The Fog is radioactive in nature, and therefore dangerous to the residents of the Island, except for certain members of the Children of Atom and the synths of Acadia, who are immune to radiation and situated above it with wind farms respectively. While the Fog has engulfed almost the entire island by 2287, fog condensers created by Acadia are capable of protecting small areas from the effects of the Fog by condensing the Fog into a liquid form. Condensed fog can then be used to craft several useful items.
Originally, the Fog was merely an occasional presence. However, ever since the arrival of the Children of Atom, it began steadily growing until it threatened to cover the entire island. The residents of Far Harbor believed that the Children of Atom were somehow responsible spreading for the Fog, while the Children believe that its growth was Atom's reward for their faith. However, Old Longfellow recounts that in the days of his early childhood, the whole island was covered in the Fog, and that many people on the island are quick to forget its threats in the years it recedes, as they have grown complacent with the relative prosperity that followed.
The native fauna of the Island was mutated by constant exposure to the Fog, creating abominations such as anglers, gulpers and fog crawlers. These creatures are highly aggressive, and sometimes attack Far Harbor. The threat of these hostile mutants, in addition to exposure to the Fog, drove many human inhabitants of the Island insane, turning them into psychotic cannibals known as trappers.
Behind the scenes
The Fog is an obvious reference to author Stephen King's The Mist, a short story that told a tale of a small Maine town engulfed in a fog bank that contained many dangerous creatures and insect-like monsters, or James Herbert’s novel The Fog, where the aforementioned Fog turns people who encounter it into homicidal maniacs, just like the Trappers.