Herbicides such as Agent Orange were sprayed in Vietnam from 1961 to 1971, defoliating the jungle and destroying enemy crops. The main ingredient, TCDD, is now known as one of the most carcinogenic dioxins in humans and other animals, according to the Environmental Protection Agency. In 1970, the United States banned dioxin. After a landmark settlement in 1984 of a class-action suit brought by more than 100,000 Vietnam veterans, Congress authorized a review of the herbicide's potential health impacts in the Agent Orange Act of 1991.

While studying the application of dioxin-based defoliants in Vietnam for the National Academy of Sciences, Jeanne Stellman of Columbia University in New York City stumbled onto previously unnoticed records in the U.S. National Archives, mostly from the U.S. Air Force and other military files. In the 17 April issue of Nature, Stellman and colleagues describe reports about the operation known as Ranch Hand. It contained data about applications that had gone unrecognized in past reviews; furthermore, the data were logged in a completely different format from other records. Stellman and her colleagues reconstructed flight paths and targeted drops of Agent Orange and similar mixtures. They were able to show that more herbicide had been dropped than cataloged previously and that dioxin concentrations in early herbicide batches were much higher than realized, effectively doubling the amount sprayed.

The paper is "going to stimulate debate on exposures that occurred in Vietnam," particularly with regard to epidemiological risk assessment for soldiers on both sides as well as civilians, and also ecological damage, says Christopher Portier of the National Institute of Environmental Health Sciences in Research Triangle Park, North Carolina. Past studies, he said, lacked the information that Stellman and her colleagues unearthed. Their analysis, he said, "raises questions that need to be followed up."

--NAOMI LUBICK