Abstract: Particle detecting apparatus for detecting other particulate matter comprises a measuring section and an input/output section removably attached together. A measuring chamber receives gaseous fluid by diffusion from a receiving chamber connected via an inlet to an area being monitored. The measuring chamber comprises a cylindrical body housing pairs of light emitting diodes positioned diametrically opposite, and directing light towards, each other through a sampling volume viewed by a photo-diode through a viewing aperture. The photo-diode detects light scattered by particles in the sampling volume. A glass tube protects the LEDs from contamination and can easily be cleaned or removed. Each LED can temporarily be de-energised in turn so as to act as a light detector, its output in response to light received from the opposite LED being thus a measure of any contamination. The measuring and input/output sections can easily be separated for servicing or replacement.