The invention relates to a smoking article (10) comprising a tobacco rod (12) and a filter, the filter comprising a filter unit (14) comprising a first segment (16) of filtration material; a second segment comprising a tubular element (18) of filtration material upstream of the first segment (16), the tubular element (18) having an outer diameter (D2) and an inner diameter (D1). An inner surface of the tubular element (18) is substantially air impermeable. The second segment further comprises a frangible or irreversibly collapsible flow restrictor (24) disposed in the tubular element (18). When the flow restrictor (24) is in a substantially unbroken or non-collapsed state, the filter unit (14) has a first RTD. When the flow restrictor (24) is broken or collapsed, the filter unit (14) has a second RTD, the second RTD being smaller than the first RTD.