That expands by about 1.82 million gallons the amount of Agent Orange and other defoliants used to thin the jungle. Much of this increase is attributed to spraying that occurred before 1965. Until then, the defoliants used had a much higher concentration of a known cancer-causing chemical called dioxin, so the new study doubles the estimate of how much dioxin was sprayed in Vietnam.
The researchers, who compared army mission logs and village resettlement activities, also estimate that more than 4 million Vietnamese men, women and children probably were exposed to dioxin in the spraying.
A precise accounting of how much plant-killing chemicals were used during the war, and where, could help public health researchers better understand the impact of dioxin, says Jeanne Mager Stellman, a Columbia University chemist and leader of the study.
"Precisely because it's 30 years later it is very, very important to tell the exposed from the unexposed," Stellman says.
Although the spraying is long past, dioxin persists in the environment and is still entering the food supply in Vietnam, says Dr. Arnold Schecter, an environmental health expert at the University of Texas-Houston School of Public Health.
"The people getting contaminated, whether Vietnamese or American veterans, are more likely to be those who are eating contaminated foods, which we're finding right now," Schecter says. "The spray records are very valuable as a starting point, but the bottom line is: Where does [dioxin] get into people and how high were the levels?"
A report on the findings appears in the April 17 issue of Nature. The scientists reached the new estimate using data from specific spraying missions, information that wasn't previously analyzed. The military deployed defoliants from airplanes, helicopters, boats and backpacks.
Earlier estimates reckoned that between 1962 and 1971, U.S.-led forces sprayed 18 million gallons of Agent Orange in Vietnam. The Army also sprayed smaller quantities of other "Agents," including Pink, Blue and Purple, to clear the lush region, which included Laos and Cambodia.
However, Agent Pink was 100 percent dioxin, so even a small amount could have had serious health consequences, Stellman says.
In December 2001, President Bush signed a bill that presumed for the purposes of health benefits that every U.S. veteran who served in Vietnam was exposed to Agent Orange. The law raised the 30-year ceiling on veterans' claims for respiratory cancers alleged linked to exposure to the herbicide, and it added Type 2 diabetes to the list of diseases associated with the chemicals. Roughly 2.7 million U.S. soldiers served in Vietnam and Southeast Asia during the war years.
John Sommer, executive director of the American Legion's Washington, D.C. office, calls the new work "extremely important" for Vietnam veterans. "There has never been a real study of the health effects of exposure to Agent Orange" during the war, largely because scientists haven't had a firm grip on how much of the herbicide was used. Now, he adds, such a study is possible.