An inflator (40) for inflating an inflatable vehicle occupant protection device includes a primary stage (50) actuatable to produce a primary flow of inflation fluid, a secondary stage (100) actuatable to produce a secondary flow of inflation fluid, and a filter chamber (150) into which the primary and secondary flows of inflation fluid are directed. The inflator (40) also includes a secondary filter assembly (120) that permits the secondary flow of inflation fluid to flow into the filter chamber (150) and blocks particulates from flowing from the filter chamber into the secondary stage (100).