Letters to the Editor Exposure estimates in epidemiological studies of Korean veterans of the Vietnam War

In the past year, several papers on health outcomes in Korean veterans of the Vietnam War have been published. All of these papers base their exposure classifications on an ‘E4 score’ which is a proximity-based measure of ‘exposure opportunity’ calculated from a model developed at Columbia University. Calculation of an Exposure Opportunity Index (EOI) or E4 score from this model requires two inputs: