[en] Problems involved in assessing health effects of chronic low-level exposures to environmental contaminants are reviewed. This problem is illustrated by a study of health effects of occupational exposure to ionizing radiation at Hanford. Hanford was established in the early forties as an installation for plutonium production. Since this time, over 33,000 workers have been employed, many in jobs involving some exposure to radiation. Yearly records of this exposure, as well as occupational data, are maintained for all such employees. The yearly exposures are measured in rems of ionizing radiation able to reach critical organs of the body. Even for those who routinely work in radiation zones, yearly readings seldom exceed one or two rems. Methods for assessing directly the health effects of continuing relatively small doses of radiation over a number of years are discussed that should be applicable to evaluating exposures to other industrial and environmental contaminants