A canopy used in rural situation with minimal atmospheric pollution will naturally need less frequent cleaning than one in a dusty industrial area with high levels of atmospheric pollution. Other factors include general dust and airborne pollutants from roads, airports and factories. The fabric’s surface geometry and local rainfall levels determine the extent of self-cleaning. Short term conditions that need considering include activities on adjacent construction sites and seasonal considerations such as autumn leaf fall. The topside of external horizontal surfaces collect the most dirt. In our experience the underside of canopies rarely need to be cleaned. Transparent canopies allow external soiling to be seen from the inside more readily than translucent or opaque fabrics do. Poor cleaning or rain streaking can emphasise dirt levels.