A monitoring system is disclosed, designed to detect the depth of deposition of a substance on a′ surface (12), such as the depth of dust in a ventilation shaft. The monitoring system includes a light source (14) and a sensor (16, 30). The light source is arranged to transmit light across a detection surface and the sensor is on the other side of the detection surface. When a substance, such as dirt or grease, is deposited on the surface it obstructs the light and the amount of light reaching the sensor decreases. A processing means (26) detects the decrease in light and from this the depth of the deposition on the surface can be calculated. Preferably the sensor comprises a CCD array (32), and the substance throws a shadow on the array. The processing means can then determine the depth of the substance from the position on the array of the edge of the shadow. Preferably the monitoring system is placed in a low power ‘sleep’ mode in between intermittent operations for detecting the depth of the substance. In this, way, it can be battery operated and the battery life is preserved.