Coal pollution mitigation, sometimes called clean coal, is a series of systems and technologies that seek to mitigate the health and environmental impact of coal; in particular air pollution from coal-fired power stations, and from coal burnt by heavy industry. The primary focus is on sulfur dioxide () and nitrogen oxides (), the most important gases which caused acid rain; and particulates which cause visible air pollution, illness and premature deaths. can be removed by flue-gas desulfurization and by selective catalytic reduction (SCR). Particulates can be removed with electrostatic precipitators. Although perhaps less efficient, wet scrubbers can remove both gases and particulates. Reducing fly ash reduces emissions of radioactive materials. Mercury emissions can be reduced up to 95%. However, capturing carbon dioxide emissions from coal is generally not economically viable. Regulations Environmental impact of coal Greenhouse gases Combustion of coal—which is mostly carbon—produces carbon dioxide as a product of combustion. According to the United Nations Intergovernmental Panel on Climate Change, the burning of coal, a fossil fuel, is a significant contributor to global warming. (See the UN IPCC Fourth Assessment Report). Burning 1 ton of coal produces 2.86 tons of carbon dioxide. Carbon sequestration technology, to remove significant quantities of carbon dioxide from the air, has yet to be tested on a large scale and may not be safe or successful. Sequestered may eventually leak up through the ground, may lead to unexpected geological instability or may cause contamination of aquifers used for drinking water supplies. Carbon capture and storage is applied at large point emitters of carbon dioxide with the objective of preventing it from entering the atmosphere. As a quarter of world energy consumption in 2019 was from coal, reaching the carbon dioxide reduction targets of the Paris Agreement will require modifications to how coal is used. Measurement of pollution and availability of pollution data In some countries, such as the EU, smokestack measurements from individual power plants must be published. Whereas in some countries, such as Turkey, they are only reported to the government not the public. However, since the late 2010s satellite measurements of some pollutants have been available. Combustion by-products By-products of coal combustion are compounds which are released into the atmosphere as a result of burning coal. Coal includes contaminants such as sulfur compounds and non-combustible minerals. When coal is burned, the minerals become ash (i.e. particulate matter or PM) and the sulfur forms