A method for filtering a gas comprises passing a gas through a compartment of a filter assembly, the filter assembly comprising: an inlet opening; a first outlet opening; a casing comprising polymeric film and bounding the compartment, the compartment communicating with the inlet opening and the first outlet opening; and a first filter at least partially disposed within the compartment. The method further comprising forming a first seal across a first section of the casing at a location between the inlet opening and the first filter to form a first sub-compartment within the casing and severing the casing at a first location.