The contribution of outdoor particulate matter (PM) to residential indoor concentrations is currently not well understood. Most importantly, separating indoor PM into indoor- and outdoor-generated components will greatly enhance our knowledge of the outdoor contribution to total indoor and personal PM exposures. This paper examines continuous light scattering data at 44 residences in Seattle, WA. A newly adapted recursive model was used to model outdoor-originated PM entering indoor environments. After censoring the indoor time-series to remove the influence of indoor sources, nonlinear regression was used to estimate particle penetration (P, 0.94 ± 0.10), air exchange rate (a, 0.54 ± 0.60 h^-1), particle decay rate (k, 0.20 ± 0.16 h^-1), and particle infiltration (F_inf, 0.65 ± 0.21) for each of the 44 residences. All of these parameters showed seasonal differences. The F_inf estimates agree well with those estimated …