The reason is that the ash left over when coal is burned contains radioactive elements, notably uranium and thorium.
理由是煤燃烧过后省些的煤灰含有放射性元素，尤其是铀和钍。
In developed countries, coal is the most hazardous (bottom left), according to the Paul Scherrer Institute in Switzerland, which studied more than 1, 800 accidents worldwide over nearly 30 years.
根据瑞士保罗·谢勒研究所对近三十年世界范围内超过1800其事故的研究结果，煤炭行业是最危险的(见左边底部)。
In developed countries, coal is the most hazardous (bottom left), according to the Paul Scherrer Institute in Switzerland, which studied more than 1, 800 accidents worldwide over nearly 30 years.
根据瑞士保罗·谢勒研究所对近三十年世界范围内超过1800其事故的研究结果，煤炭行业是最危险的(见左边底部)。
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