exposure from the complete nuclear fuel cycle from mining to waste disposal is 136 person-rem/year; the corresponding value for coal use from mining to waste disposal is "probably unknown". Oil and gas Residues from the oil and gas industry often contain radium and its decay products. The sulfate scale from an oil well can be very radium rich, while the water, oil, and gas from a well often contain radon. The radon decays to form solid radioisotopes which form coatings on the inside of pipework. In an oil processing plant, the area of the plant where propane is processed is often one of the more contaminated areas of the plant as radon has a similar boiling point to propane. Radioactive elements are an industrial problem in some oil wells where workers operating in direct contact with the crude oil and brine can be actually exposed to doses having negative health effects. Due to the relatively high concentration of these elements in the brine, its disposal is also a technological challenge. In the United States, the brine is however exempt from the dangerous waste regulations and can be disposed of regardless of radioactive or toxic substances content since the 1980s. Rare-earth mining Due to natural occurrence of radioactive elements such as thorium and radium in rare-earth ore, mining operations also result in production of waste and mineral deposits that are slightly radioactive. Classification Classification of radioactive waste varies by country. The IAEA, which publishes the Radioactive Waste Safety Standards (RADWASS), also plays a significant role. The proportion of various types of waste generated in the UK: 94% – low-level waste (LLW) ~6% – intermediate-level waste (ILW) <1% – high-level waste (HLW) Mill tailings Uranium tailings are waste by-product materials left over from the rough processing of uranium-bearing ore. They are not significantly radioactive. Mill tailings are sometimes referred to as 11(e)2 wastes, from the section of the Atomic Energy Act of 1946 that defines them. Uranium mill tailings typically also contain chemically hazardous heavy metal such as lead and arsenic. Vast mounds of uranium mill tailings are left at many old mining sites, especially in Colorado, New Mexico, and Utah. Although mill tailings are not very radioactive, they have long half-lives. Mill tailings often contain radium, thorium and trace amounts of uranium. Low-level waste Low-level waste (LLW) is generated from hospitals and industry, as well as the nuclear fuel cycle. Low-level