There are many instances where fluid and particles traveling through a narrow passage, such as a crack in a window or door, have large but sometimes unseen effects on our daily lives. For instance, in the cold months of the year a pressure gradient can exists between the inside and outside of a building which causes cold, outdoor air to flow inside through any cracks; significantly decreasing heating efficiency. This inflow of atmospheric air can bring with it dangerous contaminant particles to the inside of a building. Pollution can also occur inside a structure from internal sources of contamination, such as smoke generation from a fire. This thesis represents a two-fold examination of these phenomena. The first part of the thesis showcases a method for local measurement of air leakage flow rate, which can be used to quickly assess leakage rates across a surface, such as a window. The method uses a small local enclosure with constant volume placed about a region on the structure under investigation, which is depressurized and injected with a small concentration of carbon dioxide as a tracer gas. The time variation of the pressure and carbon dioxide concentration inside the enclosure are monitored and used to quantify the leakage flow rate as a function of pressure difference. Because of the small size of the enclosure, advanced data processing techniques are necessary to reduce uncertainty in determination of the rate of change of the carbon dioxide concentration that arises from sensor variability. Results of a laboratory demonstration of the proposed leakage detection and characterization device are reported for the problem of leakage through a circular hole in a plate with prescribed pressure differences. Experimental results from the laboratory tests are found to be in excellent agreement with results of a numerical simulation of leakage flow through a hole, as well as predictions from a number of empirical equations for this problem found in the literature. The second part of the thesis is a numerical study of particle capture in the entrance region of a crack, which is a phenomenon previously not well understood or accounted for in empirical correlations. The computational domain for laminar flow through a crack consists of the crack channel and both inlet and exit reservoirs that are much larger than the channel width. The simulations examined different mechanisms for particle capture within the channel entrance region, including collision on the inlet reservoir wall just outside the crack channel, collision within the crack channel due to cross-stream inertia imparted by the entrance flow, collision induced by Brownian diffusion both on the inlet reservoir wall outside of the channel and within the channel, and gravitational collision within the channel. A detailed study of the variation of the entrance penetration factor with parameters such as the Stokes, Peclet, and Froude numbers was performed, and comparison of the numerical predictions with different theoretical expressions were made when the latter were available. Validity of the assumption of penetration factor independence was also examined for cases where both entrance region inertia and gravitational settling are significant.