An airborne dust monitor. Photosensitive elements detect a predetermined airborne dust level at a test region adjacent the photosensitive elements. The photosensitive elements are housed within a flue which shields the test region from ambient light while it conducts ambient air through the test region. In a preferred embodiment, the flue is formed of generally vertical sidewall members which define an ambient air passage through the test region, air passage outlets positioned above the test region and a baffle generally at the inlet of the air passage for allowing the entry of ambient air to the passage while blocking light from the test region. A test system may be provided by which a member is selectively positionable within the test region. Preferably, the photosensitive detecting elements include a radiant energy source and an element responsive to that radiant energy, most preferably infrared energy, the photosensitive elements being positioned to minimize dust settlement on the acting portions thereof.