# Bacteria Filtration Efficiency for Different Types of Protective Face Masks
## 1 Introduction  
The world is currently witnessing an emerging infectious disease called coronavirus disease 2019 (COVID-19), which appeared in the form of a pandemic. It is caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), and the main transmission routes of this virus are aerosols and droplets [1]. Due to its sudden appearance, high rate of spread, and difficulties in control, it poses significant challenges both in health and other aspects. The World Health Organization (WHO) has published various recommendations and measures to prevent this spread, including the recommendation to wear protective facial masks [2]. Most outbreaks involving three or more people have been linked to time spent indoors. To reduce indoor airborne transmission, it is necessary to perform control measures through source (masking and physical distancing) and engineering controls (ventilation and filtration) [3]. Following the basic ventilation standards for indoor spaces such as schools and kindergartens, we noticed that they are designed to dilute bio-effluents and achieve basic levels of acceptable indoor air quality rather than to control infection [4].  
The main use of masks has been shown to help reduce the spread of contaminated droplets, which can cause various diseases. The prevention benefit of masking is derived from a combination of source control and wearer protection [5]. Bacterial filtration efficiency (BFE) is a parameter that classifies medical masks into two types: type I with a BFE \(\geq\) 95% and type II masks with a BFE \(\geq\) 98%, which conforms to the standard EN 14683:2019 [6]. Particle filtration efficiency (PFE), breathability and leakage, and carbon dioxide (CO2) content of inhaled air are some of the performance criteria of the EN149 standard under which filtering face piece (FFP) masks are regulated [7]. According to the PFE, FFPs can be classified into three types: FFP1, which has a PFE \(\geq\) 80%; FFP2, which has a PFE \(\geq\) 94%; and FFP3, which has a PFE \(\geq\) 99%. Nevertheless, we often see surgical and simple man-made cloth masks in everyday use that do not meet this standard. Therefore, the filtering ability of masks depends on their specifications and external factors [8]. The main goal of studies conducted in this field is to determine mask filtration efficiency in eliminating bacterial or yeast and mold contamination of indoor air [9; 10]. Many studies have reported the use of cloth, surgical, and FFP2 masks during the influenza pandemic [11; 12] or the SARS-CoV-2 pandemic [13; 14].  
However, all available data have different methodologies and, consequently, result in different filtering mechanisms [15; 16; 17]. Despite its significant limitations, bioaerosol monitoring may be a useful tool for studying occupational exposure and the transmission of infectious diseases [18]. Therefore, this study aimed to evaluate bacterial, yeast, and mold filtration efficiency using three different types of masks commonly used in the public (single-layer fabric [cotton], classic surgical, and FFP2 nonreusable [NR] masks). To collect sufficiently contaminated indoor air from schools, kindergarten facilities were selected. In addition, this study aimed to estimate the internal air quality, microbiological exposure to bioaerosols in educational facilities, and the effectiveness of preventive measures.