People outside sheltered locations in the Sierra Madre are exposed to low concentrations of the toxin. The lethal dose depends on the concentration. It smells of copper, Old World gold, with a tinge of sulfur. It is blood red in color, with the consistency of a dust cloud.
The Cloud is a self-replicating airborne toxin that's poisonous to living beings that causes a noticeable burning sensation in the lungs and can even enter the body through the skin.