A lithographic apparatus is disclosed. The apparatus includes an illumination system for providing a beam of radiation, and a support structure for supporting a patterning device. The patterning device serves to impart the projection beam with a pattern in its cross-section. The apparatus also includes a substrate table for holding a substrate, a projection system for projecting the patterned beam onto a target portion of the substrate, an infrared radiation source for providing infrared radiation into a measurement zone within the lithographic apparatus, and a detector for receiving the infrared radiation from the infrared radiation source after having passed through the measurement zone, and for outputting a signal indicative of the presence of a gas present within the measurement zone.