Q:

What is the cataclysm that befalls Earth in Wool?

In the Wool series; specifically, Wool 6: First Shift,

 it is said that the world ends with atomic bombs.

The landscape of the world definitely jives with this.
However, whenever someone goes out into the world,

 their protective suit is immediately attacked by something in the air and slowly disintegrates. First Shift makes mention of nano-technology or poison, but I don't recall if that was ever confirmed or not.

Does the series reveal what is the villain in the air?

A:

The nanos wipe out humans in a controlled, timed, DNA-based attack. The bombs served three purposes (and there were only three bombs): They ushered the survivors into the silos; they provided the initial (and fuzzy) legends about the destruction of all things; and they were the trauma that allowed the subsequent regimen of medication to divide the forgotten past from their new future.

