The Rainbow Herbicides are a group of chemicals used by the United States military in Southeast Asia during the Vietnam War. Success with Project AGILE field tests with herbicides in South Vietnam in 1961 and inspiration by the British use of herbicides and defoliants during the Malayan Emergency in the 1950s led to the formal herbicidal program Trail Dust (1961–71). Herbicidal warfare is the use of substances primarily designed to destroy the plant-based ecosystem of an agricultural food production and/or destroying foliage which provides the enemy cover.
The United States discovered 2,4-D (2,4-dichlorophenoxyacetic acid) during World War II. It was recognized as toxic and combined with large amounts of water or oil to function as a weed-killer. Army experiments with the chemical eventually led to the discovery that 2,4-D combined with 2,4,5-T (2,4,5-trichlorophenoxyacetic acid) yielded a potent dioxin-like herbicide. 
The Agents used in southeast Asia, their active ingredients and years used were as follow:
- Agent Pink: 40-60% 2,4,5-T, used during "testing" stages of spray program, prior to 1964
- Agent Green: (n-butyl ester 2,4,5-T) unclear when used
- Agent Purple: 50% n-butyl ester of 2,4-D, 30% n-butyl ester of 2,4,5-T, 20% isobutyl ester of 2,4,5-T used 1962–65
- Agent Blue (Phytar 560G): cacodylic acid and sodium cacodylate used from 1962–71 in powder and water solution
- Agent White (Tordon 101): 21.2% (acid weight basis) triisopropanolamine salts of 2,4-D and 5.7% picloram used 1966–71.
- Agent Orange or Herbicide Orange, (HO): 50% n-butyl ester 2,4-D and 50% n-butyl ester 2,4,5-T used 1965–70
- Agent Orange II:50% n-butyl ester 2,4-D and 50% isooctyl ester 2,4,5-T used after 1968.
- Agent Orange III: 66.6% n-butyl 2,4-D and 33.3% n-butyl ester 2,4,5-T.
- Enhanced Agent Orange, Orange Plus, or Super Orange (SO), or DOW Herbicide M-3393: standardized Agent Orange mixture of 2,4-D and 2,4,5-T combined with an oil-based mixture of picloram, a proprietary DOW Chemical product called Tordon 101, an ingredient of Agent White.
In Vietnam, the early large-scale defoliation missions (1962-1964) used 8,208 gallons of Agent Green, 122,792 gallons of Pink, and 145,000 of Purple. These were dwarfed by the 11,712,860 gallons of Orange (both versions) used from 1965 to 1970. Agent White started to replace Orange in 1966; 5,239,853 gallons of White were used. The only agent used on a large scale in an anti-crop role was Blue, with 2,166,656 gallons used. The bombardment occurred most heavily in the area of the Ho Chi Minh Trail.
In addition to testing and using the herbicides in Vietnam, Laos and Cambodia, the US military also tested the "Rainbow Herbicides" and many other chemical defoliants and herbicides in the US, Canada, Puerto Rico, Korea, India, and Thailand from the mid-1940s to the late 1960s. Herbicide persistence studies of Agents Orange and White were conducted in the Philippines. The Philippine herbicide test program which was conducted in cooperation with the University of the Philippines, College of Forestry and was also described in a 1969 issue of The Philippine Collegian, the college's newspaper. Super or enhanced Agent Orange was tested by representatives from Fort Detrick and Dow chemical in Texas, Puerto Rico, and Hawaii and later in Malaysia in a cooperative project with the International Rubber Research Institute. Picloram in Agent White and Super-Orange was contaminated by Hexachlorobenzene (HCB) a dioxin-like carcinogen. The Canadian government also tested these herbicdes and used them to clear vegetation for artillery training.
A 2003 study in Nature found that the military underreported its use of rainbow herbicides by 9,440,028 liters.
Vietnam remains heavily contaminated by dioxin-like compounds, which are classified as Persistent Organic Pollutants. These compounds remain in the water table and have built up in the tissues of local fauna. However, the contamination has begun to deteriorate, and the forest canopy has regrown somewhat since the Vietnam War.
Dioxins are endocrine disruptors and may have effects on the children of people who were exposed.
Soldiers exposed to Rainbow Herbicides in Southeast Asia reported long term health effects, which led to several lawsuits against the U.S. government and the manufacturers of the chemical.
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