A loss-of-coolant accident or fire suppression with water sprays would release moisture into the air within the containment building of a nuclear reactor. The resulting high air humidity can unfavorably affect the performance of the high-efficiency particulate air (HEPA) filters in the air cleaning systems. One phenomenon that can lead to filter failure or air cleaning system malfunction is the increase in filter pressure drop resulting from supersaturated airflow. To evaluate the performance and reliability of filters exposed to fog, the airstream and filter parameters that influence pressure drop are studied in tests of clean and dust-loaded HEPA filter units. A discontinuous gravimetric method employing full-size sampling filters is used to determine the average liquid water content of the airstream with an uncertainty of ≤10%. The dust loading of filters used in routine service and the liquid moisture content of the air most adversely affect the rate and extent of the pressure drop increase. The susceptibility of clean filters to such increases can be reduced by changes in parameters that enhance the drainage of water from the filter medium. However, the predominance of the adverse influence of dust loading appears to counteract the effectiveness of the improvements studied. It is also shown that relatively simple models can be used to predict the rise in pressure drop of clean filter units with increasing exposure time under fog conditions.