US troops sprayed far more dioxin over wartime Vietnam than they admitted at the time, according to new research.
Between 1961 and 1971, herbicides such as Agent Orange and other weapons of mass defoliation were used to strip mangrove swamps and forests of cover for Vietcong forces and deprive the enemy of food by destroying crops.

Jeanne Mager Stellman, of Columbia University in New York, and colleagues report in the journal Nature today that a fresh look at US military data has revealed that an extra 7m litres of dioxin-containing herbicide was sprayed over key regions of the war-torn country.

The study supersedes a US National Academy of Sciences investigation nearly 30 years ago and opens the way for a new look at the long-term effects of this form of chemical warfare.

"Large numbers of Vietnamese civilians appear to have been directly exposed to herbicidal agents, some of which were sprayed at levels at least an order of magnitude greater than for similar US domestic purposes," the scientists report.

"Other analyses carried out by us show large numbers of American troops also to have been directly exposed."

Environmental campaigners blame Agent Orange and similar substances for chronic illnesses among American veterans, and for hideous childhood deformities in present-day Vietnam.

The herbicide was banned in the US in 1970.

"The government position had been that it was impossible to do a study because military records weren't any good. We began to work on this in 1998," said Professor Stellman.

She unearthed material that had been classified as top secret during the original investigation in 1974. She also pinpointed the areas targeted for defoliant treatment.

"It [the spraying] wasn't as much of a slosh as we thought it was. In fact it was targeted," she said. "Most of Vietman was not sprayed, but the areas that were, were heavily sprayed."