A good reference is Sohrabi,M. The state-of-the-art on worldwide studies in some environments with elevated naturally occurring radioactive materials. Appl. Radiat. Isot. 49: 169-188; 1998. It includes lots of references. Bernard L. Cohen Physics Dept. University of Pittsburgh Pittsburgh, PA 15260 Tel: (412)624-9245 Fax: (412)624-9163 e-mail: blc+@pitt.edu