Coal-fired power station. Chimney stacks (top) and a gas-filtering unit (blue, foreground) at a power station. Electricity is generated by boiling water to form steam to drive turbines. The heat to boil the water is produced by burning coal. After the coal is burnt, the smoke and gases are filtered to remove coal dust and other particulate pollutants before being released into the chimney stacks. The white structures on the chimneys monitor pollutant levels. Coal dust and soot can cause lung diseases such as cancer. This is the Valley Power Plant, Milwaukee, Wisconsin, USA. It began generating in 1968-9. The filter system was added in the 1990s.