an apparatus and method are provided for detecting presence in gas of transient particulate above its normal zero or acceptable level within a duct , said apparatus comprising at least one emitter of illumination selected from infra - red , ultraviolet and visible radiation capable of being projected over essentially the entire cross section of the duct and at least one detector for detecting any sparkle of the illumination from the particulate . the apparatus and method are in particular useful for detecting monitoring particulate in areas where build up or presence of particulate can be disadvantageous , for example hospitals and electricity generating stations .