A particle detector , for detecting smoke particles in an environment, including structure defining a detection region; one or more sensors for analysing fluid in the detection region to produce sensor output; and a filter arrangement arranged to convey fluid, from the environment to the detection region, the filter arrangement including: structure defining flow paths, for conveying fluid from the environment towards the detection region, including a first flow path , which includes a filter , and a second flow path bypassing the filter; characterised in that the filter arrangement further includes a mechanism for controlling relative flow rates of fluid through the first flow path and the second flow path; the mechanism being configured or controlled to vary the relative flow rates based on a predicted level of pollution in the environment to control the concentration of pollution reaching the detection region, wherein the mechanism is configured or controlled to periodically temporarily increase the relative flow rate through the second flow path to periodically increase sensitivity of the detector whereby a time averaged exposure of the detection region to fluid from the second flow path, and contaminants carried thereby, is reduced for reduced contamination of the detection region. one or more sensors for analysing fluid in the detection region to produce sensor output; and a filter arrangement arranged to convey fluid, from the environment to the detection region, the filter arrangement including: structure defining flow paths, for conveying fluid from the environment towards the detection region, including a first flow path , which includes a filter , and a second flow path bypassing the filter; characterised in that the filter arrangement further includes a mechanism for controlling relative flow rates of fluid through the first flow path and the second flow path; the mechanism being configured or controlled to vary the relative flow rates based on a predicted level of pollution in the environment to control the concentration of pollution reaching the detection region, wherein the mechanism is configured or controlled to periodically temporarily increase the relative flow rate through the second flow path to periodically increase sensitivity of the detector whereby a time averaged exposure of the detection region to fluid from the second flow path, and contaminants carried thereby, is reduced for reduced contamination of the detection region. structure defining flow paths, for conveying fluid from the environment towards the detection region, including a first flow path , which includes a filter , and a second flow path bypassing the filter; characterised in that the filter arrangement further includes a mechanism for controlling relative flow rates of fluid through the first flow path and the second flow path; the mechanism being configured or controlled to vary the relative flow rates based on a predicted level of pollution in the environment to control the concentration of pollution reaching the detection region, wherein the mechanism is configured or controlled to periodically temporarily increase the relative flow rate through the second flow path to periodically increase sensitivity of the detector whereby a time averaged exposure of the detection region to fluid from the second flow path, and contaminants carried thereby, is reduced for reduced contamination of the detection region. <EOS>