An example additive manufacturing system comprises a first fan associated with a print chamber to flow gas from the print chamber into an exhaust conduit. A sensor is arranged to detect one or more conditions of the gas flowed into the exhaust conduit from the print chamber by the first fan and to output a signal indicative thereof. A second fan is spaced apart in a downstream direction along the exhaust conduit from the first fan and a control unit is arranged to receive the signal from the sensor and to control the second fan in dependence thereon to maintain one or more conditions of the gas extracted from the print chamber.