This work investigates public perception of the blackening of light coloured stone historic buildings in some European countries. It used on-site questionnaires to explore the perceived lightness of the façades in terms of opinions that the building is dirty or needs cleaning. There was a clear relation between opinions about the dirtiness of a building and views that it should be cleaned. Visitors to buildings found the assignment of colour or a grey scale value to buildings relatively easy despite the fact that they are typically made up of many shades. There was a strong relationship between the perceived lightness of a building and the opinion that it was dirty. This relationship was used to establish potential levels of blackening that may be publicly acceptable and propose some aesthetic thresholds. Such thresholds suggest approaches to setting limit values for elemental carbon in the air, such that significant buildings do not become unacceptably discoloured. Developments of this kind contribute to the regulation of non health aspects of air pollution and aid decision making in the management of significant buildings.