Inhalable particles include a fraction of aerosol consisting of particles of a diameter below 10 μm, which could penetrate into the airways. The respiratory human airways are divided into upper and lower airways (Cotes 2009). The upper airways include the nasal cavity and pharynx. The lower airways include the trachea-bronchial region with the alveoli, where the gas exchange, which is the most important element of breathing, takes place. The main respiratory organ is the lung. The anatomy of the respiratory system is strictly connected to the respiratory physiology and function. The upper airways constitute the main barrier separating lower airways from the possible effect of particles, contained in the air. These particles, of diameter over 10 μm are “trapped” on the surface of the respiratory epithelium and removed from the airways by cough, or by the cilia movement. The main physical determinants of the particle’s ability to penetrate the airways are the particle diameter, density, and shape (Donaldson et al. 2012). Most particles of a diameter over 5 μm are retained in the nasopharyngeal cavity and mechanically removed from the air, before they reach the lower part of the respiratory system. The respiratory epithelium is the “fi rst line” of the pulmonary

defense, since it assures the proper respiratory fi ltration. The respiratory epithelium lines the airways from the nasal cavity up to the terminal air sacks in the lungs. The main types of epithelial cells are ciliated, columnar, undifferentiated, secretory, and basal cells. The structure of the respiratory epithelium differs according to the part of the airways. In the large airways ciliated cells and secretory cells dominate, whereas in the small airways ciliated and Clara cells do. The alveolar epithelium is formed by type I and type II cells (Crystal et al. 2008). Regardless of the branch of the airways, the epithelium consists of seromucous and goblet cells. These cells produce mucin, which is the complex substance present on the surface of the respiratory epithelium. Mucin is the main component of the respiratory mucus, which also contains water, salts, and other soluble substances, free protein, and substances which chemically are carbohydrate-rich glycoproteids (Houtmeyers et al. 1999). The airway surface fl uid (ASF) is a thin layer of fl uid, covering the surface of the airways, which plays an important role in the defense of the respiratory system against the particles and chemical substances coming into human airways (Lillehoj and Chul 2002). ASF consists of periciliary fl uid and an overlying mucus layer (Gray et al. 2004). The ASF consists of proteoglycans whose function is unknown. It is suggested that these substances play a role in the mucosal host defense mechanisms (Forteza et al. 2001). Lipids are also components of the ASF fl uid. The results of numerous studies revealed that the amount of lipids in the purulent sputum is associated with the degree of bacterial infection and the metabolites of lipids present in ASF, like leukotrienes and prostaglandins, are involved in the airway infl ammation (Nadziejko et al. 1993). It has also been suggested that lipids may be involved in the modifi cation of rheological properties of mucus, modifi cation of adhesiveness, and effect on mucocilliary clearance (Widdicombe 1987). The main function of mucin is to moisturize the air coming into the airways and to “trap” particles entering the airways. Then the particles are removed from the airways by the movement of the respiratory cilia. Mucus consistency is sticky and the cillia are in constant movement (Houtmeyers et al. 1999). The cilia move in a periciliary fl uid, which determines the effective propulsion, and particles present in the upper part of the respiratory system are removed by the cilia with patches of mucous gel towards nares. The proper mechanism of mucociliary clearance depends on the size and composition of the particles. The mucociliary clearance (MCC) clears the airways through the mucin, and thus the substances which are trapped on the surface of the airways are removed. If the effectiveness of MCC fails, the cough is another defense mechanism which allows for the removing of particles with the fragments of mucin from the airways. In the lower airways, the alveolar clearance mechanism is responsible for the removing of the particles, deposited in the lower part of the respiratory tract. The MCC effectiveness is modifi ed by age, sex, exercise, posture, and sleep (Houtmeyers et al. 1999). Some environmental agents may also affect the degree of MCC clearance. It is suggested by the results of studies that the effectiveness of MCC clearance decreases with age (Mortensen et al. 1994). The studies on the effect of gender on MCC clearance did not show the univocal effects (Hasani et al. 1994). Exercise is another factor, increasing the degree of MCC clearance in healthy, non-smoking males (Wolff et al. 1977). The postural factors are associated with the gravitational factors, especially in subjects with existing infl ammation in the airways. The proper postural drainage of respiratory secretion ensures the proper purifi cation

of the airways. Sleep is the physiological factor which has a depressant effect on MCC clearance (Hasani et al. 1993). The osmotic agents, like hypertonic saline or mannitol enhance the mucus clearance in subjects with MCC dysfunction (Daviscas et al. 2010). Exposure to environmental factors, like tobacco smoke or gases like sulphur dioxide (SO2), ozone (O3), or nitrogen dioxide (NO2) impair the proper MCC clearance (Houtmeyers et al. 1999). Smoking, both active and passive, affects the proper mucociliary clearance in the airways (Habesglou et al. 2012).