The researchers studied how salt and biological factors affect the weathering of sandstone in ancient buildings in Aachen City, Germany. They found that salt weathering, caused by air pollutants reacting with mortar, has a big impact on the stone. Lichen growth also plays a role in the deterioration of the sandstone. The rate of weathering is high in this humid region, especially in areas with frequent intense weather cycles. The type of rock and environmental conditions also influence how quickly the sandstone weathers.