More than three decades after the Vietnam War ended, scientists have uncovered long-forgotten government documents describing just how much Agent Orange (and its carcinogenic component dioxin) was used during the war. From those records they have, for the first time, re-created the flight paths of U.S. military aircraft that distributed the millions of gallons of herbicide across Vietnam, Laos and Cambodia between 1961 and 1971. Their findings, published today in the journal Nature, reveal that far more herbicides were used during the early years of the war than has been reported--including the more dioxin-rich herbicides Agent Purple and Agent Pink.

Defoliant sprays like Agent Orange were employed in Vietnam to clear dense forests and mangroves for troop movement and to destroy enemy food crops. They were sprayed around U.S. military stations to hedge fast-growing plants, which could provide cover for enemy snipers.

Very little has been done to document the effect of the widespread spraying on veterans or the Vietnamese people because no accepted means for determining exposure existed, says lead study author Jeanne Mager Stellman of Columbia University. Her team thus undertook a historical reconstruction of herbicide dispersal using earlier National Academy of Sciences reports based on a U.S. military record of flight paths and sprayings known as the Herbicide Report System (HERBS).

As the work progressed, Stellman decided that it would make more sense to use the original data contained in the HERBS files, rather than basing the investigation on the summary reports alone. When the investigators accessed the original military records they found that the HERBS file had five different versions of what was supposed to be the same information. The scientists discovered that the data used to compile the previous National Academy of Sciences reports were rife with error: some versions of the HERBS file included spraying missions absent from other records; others contained duplicate entries. After sorting carefully though that information and comparing it with air force flight mission records, the team was able to determine the exact flight paths and targets of the aircraft that dispersed the herbicides. "The air force wasn't out there crisscrossing the country, spraying every day; they had specific targets and missions," Stellman says.

The team found that the most recent inventory of herbicide use, conducted in 1974, excluded 9,440,028 liters of herbicide. The HERBS files, once thought to include all the data relevant to herbicide spraying, in fact failed to mention about 200 missions flown during the war prior to 1965. Those newly discovered mission records reveal that about 1.9 million previously unaccounted for liters of Agent Purple were sprayed between 1962 and 1965. The researchers further determined that Agent Purple was likely to have had a dioxin content of as much as 45 parts per million. Agent Orange, in contrast, is thought to have contained some 13 parts per million, revised upward from the previous estimate of three parts per million. Stellmans team figures that the amount of dioxin sprayed was almost double that of previous estimates.

More than 20,585 towns lay within spraying regions and, of those, 3,181 had some population data available. According to Stellman, as many as 4.8 million people could have been present when herbicide rained down on their towns. The team also found records that reveal spraying aimed at clearing the Ho Chi Minh Trail--the major reinforcement and supply route through Laos--and figured out which agents were dropped and when. In a related paper published last week in another journal, the team described a sophisticated computer program they developed using a Geographic Information System called the Vietnam Herbicide Exposure Assessment System. The new program will allow scientists to determine herbicide exposure for towns, troop locations, army bases or individuals known to have been in a certain place at a certain time during the spraying missions.

Stellman is now working on determining the rate of prostate cancer among Vietnam veterans who were exposed to Agent Orange or other herbicides. "I'm excited that we've finally gotten through the conceptual barrier that military data aren't useful," Stellman says. "Now, after 30 some odd years, we owe it to ourselves and to the veterans to see if the proximity of spraying has an effect [on health]."