Abstract: An improved trunking system is provided for ventilation systems in which air or fumes are to be supplied to or exhausted in adverse environments from travelling equipment such as crane cabs and fume hoods in steel works and coke ovens. The trunking is of self-supporting modular construction, each module having a longitudinal opening, a row of metal strips in end to end interengagement extending from at least one side of said opening so as normally to close the latter, the strips being individually located against longitudinal and lateral movement relative to the opening, and so that the pressure differential between the trunking and the ambient atmosphere tends to bias the strips to positions closing the opening. The trunking includes rails to guide a tap which travels along the trunking in sealing relationship thereto and has roller means locally engaging the flap or flaps to open the latter.