[en] Autoradiography has been used to determine the distribution and the nature of plutonium particulate material on a high-efficiency particulate air (HEPA) filter used to filter ²³⁹Pu-containing dust. Higher concentrations of alpha-active material on upstream and downstream folds of the filter indicate uneven airflow through the filter. Observations of aggregate recoil particles on the downstream face of the filter suggest that aggregate recoil transfer, a mechanism which may reduce long-term HEPA filter efficiency, may be occurring. Amounts of alpha activity found on downstream filters confirm this supposition