Down syndrome clusters in Germany after the Chernobyl accident

In two independent studies using different approaches and covering West Berlin and Bavaria, respectively, highly significant temporal clusters of Down syndrome were found. Both sharp increases occurred in areas receiving relatively low Chernobyl fallout and concomitant radiation exposures. Only for the Berlin cluster was fallout present at the time of the affected meioses, whereas the Nuremberg cluster preceded the radioactive contamination by 1 month. Hypotheses on possible causal relationships are compared. Radiation from the Chernobyl accident is an unlikely factor, because the associated cumulative dose was so low in comparison with natural background. Microdosimetric considerations would indicate that fewer than 1 in 200 oocyte nuclei would have experienced an ionizing event from Chernobyl radioactivity. Given the lack of understanding of what causes Down syndrome, other than factors associated with increased maternal age, additional research into environmental and infectious risk factors is warranted.