W. H. ADAMS: LATE MEDICAL CONSEQUENCES OF EXPOSURE TO RADIOACTIVE Miscarriages It is not knownif there was an increase in the rate of miscarriage of conceptions following fallout exposure. Although medical program data were interpreted as showing an increase in adverse pregnancy outcomes during thefirst five years, it is now knownthat normally up to 40% ofall conceptions abort, usually in the first few weeks of pregnancy. Techniques to determine this rate were notavailable in the 1950s. Therefore, all that can be said about miscarriages in those early years is that, whereas the birthrate in the Marshall Islands in 1957 was 37.3 per 1000 population,that of the Rongelap population, averaged over a six-year period, was 48 per 1000 per year. Growth and Development The exposed children were followed closely for evidence of developmental delay. Stature, weight, and osseous maturation were assessed over many years. Growth studies on young people exposedto radiation from atomic bombs in Hiroshima and Nagasaki showedthat their adult heights were significantly lower than those of unexposed children. It was determined that there was no significant difference in mean adult stature between those who were exposed on Rongelap and Ailingnae and those who were not. Nevertheless, two young boys clearly had retarded growth (11). It was subsequently determined that this was due to hypothyroidism. When thyroid replacementwasinstituted, their growth accelerated, thus confirming thatthis abnormality was secondary to radiation-induced thyroid damage rather than to some otherless well-defined effect of radiation. Chromosomal and Genetic Effects In 1964 chromosomepreparations were obtained from lymphocytes cultured from the peripheral blood of many of the exposed population (12). When compared to an unexposed group, an increase in chromosomalaberrationsattributable to radiation were detected in the Rongelap population. Thereis little of clinical significance in this finding, for radiation-induced chromosomal abnormalities in man have not been associated with any disease state. On the other hand, they are knownto be useful as a dosimeter. However, in the situation which prevailed on Rongelap, much more accurate estimates of dose were obtained using direct readings near the time of the exposure. Noincrease in birth defects has been found in the years following the exposure. This is not surprising in light of the evidence from the follow-up of Japanese A-bomb survivors; Radiation Effects Research Foundation workers, using eight parameters, were unable to detect a statistically significant increase in genetic abnormalities in the exposed population despite its large size and the great range ofradiation doses received (13); i.e., a genetic effect of radiation on man hasyet to be demonstrated, even though studies of experimental animals and microorganisms have clearly shown such an effect. 274