The outbreak of the Severe Acute Respiratory Syndrome coronavirus 2 (SARS-CoV-2) in China in late 2019 has led to a pandemic and a serious global problem of both public health and the economy. Initially, the virus was named the novel 2019 coronavirus, and the disease it causes was called 2019-nCoV. The World Health Organization identified the disease as COVID-19 on March 11^th, 2020. Since the first recorded local outbreak of the SARS-CoV-2 coronavirus in Wuhan, China, in December 2019, the virus and the disease it causes have spread rapidly. Due to the possibility of travel and migration of people, the SARS-CoV-2 virus has spread not only to China but also to many countries on all continents [1]. Hence, on January 30, 2020, WHO announced the outbreak of COVID-19 as the sixth public health emergency of international concern beyond H1N1 (2009), polio (2014), Ebola in West Africa (2014), Zika (2016), and Ebola in the Democratic Republic of the Congo (2019). By June 2021, the WHO had reported more than 172 mln confirmed cases and almost 3,7 mln of deaths worldwide [2]. Based on numerous reports at present, a rapidly increasing incidence of infections has been demonstrated and the possibility of virus transmission by asymptomatic carriers. Due to the constant search for targeted therapy and the development of vaccinations, the greatest emphasis is placed on COVID-19 preventative measures. Therefore, to stop the spread of the virus, social regulations and global cooperation are expected not only from health professionals but from governments and societies. In the face of the global spread of the virus and the lack of effective treatment and vaccine, the main goal is to prevent the still not fully understood effects of COVID-19. Therefore, measures of public health control and prevention of infection are essential. WHO recommends avoiding close contact with people suffering from acute respiratory infections, hand hygiene (frequent washing of hands, especially after direct contact with patients or their surroundings), and avoiding unprotected contact with livestock or wild animals. In addition, in the case of people with symptoms of acute respiratory infection, the so-called cough etiquette (distance, covering the mouth and nose when coughing and sneezing, and washing hands) and improved standard PPE (personal protective equipment), as well as strict infection prevention and control procedures in hospitals (especially in emergency departments), need to be practiced [1].

Due to the need to use prophylactic measures in the form of nose and mouth covers, such as protective masks, attention should be paid to the length of their use in the context of possible secondary infections with own and opportunistic microflora of the mouth and upper respiratory tract and the development of skin infections or their exacerbation. Microorganisms are present on the surface of many human tissues, for example, on the skin, in the mouth (oral cavity microbiome), and the respiratory tract. Extensive characterization developed in the Human Microbiome Project (HMP1) is an interdisciplinary effort of centers, such as Broad Institute, Baylor College of Medicine, Washington University School of Medicine, and the J. Craig Venter Institute, a Data Analysis and Coordination Center (DACC), and several independent investigators. HMP1 identified the diverse habitats of microorganisms in the oral cavity, including the back of the tongue, hard palate, sub- and supragingival plaque, cheek mucosa, palatine tonsils, keratinized gingival tissues, and saliva [3]. The number and variety of microbes in the oral cavity are influenced by many parameters, including age, diet, genetic factors, and individual hygiene. The human microbiota, the so-called physiological flora, is subject to changes as a result of exposure to a number of factors, including antibiotic therapy, the use of vaccines, and even active or passive smoking [4]. The mouth can be divided into three similarly colonized habitats: gingiva and hard palate, tongue and pharynx, and dental plaque [5]. Based on the analysis of the composition of the oral microbiome, its relative stability over time is suggested [6]. Oral microbiome growth is slow, and critical parameters include nutrients and the sensitivity of the microbes to oxygen availability. The oral cavity and mouth, in particular, are exposed to close contact with the surrounding environment through continuity and thus are susceptible to numerous airborne and foodborne microorganisms through food. This can lead to the development of pathobionts from other niches of the upper respiratory tract [3].

The announced COVID-19 pandemic caused by Sars-CoV-2 is a public emergency worldwide (WHO, 2019). Social distance, hand hygiene, surface disinfection, and mouth and nose coverage are paramount practices for infection control during the COVID-19 pandemic. There are global discussions on the legitimacy of the general use of non-medical masks to limit the transmission of SARS-CoV-2 [26]. Thus, both global and local organizations recommend, especially in the case of health care workers and people who may be infected, the use of prophylaxis in the form of protective masks covering the nose and mouth [27].

In the context of the current situation, many studies performed to date, conducted not only in recent months but also in recent decades [26], indicate the dependence of filtration efficiency on the materials of the protective masks. Since the 1970s, experiments have been carried out using various techniques (including sedimentation on agar plates) and materials (including medical, cotton, muslin, flannel, and admixture fabrics) for the production of protective masks, in which similar results were obtained. Examples of the masks used most commonly in the COVID-19 pandemic are presented in Fig. (1). In the case of bacteria, the filtration efficiency of medical and fabric masks was assessed up to 99%, and for aerosols (<3.3 μm) up to 89%. The ability of the material to block the transmission of microorganisms is determined by assessing the filtration efficiency expressed as a percentage using surrogate indicators, e.g., biological aerosols, included by the ASTM (American Society of Testing and Materials) in standards for masks. The filtration efficiency depends on the physical retention of particles of varying sizes, regardless of the microorganisms they contain [26]. The efficiency of blocking droplets and aerosols of masks made of cloths may increase with the number of layers [27, 28]. In the analysis of the filtration efficiency of single-layer cotton fabric masks in bioaerosol (0.2 μm), it was estimated at 43% to 94%, and in the case of disposable surgical masks, it was estimated at 98% to 99% [26, 29]. Experience suggests that many (but not all) cloth masks reduce droplet and aerosol penetration. Hence, although there is still a lack of conclusive and unequivocal evidence due to difficulties in developing control groups, nose and mouth protection with protective masks may reduce the risk of contamination of the environment by any virus, including SARS-CoV-2. In the course of the current COVID-19 pandemic and the difficulties in controlling it, the potential advantage of benefits over possible risks is emphasized [26, 30].

Fig. (1). A – Examples of face masks (left – FFP2 mask, right – surgical mask (top), cotton mask (bottom)). B – Examples of face masks being worn.

The disadvantages of wearing masks, sometimes for a long time, which should be kept in mind, are possible exposure and the risk of developing respiratory tract infections (especially lower respiratory tract can be affected). The source of infection is the secondary acquisition of the patient's own microbiome of the upper respiratory tract, including pathobionts from the patient's exhaled air, depositing, e.g., in the form of droplets on the surface of the mask fabrics. Hence, it is worth paying attention to its use, considering the time, especially long-term (e.g., health care workers). Persistent contact with contaminated and moist cloth/mask material (also with the lowest degree of filtration), apart from respiratory system infections, may lead to extensive skin lesions of the face and exacerbation of existing lesions.

Synthetic fibers have a poor adsorption capacity due to their molecular structure. Cotton is a natural fiber composed almost entirely of cellulose with a high adsorption capacity [31]. Some materials additionally promote the development of certain microorganisms, including Micrococcus spp., Staphylococcus spp., and Propionibacterium spp. Confirmed by in vitro studies, a high affinity of Staphylococcus spp. for cotton and polyester was found [32, 33]. In studies by Callewaert et al., colonization of various materials (depending on composition) by microorganisms belonging to the human microbiome that had contact with them was shown [22].

The influence of the microbiome on the possibility of developing both upper and lower respiratory tract infections is increasingly emphasized. In both cases, a significant role of the so-called pathobionts, the carrier of which is usually asymptomatic, has been indicated in recent studies. Uncomplicated infections of the upper respiratory tract are common worldwide and are characterized by a large incidence. Lower respiratory tract infections are estimated to be less common but are associated with a high mortality rate. Pathobionts that can potentially lead to the development of a number of respiratory diseases, such as S. pneumoniae, M. catarrhalis, H. influenzae, and S. aureus, are often considered to be residents of the upper respiratory microbiome, especially in children under 2 years of age. It has also been shown that the carrier status of these potentially pathogenic microorganisms is not synonymous with the development of respiratory diseases but still poses a risk of their occurrence. There is also a correlation in which the presence of pathobionts enhances the acquisition and replication of viruses in humans. An example is S. pneumoniae, the preincubation of which with human bronchial epithelial cells leads to increased susceptibility to human metapneumovirus infections [3].

Risk factors that may facilitate the development of respiratory infections include new human-acquired strains of microorganisms or the state of bacterial symbiosis. Hence, the stimuli to induce disease may be exo- or endogenous. In addition, it is believed that the composition of the upper respiratory microbiome may also influence the potential for the development of infections. Studies on the tonsil crypt microbiome in children and adults suffering from recurrent tonsillitis showed that Proteobacteria (mainly Haemophilus spp.) were associated with the disease, while high numbers of Bacteroidetes (especially Prevotella spp.) limited disease progression. In analyses of the oropharyngeal microbiome in elderly pneumonia patients, the absence of Bacteroidetes, including Prevotella spp. and other anaerobic bacteria such as Leptotrichia and Veillonella spp., was associated with pneumonia. Based on conventional microbiological tests, a viral etiology of infections is suggested in the majority of acute rhinitis and laryngitis cases. The most common bacterial etiological factors of these infections include S. pneumoniae and H. influenzae, and in the case of laryngitis, additionally S. aureus, beta-hemolytic streptococci, M. catarrhalis, and Klebsiella pneumoniae are considered. Additionally, studies suggest that early colonization of S. pneumoniae, H. influenzae, and M. catarrhalis causes a long-term increased risk of pneumonia and bronchiolitis in healthy newborns, and elderly and immunocompromised people may also cause pneumonia or acute exacerbations of COPD (chronic obstructive pulmonary disease). H. influenzae is an invasive burden on the elderly, with the frequency of occurrence increasing with age.

Due to the need to define preventive strategies for the development of severe diseases in patients >65 years of age, attention should be paid to the potential sources of the invasive microorganisms, such as upper respiratory tract infection, and also on the colonization of the nasopharynx and hygiene when using face masks [34-36]. In addition, studies on the oral microbiome in the context of the development of periodontal diseases, caries, and the cardiovascular system, draw attention to the impact of microbiota imbalance on the potential development of respiratory diseases due to physiological micro-aspiration episodes. The significant importance of the influence of the oral microbiome and microaspiration has been demonstrated in studies in nursing home patients, where neglecting oral hygiene measures led to a significant increase in the incidence and then mortality due to pneumonia. It has been suggested that imbalances in the microbiome of the upper respiratory tract play a key role in the pathogenesis of acute infections such as AOM (acute otitis media) and pharyngitis and may also play a role in the development of lower respiratory tract infections, leading to pneumonia. Composition, biodiversity, host factors, and viral infections all influence the health of the microbiome of the upper respiratory tract. Under certain conditions (e.g., wearing masks for a long time), the balance of commensal microorganisms may be upset, leading to an increased share of pathobionts, the most common of which are: S. pneumoniae, H. influenzae, S. pyogenes, M. catarrhalis, and S. aureus, resulting in respiratory diseases of various course [3].

The studies by Johnson and Morawska [37] showed that when speaking, exhaling, and especially coughing, droplets of 5–100 μm come out of the mouth, and as shown in the studies by Wei and Li [38], masks suppress and reduce the possibility of transmission of infectious agents during close contact and by air in confined spaces. Moreover, in the analyses of Rengasama et al. [39], it was found that masks made of 100% cotton or with 30% polyester admixture showed 40-60% filtration efficiency for polydispersed/dispersed sodium chloride (NaCl) aerosol particles (75 ± 20 nm diameter of the Count Median Diameter (CMD) and a Geometric Standard Deviation (GSD) not exceeding 1.86) at a frontal speed of 5.5 cm / s. In studies by Ho et al. designed for cotton and surgical masks, it was shown that at a speed of 5.5 cm / s, the filtration efficiency was rated at 86.4% and 99.9%, respectively. These results confirmed earlier analyses by van der Sande et al. [40], as well as similar studies by Davies et al. Healthy adult volunteers and children [40], in a specific procedure (identical for all of them) at the appointed time (10-15 minutes or 3 hours for adults) wore filtering masks (FFP2), surgical masks, and cloth masks (kitchen towels), and the test protocol included the measurement of particles concentration on both sides of the mask. All free-floating particles in the air were analyzed using a portable electrostatic classifier and counter that recorded particles ranging in size from 0.02 mm to 1 mm, covering most of the infectious respiratory aerosols [40]. A reduction in aerosol exposure was observed for each type of mask, although FFP-2 masks were the most effective, and towel masks were found the least effective. The studies by Davies et al. [41] were based on the analysis of masks made of cotton T-shirts, surgical masks, and people without masks. In various analyses of protective masks made of single layers of scarves, sweatshirts, T-shirts, and towels, the filtration efficiency was estimated at 10% to 40% with the use of a NaCl aerosol (0.075 μm) [39]. In experiments using a bacterial marker with aerosol-sized particles, the filtration efficiency of a single-layer towel fabric was found to be 83%, and with 2 layers 97%, compared to 96% for surgical masks [41]. The filtration efficiencies of different materials used to produce face masks (based on several studies) are summarized in Table 1.

It is confirmed that surgical masks are three times more effective in blocking the transmission of microorganisms than cotton masks; however, each form of airway isolation significantly reduces the number of microorganisms released [27]. It should be noted that the fabric does not retain individual virions, but virus transmission is mostly via larger particles in the secretions, in the form of an aerosol (<5 μm) or droplets (> 5 μm) when talking, eating, coughing, and sneezing, and the evaporation of droplets creates aerosols. Experimental analyses of the filtration efficiency for viruses showed 72% efficiency for single-layer towel fabrics, 51% for t-shirt fabrics, and 90% for surgical masks [41].

Improper fit of the mask can result in over a 60% decrease in the filtration efficiency, implying the need for future cloth mask design studies to take into account issues of “fit” and leakage while allowing the exhaled air to vent efficiently [28].

In general, according to the newest studies, the clinical usefulness of masks, also non-medical ones, is strongly advocated because even fabrics in a single layer restrict the transmission of aerosols, and thus of microorganisms [28]. The main purpose of the use of face masks is, therefore, the ability to retain certain particles that have a limited escape route, especially beyond the mask, so that virus carriers do not penetrate the air in the form of aerosols and have no chance of settling on potentially human-touched surfaces [26]. In this way, the possibility of transmission of infectious agents in the population is reduced with the proper use of masks.

Time is needed to characterize COVID-19 and the SARS-Cov2 virus, especially since new mutations of this virus appear in a short time. Despite the vaccines developed, every effort is made to slow the spread of the disease, allowing time for a better and more effective capacity for the health system and society to prepare since no effective treatment has been developed to date [1].

Moreover, it also takes time to vaccinate large parts of the world population and observe the results, and ending the pandemic still seems a long way off. Hence, the use of the currently developed prophylaxis will find its long-term application.

The use of protective masks may reduce the likelihood of infection but will not eliminate the risk of infection. However, it should be remembered that any mask, no matter how effective the filtration is or how well it seals, will have little effect if not used in conjunction with other preventive measures, including isolation of infected people, immunization, proper respiratory culture, regular, frequent replacement of masks, and hand hygiene [41].

Cotton masks can be a potential substitute for surgical masks for people with respiratory tract infection, living in an air-conditioned microenvironment, or in human communities that do not provide social distance. Due to the multiple-use, availability of materials, and the possibility of maintaining the hygiene of this type of mask, it is an alternative to everyday protection for healthy people during a pandemic of respiratory infections [27]. However, certain risks associated with this form of prophylaxis should be taken into account, which, unfortunately, may also aggravate or even constitute a source of serious respiratory infections and lead to the development and aggravation of skin problems. Moreover, educating society not only on hand hygiene but also on the topic of the value of nose and mouth covers, as well as the frequency of their replacement and/or disinfection, is becoming a significant issue.