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More than 30 years have passed since the United States released 20 million gallons of herbicides over the forests of South Vietnam to remove the dense foliage that hid enemy troops. Agent Orange, named after the orange stripe on the barrels of the herbicide, was the most common product of several used during the Vietnam War, with 12 million gallons sprayed between 1962 and 1970.

Despite speculation that exposure to Agent Orange has caused health problems among both Vietnam veterans and Vietnamese people, no major studies have addressed the question because there has been no accepted method for estimating veteran or Vietnam resident exposure that could be used in large-scale epidemiology studies.

Mailman researchers have created a geographic information system (GIS) that lets researchers calculate the relative exposure opportunities of the troops to military herbicides, including Agent Orange. In the process of compiling data for the GIS, the researchers discovered that at least twice as much dioxin—the toxic contaminant in the herbicides—was dropped during the war than was previously thought.

"Our research shows patterns of exposure that justify future studies of Agent Orange on the health of military and civilian populations," says the study's husband and wife team of lead investigators Dr. Jeanne Stellman, professor of health policy and management, and Dr. Steven Stellman, professor of epidemiology. The research was published in the March Environmental Health Perspectives and was the cover article of the April 17 issue of Nature.

Though Agent Orange exposure has been linked to several health conditions, including chronic lymphocytic leukemia, non-Hodgkin's lymphoma, and soft tissue sarcoma, these links have been established from studies of Canadian farmers, Swedish lumberjacks, and other groups exposed to the herbicide. The health effects on 2 million to 4 million veterans and Vietnamese civilians remain unknown. Despite the uncertainty, the Agent Orange Act of 1991 directs the Veterans Administration to compensate any Vietnam veteran who is disabled by a disease related to Agent Orange exposure.

The same act also instructed the Institute of Medicine (IOM) to determine if a health study of Agent Orange on Vietnam veterans was feasible. In 1994, the IOM decided more accurate exposure indices could be developed and used for a large epidemiological study. In 1998, the National Academy of Sciences awarded a contract to Dr. Jeanne Stellman to create a methodology based on military records that could estimate a veteran's relative exposure to Agent Orange and other herbicides.

The Stellmans used modern mapping and database technologies to refine their previously published exposure opportunity model. The data were entered into a GIS database that divides South Vietnam into a grid of 176,000 squares, each around one square kilometer in area.

During the course of verifying the spray data, they discovered important inconsistencies in the files and surmised that important Air Force operational data had been overlooked. Over the next few years, the researchers dug through the National Archives for records documenting Operation Ranch Hand flights that were responsible for 97 percent to 98 percent of the spraying. They found that although less herbicide was sprayed between 1961 and 1965, the main herbicides used at the time, Agent Purple and Agent Pink, contained the highest levels of dioxin and no systematic records existed of the amounts and locations sprayed. They applied their mapping and modeling techniques to the original reports and maps and also reassessed the records used to calculate the amount of dioxin in each herbicide used in Vietnam.

They concluded that much of the Agent Orange dioxin contamination probably averaged 13 parts per million instead of the original estimate of 3 parts per million and that the early years of the war were probably responsible for a great deal of the dioxin dispersed. With the new average, and their discovery of an additional 7 million liters of sprayed herbicide, Drs. Stellman and Stellman say twice as much dioxin was dropped on Vietnam as previously estimated.

Using the GIS system and software they developed, researchers can calculate herbicide exposure in troops that were stationed in, or moved through, the squares. The model built by the Mailman team factors in how much herbicide was present, how close the troops were to exposed areas, and how much time the troops spent in exposed regions. Troops who were directly sprayed with the agents receive a higher exposure index than troops who moved into the sprayed areas weeks later. Researchers who use the database, however, are not required to employ this exposure index and can develop their own models.

The GIS databases show that parts of South Vietnam were heavily sprayed and exposure levels in some troops and 2 million to 5 million Vietnamese warrant more health studies. "Special Forces or commandos living in the heavily wooded Iron Triangle near Saigon were heavily exposed, the 101st Airborne was directly sprayed many times, and chemical corps and Ranch Hand Air Force personnel were also pretty heavily exposed," Dr. Jeanne Stellman says.

But other parts of Vietnam were left untouched. "That means in an epidemiological study, you would want to look at troops in the highly exposed regions, compared to troops where there was no spraying," she says.

Funding for those epidemiological studies may be forthcoming now that the IOM's Committee on the Assesment of Wartime Exposure to Herbicides recently commended the project and recommended that government agencies facilitate additional epidemiologic studies of veterans by non-governmental organizations and independent researchers.

"Thirty to 40 years ago, we sent young men to Vietnam to fight a war and we've been ignoring them ever since," Dr. Jeanne Stellman says. "Hopefully, because of our work, that will end."

The work was supported by the National Academy of Sciences and the U.S. Public Health Service.


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