To examine the performance of the high-efficiency particulate air (HEPA) filter, demonstration tests were performed under several severe conditions. The HEPA filter tested is a 610- × 610- × 292-mm conventional deep-pleat filter, normally used in a fuel reprocessing plant. It was tested under a variety of conditions: in air with concentrated dust (100 mg/m3), at high temperatures (maximum 240°C), in humid air (relative humidity 95% and water mist of 100 mg/m3), in a shock transient (overpressure up to 50 kPa), in a large air flow (pressure drop up to 20 kPa), under severe earthquake conditions (acceleration up to 50 m/s2), and in acid and alkaline mists (6 N HNO3, 5% NaOH, 5% Na2CO3). For reference, the performance of HEPA filters in normal conditions was also measured. The HEPA filter performed efficiently enough, even in such severe conditions as would be encountered in a waste air purification system in the nuclear industry. Some empirical formulas are proposed to express the performance of the filter.