A lithographic apparatus is disclosed. The apparatus includes a projection system configured to project a patterned radiation beam onto a target portion of a substrate. The projection system includes a housing and a plurality of optical elements arranged in the housing. The apparatus also includes an inlet for feeding conditioned gas to the housing and a gas exhaust for exhausting the conditioned gas from the housing for providing a gas conditioned environment in the housing. At least one gate is provided for providing communication of the gas conditioned environment with ambient atmosphere. The gate is arranged to provide a predetermined leakage of the conditioned gas to the ambient atmosphere.