About 55 percent was Agent Orange - Scientists have increased their estimate of how much Agent Orange and other dioxin-tainted defoliants the U.S. military sprayed during the Vietnam War. But researchers say it remains unclear whether the new estimate means some U.S. veterans and Vietnamese face increased risks of cancer and other illnesses.
By re-examining military records, including the logs of pilots who flew spraying missions, public health researchers at Columbia University determined that nearly 2 million additional gallons of the herbicides were sprayed from 1961 to 1971. The new figure adds about 10 percent to the 19 million gallons previously known to be sprayed.
nicknamed for the color of the identification band on its storage containers. Scientists said the other herbicides, such as Agent Pink and Agent Purple, were closely related to Agent Orange but even more potent.
Scientists said at least one-quarter of the extra herbicides used were the more potent blends, and the total amount of dioxin left in Vietnam's forests, fields and mangrove swamps could be four times higher than previously estimated.
Much of the dioxin was TCDD, which is linked to some cancers and diabetes, spina bifida in children and other conditions.
"That was a major surprise," said Columbia epidemoliologist Steven Stellman, one of the study's co-authors.
"The dioxin contamination in Agent Purple was much higher than in Agent Orange," he said. "So even if it didn't contribute to the total herbicide usage, it contributes disproportionately to the total amount of dioxin."
The study appears in Thursday's issue of the journal Nature. The research was authorized by the National Academy of Science's Institute of Medicine, which has issued a series of reports looking at the effects of herbicides used in Vietnam.
About 10,000 Vietnam veterans receive disability benefits related to Agent Orange exposure. One million Vietnamese exposed to wartime spraying and their offspring are reported to suffer from the same ailments.
Researchers disagreed as to how the Columbia study might be used.
NAS recommended that it "serve as a foundation for new public health studies to more accurately determine which U.S. veterans might have been exposed." But some scientists who did not work on the study said it "does little" to determine Agent Orange's health consequences because it does include toxicological measurements from blood and tissue samples.
"What is important from a health perspective is what gets into humans, not what is sprayed," said Arnold Schecter of the University of Texas School of Public Health in Dallas.
Thirty-five years after defoliant spraying, it is getting harder to determine whether a veteran was exposed and how that might be linked to disease.
That's because dioxin levels decline in the bloodstream over time; an exposed veteran now might carry less than 2 percent of their wartime dioxin level. And dioxin analysis is expensive - as much as $1,000 for a single test.
Using geographical information, Stellman said the Columbia study determines which soldiers might have been heavily exposed to defoliants by comparing records of troop movements and the coordinates of 20,000 spraying missions.
Such information also could be useful in Vietnam.
The Columbia researchers calculated that up to 4.8 million Vietnamese were living in 3,181 villages that were directly sprayed. And unlike U.S. veterans, their dioxin exposure persists. That's because the herbicide remains in the soil and water, and migrates into the tissues of fish and fowl that local residents eat.
Blood samples collected by Schecter show some Vietnamese still carry dioxin at levels 135 times higher than people living in unsprayed areas.
"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."