6.11.07

Rainbow herbicides: How the US waged war on the forests of Southeast Asia

During the Vietnam War, amidst carpet bombing and My Lai, the United States engaged in a unique form of chemical warfare known as herbicidal warfare. Between 1962 and 1971, Air Force aircraft were employed as souped-up crop dusters to spray defoliants throughout Southeast Asia as part of Operation Ranch Hand. Defoliants are a particular variety of herbicides that kill plants by causing them to lose their leaves. This mode of action made them particularly good at destroying both food crops (primarily rice, which is difficult to incinerate) and jungle canopy, which the Viet Cong took advantage of to hide their activities.

A number of different defoliants, either single chemicals or mixtures, were developed and utilized by the US military in 'Nam. Six of these were assigned a particular colour code, and this colour was painted on the barrels in which the particular agent was stored.

Agent Orange is by far the best known of these "rainbow herbicides", largely due to the fact that it accounted for over half of the roughly 19 million US gallons of defoliant that was sprayed in Vietnam. It consists of a 1:1 mixture of 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). Both of these chemicals are chlorophenoxy synthetic auxin herbicides, which means that they resemble in structure a class of plant growth hormones called auxins (particularly indoleacetic acid (IAA)), enabling them to stimulate rapid and uncontrolled plant cell growth. This causes plants to lose their leaves as they literally grow themselves to death. Chronic exposure to high levels of chlorophenoxy herbicides can potentially lead to kidney damage, reproductive effects, and teratogenicity.

Agent Purple, like Agent Orange, is a mixture of 2,4-D and 2,4,5-T. It was only used between 1962 and 1964.

Agent Pink and Agent Green both contain only 2,4,5-T. Boring.

Agents Purple, Pink, Green, and Orange were all contaminated with trace amounts of TCDD, a dioxin. Dioxins, more technically referred to as polychlorinated dibenzodioxins, are a group of incredibly toxic by-products associated with the production of chlorine-containing herbicides including 2,4,5-T. TCDD is suspected of being responsible for the deaths of millions of Vietnamese. They are capable of causing a number of different cancers, neuropathies, diabetes, and birth defects (e.g. spina bifida) in children born of those exposed. It's a huge problem in Vietnam that continues to this day.

Agent White is a little different. It consists of a 4:1 mixture of 2,4-D and picloram, another synthetic auxin herbicide. Since there is no 2,4,5-T, there is no TCDD. But not to fret, faithful reader, for the picloram that was used was reportedly contaminated with hexachlorobenzene and nitrosamines, both of which can cause cancer. On its own, picloram is a mucous membrane irritant.

Agent Blue contains cacodylic acid, an arsenic-containing herbicide that is very different from the chlorine-containing synthetic auxin pesticides found in the other five agents. Arsenic compounds, like inorganic arsenic, are able to disrupt ATP production in both plants and animals. For plants, this can lead to, among other things, the interruption of water transport, resulting in desiccation and the loss of leaves. For people, multi-system organ failure and cancer are possible toxic outcomes.

References
- http://en.wikipedia.org/wiki/Operation_Ranch_Hand
- http://toxnet.nlm.nih.gov/index.html

3 chemically inspired comments:

Saharath said...

i'm very intrigue to use this on my research report but i'm not exactly sure if i can actually cite this XD sort of afraid my professor would disregard this as a source haha

deserae said...

Ditto on Saharath!!

The Curious Chemistry Grad said...

love this one :)