This work describes results of measurements of gas flow resistance, efficiency and structural properties of composite aerogel filters. To produce aerogel filters HEPA filters were impregnated with silica gel obtained by hydrolysis and condensation of alkoxysilanes and dried by extraction of ethanol with liquid CO2 followed by supercritical conversion of the latter. The resulting composite aerogel filters offer extremely high efficiency in the < 10 nm range at reasonable pressure drop.