A system for detecting a plurality of conditions in an environment, the system including: a particle detector configured to detect particles in air; a sampling pipe network including a plurality of air sample inlets in fluid communication with the environment and the particle detector; an aspirator to draw an air sample from the environment to the particle detector through the sampling pipe network; and at least one gas detection apparatus (10, 702, 922, 942) having a housing and being arranged in fluid communication with the sampling pipe network at a location upstream of the particle detector, characterised in that said housing contains: an inlet port arranged to draw a sub-sample from the air sample flowing in sampling pipe network, or duct portion of the housing, for analysis by the gas detection apparatus (10, 702, 922, 942); an outlet port arranged to exhaust the sub-sample of air; a flow path defined by a sample passage, said flow path extending from the inlet port to the outlet port, at least one gas detector (40, 42, 43) sensitive to a target species, said at least one gas detector (40, 42, 43) being in a test region and arranged in fluid communication with the flow path to detect the presence of the target species in the sub-sample; and at least one membrane (44, 45) providing fluid communication between the sample passage and the test region such that the target species in the sample is capable of passing through the membrane (44, 45) and into the test region; and wherein the sample passage includes a flow control structure (50, 52, 53) to direct at least a portion of the sample gas towards the membrane (44, 45). a particle detector configured to detect particles in air; a sampling pipe network including a plurality of air sample inlets in fluid communication with the environment and the particle detector; an aspirator to draw an air sample from the environment to the particle detector through the sampling pipe network; and at least one gas detection apparatus (10, 702, 922, 942) having a housing and being arranged in fluid communication with the sampling pipe network at a location upstream of the particle detector, characterised in that said housing contains: an inlet port arranged to draw a sub-sample from the air sample flowing in sampling pipe network, or duct portion of the housing, for analysis by the gas detection apparatus (10, 702, 922, 942); an outlet port arranged to exhaust the sub-sample of air; a flow path defined by a sample passage, said flow path extending from the inlet port to the outlet port, at least one gas detector (40, 42, 43) sensitive to a target species, said at least one gas detector (40, 42, 43) being in a test region and arranged in fluid communication with the flow path to detect the presence of the target species in the sub-sample; and at least one membrane (44, 45) providing fluid communication between the sample passage and the test region such that the target species in the sample is capable of passing through the membrane (44, 45) and into the test region; and wherein the sample passage includes a flow control structure (50, 52, 53) to direct at least a portion of the sample gas towards the membrane (44, 45). an inlet port arranged to draw a sub-sample from the air sample flowing in sampling pipe network, or duct portion of the housing, for analysis by the gas detection apparatus (10, 702, 922, 942); an outlet port arranged to exhaust the sub-sample of air; a flow path defined by a sample passage, said flow path extending from the inlet port to the outlet port, at least one gas detector (40, 42, 43) sensitive to a target species, said at least one gas detector (40, 42, 43) being in a test region and arranged in fluid communication with the flow path to detect the presence of the target species in the sub-sample; and at least one membrane (44, 45) providing fluid communication between the sample passage and the test region such that the target species in the sample is capable of passing through the membrane (44, 45) and into the test region; and wherein the sample passage includes a flow control structure (50, 52, 53) to direct at least a portion of the sample gas towards the membrane (44, 45).