently recommended to prevent transmission of respiratory diseases for medical staff, contagious patients, and, in some cases, the general population. Obviously, any mask is better than no mask, especially in terms of protecting others. Wearing a mask retains a relatively large proportion of the viral droplets emitted by the mask wearer, thus providing a high degree of protection against bioaerosol emission. Although masks are designed to primarily retain pathogen-laden micrometer-sized aerosol particles when exhaled, they also likely provide some degree of self-protection during inhalation (usually much less due to shrink of liquid aerosol particles between exhalation and inhalation). All things considered, face masks contribute significantly to decrease the risk of infection to those in the vicinity, and may also reduce the risk of infection to the mask wearer, especially if the pathogen is transmitted by larger aerosol particles. For example, the understanding in the nineteenth century on the contagion of tuberculosis caused by the Mycobacterium tuberculosis pathogen helped to limit its spread by developing the first mask covering the nose and mouth6,7. It was well demonstrated that face masks worn by patients infected with tuberculosis could significantly reduce transmission rates to uninfected patients8.
More recently, the global pandemic of coronavirus (COVID-19) has raised the question of the issue of transmission of viral respiratory diseases. At the beginning of the pandemic, the first epidemiological and virological studies favored droplet and surface transmission. Subsequently, numerous studies have shown that transmission by fine aerosol particles containing viable viral particles represents one of the main routes of transmission of severe acute respiratory syndrome Coronavirus 2 (SARS-CoV-2) in poorly ventilated indoor environments1,9,10. Early on, given the knowledge of airborne diseases, the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) advocated universal mask use to reduce the risk of SARS-CoV-2 transmission11. Masks were indicated to prevent others by limiting the exhalation of potentially infectious respiratory droplets containing SARS-CoV-2 into the air stream, but also to protect the wearer in many cases3. The emergence of COVID-19 has thus confirmed the mask effectiveness. Indeed, accumulated evidence shows that face masks are a critical barrier, reducing the number of infectious viruses in the exhaled air, especially in asymptomatic or pre-asymptomatic individuals12,13,14. A study by Bagheri et al. (2021) showed that if an uninfected person wears a surgical mask and an infected person speaks without a mask, the maximum risk achieved in this case is 90% after 30 min for the uninfected person. However, if both people wear surgical masks, the maximum risk is less than 30% even after one hour15.
Among the different categories of face masks, medical face masks (MFMs) are single-use medical devices specially designed to prevent the dissemination of bioaerosols from the wearer into the environment. They are regulated by specific standards such as the European standard EN14683:201916 and must therefore meet performance requirements, particularly in terms of bacterial filtration efficiency (BFE). MFMs are classified into two types according to their BFE values. Type I masks (BFE ≥ 95%) and Type II/IIR masks (BFE ≥ 98%). By contrast, community face masks (CFMs) or cloth masks come in various designs and are made with a large variety of fabrics and whilst not as effective as MFMs. CFMs are not designed to be used in an environment requiring high level of sanitary protection. Numerous studies performed on CFMs have shown that various characteristics of the fabrics (material type, fabric type (woven or knitted), fiber characteristics) may influence their filtration17,18. CFMs can be considered as simply anti-spray masks and are typically reused by washing. Unlike the MFMs that are strictly regulated and certified, CFMs are not standardized nor strictly regulated. Currently CFMs are intended for the general population and divided into two categories according to their ability to filter particles of a size of 3 ± 0.5 μm. According to AFNOR SPEC S76-001 requirement19, the CFMs of category 1 must have a filtration efficiency higher than 90%, whereas the CFMs of category 2 must have a filtration efficiency higher than 70%.
Mask filtration is based on different mechanisms: gravity sedimentation, inertial impaction, interception, diffusion and electrostatic attraction20. Due to the significant effect of many factors on the performance of mask filtering (and mainly the type of mask, CFMs vs. MFMs for instance), clarifying the mechanism of bioaerosol penetration into the mask has high importance. The first and dominant mechanism is the filtration of pathogen-laden droplets directly through the filter material. This first step is mainly dependent on the size and speed of the airborne droplet for a given filter material design. But when contaminated aerosol liquid particles reach the outer surface of the mask, if the surface does not destroy the pathogen initially contained on it, microorganisms can then penetrate the mask during breathing by various mechanisms (including capillaries)20. This second step is mainly dependent on the size and the number of pathogens accumulated at the external surface of the mask if the exposure time is long enough for a given filter material design. Thus, a mask can often become a pathogen collector, particularly when its outer surface is exposed to contaminated aerosol particles. Since viruses and bacteria can remain on the surface of the mask, or even in the textile structure of the masks, it is obviously dangerous and undesirable that they can migrate through the mask once the filtration of the liquid aerosol particles by the filtering material has been achieved.
The determination of the penetration and propagation capacities of micro-organisms through the mask therefore appears to be a major challenge for evaluating the protection provided by the mask. It is essential to analyze the filtration of bioaerosols via the prism of the transport of pathogens contained in the airborne liquid vectors through the mask, rather than stopping only at the study of the filtration of these aerosol particles at the surface of the filtering material. This work was carried out in order to study, for a fixed size of aerosol liquid particles (in the range of aerodynamic diameter between 2 and 3 µm), and for different qualities of mask (a MFM, a CFM with excellent performances, and a CFM with a low filtration efficiency), the impact of the size of the pathogen (a virus of 100 nm or a bacterium of 1 µm) on the penetration through the mask. In other words, since MFMs are evaluated using bacterial and non-viral bioaerosols, this study allows us to study whether an extrapolation (for a fixed aerosolized vector size of a few microns of aerodynamic diameter) can easily be carried out between Bacterial Filtration Efficiency (BFE) and Viral Filtration Efficiency (VFE).