Excessive combustion, air pollution and changes in physical properties can change the ambient temperature and affect the natural, ecological balance locally, regionally or globally. Power plants and industrial cooling water are the most common water pollution sources. Cooling water directly discharged into rivers and lakes will raise the water temperature, speed up the metabolism of water organisms and reduce dissolved oxygen, thus affecting fisheries and destroying the natural balance. Circulating cooling water systems can be used to reduce cooling water discharges or cool down through cooling ponds before being discharged into natural waters.