TCDD is 2,3,7,8-Tetrachlorodibenzodioxin, a chemical in a group of compounds named dioxins. It is what makes Agent Orange as notorious as it is, and is arguably the most toxic of all the dioxins, which is saying a lot, as dioxins are notoriously toxic.
TCDD wasn’t purposely part of Agent Orange, it’s a by-product of the process by which creates organochloride herbicides. Agent orange was intended to be a mixture of 2,4,5-Trichlorophenoxyacetic acid (2,4,5, T) and 2,4-Dichlorophenoxyacetic acid, (2,4-D) which were commonly used herbicides. There was already some controversy related to the health effects of these herbicides (particularly 2,4,5-T,) but it was the contamination of Agent Orange somewhere during the manufacturing process with TCDD that really made it so deadly.
Often, I hear people talk about illnesses related to Agent Orange as cause by “herbicide.” TCDD isn’t an herbicide, and wasn’t placed in there to kill plants, it was simply an accident, likely caused by Monsanto overheating the mixture during manufacture. (It’s interesting to note that other manufacturers, and even published, peer-reviewed studies had already warned that TCDD could be a contaminant when high temperature processes were used, but Monsanto used this method anyway, perhaps because it was cheaper or quicker.)
Even if the batch hadn’t become contaminated with TCDD, it would likely be the case that vets and civilians would have become ill, just due to the health problems associated with the herbicides, and the crazy amount they were spraying overhead. But TCDD is nasty, nasty stuff, even in small amounts.
Basically, TCDD activates proteins in our bodies called aryl hydrocarbon receptors, which help us protect ourselves from naturally-occurring toxic chemicals like benzene. These receptors trigger the production of enzymes called Cytochrome P450 to break down these hydrocarbons to protect our cells from the damage they cause. This is actually a GOOD thing, and we actually see with miniscule doses, LESS cancer and LESS diabetes in TCDD-exposed rats. However, as soon as you’re exposed to a decent dose for a period of time, everything goes haywire. The body can’t produce more P450 enzymes, so we are unable to break down toxins, and you see huge increases of cancer and the whole mess of health problems that we associate with Agent Orange.
It’s actually a very similar to the way heroin/morphine work and cause withdrawal. Heroin is an opiate receptor agonist, and TCDD is an aryl hydrocarbon agonist. When you take away the morphine, you get withdrawal symptoms because your brain stops making dopamine, since the heroin has been clogging up all the opiate receptors. With TCDD, instead of dopamine, you stop producing P450, whose job it is to break down all these cancer-causing hydrocarbons. It’s like letting loose a cancer free-for-all.