The U.S. military sprayed more Agent Orange and other dioxin-containing herbicides during the Vietnam War than previously thought, scientists say.

Researchers at Columbia University in New York re-examined military records using more complete data. They found the amount of Agent Orange was underestimated by seven million litres and almost twice as much of the carcinogen dioxin was released.

U.S. forces sprayed the herbicides to defoliate large areas of forest. From 1961 to 1971, the researchers determined 1.84 million gallons, or 10 per cent, more herbicides were sprayed than earlier estimated.

The defoliant was sprayed over large areas of forest

Agent Orange got its name because of the coloured stripes on the containers. Use of the herbicide was stopped in 1971 after it was discovered to contain dioxin.

The toxins persist in soil and water in parts of the southern half of Vietnam. Tree cover has regrown in many places but the chemicals have made their way into the tissues of fish and fowl eaten by local residents.

"Cancer, miscarriages and birth defects in the sprayed areas are always higher than in the areas not sprayed," said Tran Manh Hung of the special committee on Agent Orange in Vietnam's Ministry of Health. "It might take another 50 years before those rates become equal."

The study by Jeanne Mager Stellman of Columbia and her colleagues provides detailed computerized maps of the spraying. The researchers relied on an electronic record compiled by the U.S. military on the flight paths, herbicides and volumes sprayed.

The team also stumbled on archived daily logs made by pilots after missions, which allowed them to piece together details of the missions' targets.

"Areas sprayed during the early years and in the various test sites around the world may be of particular interest for follow-up ecological and epidemiological studies," the researchers wrote in Thursday's issue of the journal Nature.

Some scientists agreed the new study could aid assessments of wartime herbicide exposure for Vietnam veterans. Others noted from a health perspective, it is important to distinguish between the amount sprayed and the amount that actually gets into humans.

Last year, U.S. and Vietnam agreed to investigate the effects of Agent Orange and conducted their first joint conference on exposure.