Indoor bioaerosol dynamics

Nazaroff, William W

For bioaerosol particles, arguably the most important exposure pathway is inhalation followed by deposition in the respiratory tract. The probability of deposition varies with particle size, with lung morphology, and with breathing characteristics. Figure 7 , which is based on semi-empirical modeling originally developed for radiological protection (Yeh et al., 1996) , illustrates some of these features. In these model calculations, the respiratory tract is divided into three zones: the head region (NOPL), the tracheobronchial or conducting airways (TB), and the pulmonary or gas-exchange region (P). The information presented in this figure reflects two dominant characteristics of the system. First, the three regions of the respiratory tract are exposed to bioaerosol particles sequentially. For the largest particles considered, the high deposition effi- Fractional particle deposition in different regions of the respiratory tract in relation to particle size. The results are from the NCRP/ITRI semi-empirical model (Yeh et al., 1996) . Particle density is assumed to be 1 g/cm 3 . The results assume nose breathing, tidal volume of 0.77 l, breathing frequency of 13/ min, and a functional residual capacity of 3 l. In each frame, the deposition fraction is referenced to particle concentrations in the inhaled air. Total deposition fraction in the respiratory tract for a given particle size would be obtained by summing results for the three regions ciency in the head protects the distal airways from exposure. Second, two different mechanism classes control particle deposition. The behavior of the larger particles is dominated by their inertia. Larger particles have a higher mass-to-drag ratio, and so the larger the particle, the more efficient the deposition. However, for submicron particles, inertial processes are relatively unimportant. For the smallest particles in this figure, Brownian diffusion is the dominant transport mechanism. This is a slow process, important only in the smallest airways: Deposition efficiency of 0.1-1 lm particles is small in the head region, yet substantial in the pulmonary region. Similarly, the deposition efficiency increases with decreasing particle size when Brownian diffusion dominates.