Filtration of submicrometer aerosol particles by nonhomogenous porous filters is studied analytically. Analytical analyses are performed for a homogeneous bulk filter material containing a small volumetric fraction of spatial inhomogeneities (inclusions). An expression is derived for the effective filtration length of such a composite filter material, relative to the comparable length of the corresponding uniform matrix and the filtering properties of the inclusion. The model expression is tested by experiments performed for submicrometer aerosols, collected in a granular bed filter. The model predicts a nonmononic α-dependence of the effective filtration length, which qualitatively agrees with experimental data.