This is the tenth in a series of substantive reports of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR)" to the General ~ssembl~~. The preparation of this Report and its scientific annexes took place from the thirtyfirst to the thirty-seventh sessions of the Committee. The material of this report was developed at annual sessions of the ~ommirtee, based on iorking papers prepared by the Secretariat that were modified and amended from one session to the next according to the Committee's requests. During the period of preparation of this Ke~ort. which contains seven scientific annexes. another Report containing three scientific annexes was completed at the thirty-fifth session of the Committee. These two reports, referred to as the 1986 and 1988 Re~orts. constitute the latest comprehensi\e assessment by the Committee of the sources. effects and risks of ionizing radiation