of energy per capita in the nation. The state's average retail price of electricity is 5.43 cents per kilowatt hour, the 3rd lowest rate in the nation. In 2004 coal-fired power plants produced approximately 92 percent of the electricity generated in Kentucky. Environmental impact Starting in the 1960s coal seams in both Kentucky coal fields have been increasingly accessed via a method known as Mountaintop Removal Mining, which is a form of surface mining that involves the topographical alteration and/or removal of a summit, summit ridge, or significant portion of a mountain, hill, or ridge in order to obtain a desired geologic material. The process involves blasting with explosives to remove up to 400 vertical feet (120 m) of overburden to expose underlying coal seams. Excess rock and soil laden with toxic mining byproducts are often dumped into nearby valleys, in what are called "hollow fills" or "valley fills." This method allows, through the use of explosives and large machinery, more than two and a half times as much coal to be extracted per worker per hour than in traditional underground mines, thus greatly reducing the need for workers. In Kentucky, for example, the number of workers declined over 60% from 1979 to 2006 (from 47,190 to 17,959 workers). The industry overall lost approximately 10,000 jobs from 1990 to 1997, as MTR and other more mechanized underground mining methods became more widely used. The coal industry asserts that surface mining techniques, such as mountaintop removal, are safer for miners than sending miners underground. Yet, the environmental implications of mountaintop removal have been widely accepted as a threat to Kentucky communities and ecosystems. Valley fills have been found to cause the permanent loss of critical ecosystems through water pollution and the burial of headwater streams. Furthermore, vegetation removal and soil compaction from mining equipment both contribute to stronger and more frequent flooding from storm runoff. As for human health in counties involved in mountaintop mining, there are elevated rates for mortality and lung cancer as well as chronic heart, lung and kidney disease. These threats do not appear to go away after mining has ceased nor after land reclamation has taken place. Coal ash pollution Ash is the waste product of coal that has been used to boil water. Typically it is stored in pills next to the power plant and then recycled through use in cement mixing. A problem is