After a series of powerful eruptions from Tungurahua volcano in Ecuador in August 2006, the village of Bilbao was one of at least a dozen villages on the volcano's western slope that were seriously damaged or destroyed by ashfall. Ashfall is one of many volcanic hazards. It is made up of tiny jagged particles of rock and glass, which can cause severe illness and death in people with heart and lung ailments. During major eruptions, ash can accumulate on rooftops, causing them to collapse, which can kill or injure people inside. According to the USGS, a dry layer of ash four inches thick weighs 120 to 200 pounds per square yard, and wet ash can weigh twice as much. The load of ash that different roofs can withstand before collapsing varies greatly; flat roofs are more likely to collapse than steeply pitched ones.