Researchers have created experiments and a model has been developed to measure the effects of mass loading a High-Efficiency Particulate Air (HEPA) fiber filter during filtration of sub-micron liquid aerosol particles. It was found that penetration of the test medium increased during clogging by a liquid aerosol, and this was irrespective of particle size. The affect was not changed by the physicochemical properties of the test aerosol. Application of a non-stationary filtration model shows that the increase in penetration is in part caused by an increase in interstitial velocity and in part by a decrease in the number of fibers available for capture of particles.