A pressurized metered dose dispenser (100) for dispensing an aerosol formulation comprising particles of a medicament suspended in liquefied propellant, optionally in combination with one or more excipients, the dispenser comprising an aerosol container (1) equipped with a metered dose valve (10), where a formulation chamber is defined in part by the internal walls of the container, and wherein the dispenser further comprises a porous, fluid permeable, particulate semi-permeable body (40) located within the formulation chamber adjacent to the metered dose valve.