Indoor bioaerosol dynamics

Nazaroff, William W

Consider particle penetration through leakage paths. As air flows into a building through a leak in the envelope, particles suspended in that airstream may contact a surface bounding the leak, adhere, and be lost from the airstream. The penetration factor, p, represents that portion of particles in the outside air that make it through the leaks to enter the indoor environment. Large particles may deposit because of gravitational settling or inertial impaction. Small particles may deposit because of Brownian motion. Figure 11 presents the results of model calculations showing how the Fig. 10 Deposition velocity (v d ) measured to the vertical wall of a 2-m 3 chamber as a function of particle diameter (d p ) (Lai and Nazaroff, 2005) . The deposition velocity is linked to the loss rate coefficient as follows: b w = v d S w /V, where b w is the contribution to the total loss rate coefficient attributable to deposition on the walls (h À1 ), S w is the wall area, and V is the room volume. The model equations are linear regressions to the log-transformed data, utilizing three measured points (v d1 ) and six measured points (v d2 ), respectively penetration factor varies with particle size for idealized crack geometry. Different parameter values are assumed for the indoor-outdoor pressure drop (4 or 10 Pa) and the height of the crack (0.1, 0.4, and 1.0 mm). An important message from this figure is that cracks must be quite fine for any meaningful attenuation of the airborne particles during infiltration. Specifically, penetration is essentially complete across the full diameter range 0.1-10 lm for any crack whose minimum dimension exceeds~1 mm (given a 4 Pa or higher pressure drop and assuming that the flow channel through the crack is no longer than 3 cm). The distribution of leak dimensions in any real building is not known. However, it seems likely that a normal case would feature most of the infiltrating air flowing through cracks larger than 1 mm in minimum dimension. Hence, there is an expectation that p~1 for bioaerosol particles.