Exposure to fine particles of outdoor origin has garnered increased interest of late. A number of recent studies have shown a correlation of negative health effects with increases in outdoor fine particles. Since people spend up to 90% of their time indoors, the relationship between indoor and outdoor fine particles has taken on added significance. This paper describes some results from a study in which the processes of particle removal from infiltrating air by building envelopes are simulated in a chamber. The chamber consists of two compartments, each having a volume of 19 m3. Particles with aerodynamic diameters in the range of 0.015 to 5 um are generated in one compartment and then transported through simulated leakage paths to the other compartment under the action of applied pressure differentials. The simulated leakage paths described in this paper consist of horizontal slits (0.508 mm high, 102 mm deep, and 433 mm wide) between aluminum plates. The penetration factor for each size particle is determined by simultaneously measuring the concentrations in the two compartments as a function of time. The penetration factor is obtained through a mathematical solution of the mass balance equations. The measured values of penetration are compared to predictions of a mathematical model describing deposition by the mechanisms of settling and diffusion. At applied pressures of 2 Pa, only 5% of 0.01 um particles and 60% of 0.025 um particles pass through the 0.508 mm high slits. At a pressure of 5 Pa, 30% of 0.01 um particles and 80% of 0.025 um particles pass through the slits. At 10 Pa, 54% of 0.01 um particles and 90% of 0.025 um particles pass through the slits. At 20 Pa, 72% of 0.01 um particles and 94% of 0.025 um particles pass through the slits.