This study evaluates the role of small mesopore volume fraction in aerogel monoliths in achieving high efficiency airborne nanoparticle filtration. The diffusional flow in mesopores (diameter 2–50nm) is more effective in nanoparticle capture than the viscous flow regime encountered in macropores (diameter>50nm). Substantial macropore fraction, however, is essential for achieving high air permeability. This idea is tested by evaluating the filtration efficiency of 25–150nm diameter airborne sodium chloride nanoparticles and the permeability of air through hybrid aerogel monoliths with about 2–4% by volume mesopore content. The hybrid aerogels are prepared by growing discontinuous silica aerogel particles in the macropores of δ-form syndiotactic polystyrene (sPS). The results show that air permeability is not much affected by the silica content although the particle capture efficiency is increased to >99.95% due to the contribution of diffusional deposition aided by the mesopores. The aerogels develop cracks and reduce filtration efficiency at high silica content due to brittle nature of silica particle networks.