events. All present U.S. sites employ shallow land burial, the simplest and cheapest method that is allowable under these regulations. Metal containers containing waste are placed in long trenches at least 7.5 m deep and are covered with a clay cap suitably contoured for drainage and erosion control. Future sites are likely to be much more elaborate structures of concrete and steel, because a number of states have already banned the shallow land burial method of disposal.

Radiation is very poorly understood by the general public and, as a result, seems to generate fears out of proportion to objective risk. The fact that some radiation injuries become apparent only years after exposure no doubt contributes to this wariness of the public, but radiation standards have been promulgated by various national and international groups for more than 50 years. In the present case, the U.S. NRC has decreed that releases of radiation from any artificial source, including LLRW disposal sites, should not exceed 0.25 milliSieverts (mSv; 25 mrem) per year to the whole body or any organ other than the thyroid, which is given a limit of 0.75 mSv (75 mrem) per year. Eisenbud (1980) estimated that LLRW from isotope use in biology and medicine contributes less than 0.01 mSv (1 mrem) to each person's annual radiation exposure, and the U.S. Environmental Protection Agency estimates that the annual exposures of people living near a disposal facility would be something under 0.1 mSv (10 mrem) (Council on Scientific Affairs, 1989).

Although scientists working with radiation have traditionally taken a very conservative no-threshold view of safety, that is, all radiation is assumed to be injurious, it might be well to put these exposure limits in perspective by considering some of the other sources of radiation to which the general public is exposed. As Table A-2 illustrates the average person in the United States today receives about 3.6 mSv (360 mrem) of radiation annually, of which roughly 3.0 mSv (300 mrem) comes from natural and largely unavoidable sources (National Council on Radiation Protection and Measurements, 1987). Radon gas from radium in the soil is an example of such sources. Cosmic rays are another. Internal radionuclides carried naturally in the body include potassium-40, lead/polonium-210, carbon-14, and radium-228/224. Subsamples of the population can be exposed to much higher doses. Denver residents, who have less atmosphere above them than those who live at sea level, get close to twice the average dose of 0.27 mSv/year (27 mrem/yr) from cosmic rays. Smokers' lungs are thought to absorb as much as 160 to 200 mSv/yr (16,000–20,000 mrems/yr) from polonium in tobacco smoke. Artificial sources of radiation other than from waste may vary greatly as well. Diagnostic radiology is estimated to contribute about 0.5 mSv (50 mrem) to each person's annual exposure, but this obviously could be much greater in those with poor health or many injuries. The LLRW contribution, even for people living near disposal sites and the current exposure standards, are thus