Electricity generation produces a large share of Canadian nitrogen oxides and sulfur dioxide emissions, which contribute to smog and acid rain and the formation of fine particulate matter. It is the largest uncontrolled industrial source of mercury emissions in Canada. Fossil fuel-fired electric power plants also emit carbon dioxide, which may contribute to climate change. In addition, the sector has significant impacts on water and habitat and species. In particular, hydro dams and transmission lines have significant effects on water and biodiversity." Coal mining practices in the United States have also included strip mining and removing mountain tops. Mill tailings are left out bare and have been leached into local rivers and resulted in most or all of the rivers in coal producing areas to run red year round with sulfuric acid that kills all life in the rivers. The efficiency of some of these systems can be improved by co-generation and geothermal (combined heat and power) methods. Process steam can be extracted from steam turbines. Waste heat produced by thermal generating stations can be used for space heating of nearby buildings. By combining electric power production and heating, less fuel is consumed, thereby reducing the environmental effects compared with separate heat and power systems. Electric cars burn no petroleum, thereby shifting any environmental impact from the car user to the electric utility. In South Africa an electric car, will be powered by coal generated electricity and harm the environment. In Norway an electric car will be powered by hydroelectricity and be harmless. Electric cars by themselves are neither beneficial or harmful, it depends how your region generates electricity. Homeowners can get 90% efficiency using natural gas to heat their home. Heat pumps are very efficient and burn no natural gas, shifting the environmental impacts from homeowners to electric utilities. Switching from natural gas to electricity in Alberta Canada burns natural gas and coal at about a 40% efficiency to supply the heat pump. In Quebec Canada where electric resistance heating is common, the heat pump will use 70% less hydroelectricity. Heat pumps may be beneficial for the environment or not, it depends how your region generates electricity. Nuclear power plants do not burn fossil fuels and so do not directly emit carbon dioxide; because of the high energy yield of nuclear fuels, the carbon dioxide emitted during mining, enrichment, fabrication and transport of fuel is small when compared with the carbon dioxide emitted by fossil fuels of similar energy yield. A large nuclear power plant may reject waste heat to a natural body of water; this can result in undesirable increase of the water temperature with adverse effect on aquatic life. Emission of radioactivity from a nuclear plant is controlled by regulations.