Defoliant exposure is a whole group of effects caused by breathing in the toxic air around the Great Black Swamp area.
According to the dialog with one of the DMC Guards (available if the character is in the good relations with the DMC and offers to talk when encountering one of the guards) the whole Great Black Swamp area was purposefully created by deploying an enormous amount of chemicals to stop the growth of out-of-control, genetically-engineered crops.
Breathing in the fumes of this chemical defoliant can cause irreversible damage to a person's body. Entering the swamp area without a gas mask carries a risk, as the player's character has a small chance of suffering this effect with every passing turn.
The effects of being exposed are periodical, and can come and go. They include:
- Abdominal Pains
- Eye Irritation
The syndrome can lead to the development of Pneumonia.
Once the initial damage is done, there is no way to reverse the changes. The character will, periodically, suffer from one or more of the effects.
From that moment on, all that the player can do is to mitigate the effects by keeping his character extra-safe and secure.
The effects of the defoliant exposure (including it being incurable) are meant, to some extent, to be similar to those of exposure to the real-world (and controversial) defoliant Agent Orange, used during the Vietnam War to destroy foliage and deny enemies concealment.