Respiratory masks are usually made of non-woven materials consisting of polypropylene (PP) or polyester fibers (PES). Their main function is to protect the wearer from inhaling aerosol (dust particles, gasses, micro-organisms, etc.) in the environment and, conversely, to protect the environment from micro-organisms present in the exhaled air of the wearer of the mask.
Filtering the aerosol through the respirator is a complex process. To achieve the highest filtration efficiency, the pores must be small enough and wind like a channel that changes direction frequently as the air flows through the mask. Only nonwovens consisting of microfibers with a diameter of 1.5-2.5 µm can meet these requirements (the diameter of conventional synthetic fibres is about 20 µm, for example). In addition, respiratory masks are made of three layers to ensure a sufficiently small pore size (e.g. the size of the bacterial cell is 0.5-5.0 µm, while the size of the virus varies 20-300 nm), which would effectively block the passage of aerosol through the porous structure of the non-woven fabrics. The filter layer is located in the middle between the upper and lower layers.
Based on the degree of filtration and thus the protective effect, respiratory masks can be divided into medical masks and respirators. Medical masks are loose-fitting and do not provide complete protection against aerosol. In contrast, respirators adapt completely to the face and thus ensure good protection for the wearer. Medical masks are also considered to be rapidly humidified due to the moisture in exhaled air and should therefore be changed every 2 to 4 hours, while respirators should be changed every 8 hours due to the more effective filtering layer.
According to the standard EN ISO 14683 Medical face masks – Requirements and test methods, medical masks are classified in the following categories: Type 1, Type 2 and Type 2R. Their characteristics are shown in Table 1. Type 1 medical masks provide the lowest level of protection. They block more than 95% of micro-organisms and are not water-repellent, i.e. they do not protect the wearer from the liquid drops in the environment (drops when coughing, sneezing). Type 1 medical masks should only be used for patients and others to reduce the risk of spreading infections, especially in epidemic or pandemic situations, and are not intended for use by healthcare professionals. Type 2 and 2R medical masks guarantee a filtration efficiency of over 98% bacteria. Type 2R medical masks are also water-repellent, providing protection against liquids. Both types are intended for use by medical personnel in clinics, operating theatres, etc. Irrespective of the type of medical mask, it must also comply with the microbiological standard, as the surface of the mask may not contain more than 30 colony units (CFU) per 1 g of textile material after manufacture.
Table 1: Properties of medical masks as ascribed by EN ISO 14683 standard.
Respirators are designed to protect the wearer from inhaling dust, airborne microorganisms and hazardous atmospheres (fumes, vapours, gases ). There are two main categories: the air-purifying respirators and air-supplied respirators. The latter are intended for specific use, so the first type of respirator is discussed below. Air-purifying respirators are divided into three categories according to standard EN 143 Respiratory protective devices – Particle filters – Requirements, testing, marking: FPP1, FPP2 and FPP3. Their filtration efficiency increases as the type number increases (Table 2). Accordingly, FPP1 respirators offer the worst protection, as they block at least 80% of the airborne particles at an air flow rate of 95 l/min. In contrast, FPP3 respirators offer the best protection at the same air flow rate, as they retain at least 99% of the particles. An important parameter is also the total internal leakage, i.e. the amount of air that does not pass through the filter and is therefore not filtered.
In epidemic or pandemic situations, FFP3 respirators are primarily required by healthcare professionals who are in repeated and prolonged contact with sick people or patients.
Table 2: Properties of different types respirators according to the EN 143 standard.
NTF, OTGO, Chair of Textile and Clothing Engineering