(HCl). In addition to Estonia, this pollution also affects Finland and Russia. The industry emits into the atmosphere annually about 200,000 tonnes of fly-ash, including heavy metals, carbonates, alkaline oxides (mainly calcium oxide (CaO)), and harmful organic substances (including PAHs). About 30% of the fly-ash is CaO, a portion of which is neutralised by atmospheric . Alkaline fly ash has raised the pH value of lake and bog water. This has caused the invasion of eutrophic plants in the area of the oil shale industry, leading to the degradation of those waterbodies. Another source of air pollution is the dust that arises during deposition of oil shale ash and semi-coke. According to a 2001 study, the concentration of particulate matter in the fly-ash is 39.7 mg per cubic metre. The most hazardous particles are those with a diameter of less than ; these particles are associated with an increase in cardiovascular mortality and premature deaths in Estonia. The combustion of oil shale releases more into the atmosphere than any other primary fuel. Generating 1 MWh of electricity in modern oil shale-fired boilers creates 0.9–1 tonnes of . In 2017, oil-shale related greenhouse gas emissions in Estonia accounted for 12.9 million tonnes or 69.1% of total energy-related emissions. Of the all energy-related emissions, heat and power generation accounted for 76%. According to the OECD, emissions in Estonia could be reduced by two-thirds if oil shale would be used for production of lighter oil products, instead of burning to generate electricity. Mitigation Various efforts have reduced the industry's environmental impact. Fluidised bed combustion generates fewer , , and fly-ash emissions, including PAHs, than the earlier technologies that burned pulverised oil shale. Reclamation and reforestation of exhausted mining areas have been carried out since the 1970s. In 2010–2013, a €38 million project was implemented for the environmentally safe closing of of semi-coke and ash heaps. In accordance with a European Union waste framework directive, the heaps were covered with waterproof material, new topsoil, and sod. In Kiviõli, a semi-coke heap, the highest artificial hill in the Baltic countries, was converted into a ski centre. The former Aidu open-pit mine was converted into a rowing course. A part of the former Sirgala open pit mine has been used as a military training area. An oil shale sector health impact study conducted in 2014–2015 show that the residents of the region complained significantly more frequently