Earlier comparisons of epidemiological and geochemical maps of China disclosed associations between high mortality rates from nasopharyngeal carcinoma (NPC) and low contents of magnesium in the soil. More recent work has shown that high rates of NPC are also associated with high levels of uranium and thorium in the soil. These latest findings are consistent with the earlier ones, since uranium and thorium occur predominantly in acid rocks poor in magnesium. Uranium and thorium are radioactive, and their progeny include radioactive elements such as radium and radon. Whether or not naturally occurring radioactive elements could be risk factors in NPC should, therefore, be further investigated. Direct carcinogenic properties of radon are well known, but in connection with NPC possibilities for reactivation of virus by natural ionizing radiation should also be considered.
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