The high-efficiency particulate air (HEPA) filters within the air cleaning systems of nuclear facilities form part of the barrier between contaminated zones and the ambient environment. Consequently, they are of the utmost importance in protecting the public from the risks of radiation, not only during normal operation but particularly during accident situations. Although HEPA filter media have excellent particle removal efficiencies, they are rather brittle, fragile, and weak materials. As a result, structural damage followed by significant losses of filtration efficiency can easily occur in handling, transport, and even normal operation of these filter units. The behavior of commercial HEPA filter units is investigated in dry air at flow velocities up to 35 m/s and at rated flow under extended exposure to high-humidity air. For typical deep-pleat units with wooden frames, the structural limits lie between 4 and 23 kPa in dry air and between 1 and 9 kPa during exposure to fog. The failure modes and underlying failure mechanisms are thoroughly studied. The structural strength of deep-pleat units is considerably improved by reinforcing the filter medium and increasing pack stability. As verified by removal efficiency tests, differential pressures up to 56 kPa in dry air and 15 kPa after extended operation under fog conditions can be sustained without mechanical damage to the filter medium. Increased safety margins, particularly under unfavorable operating conditions, are thus attained. Three new nuclear power plants in the Federal Republic of Germany have already been equipped with the improved HEPA filters. The German licensing authorities are now considering modifications of filter performance specifications to reflect the increased strength of these new filter units.