"Dark Magic" and the Feywilds

Much of the surface and most of the subterranean realms are covered with some amount of mysterious "dark magic," a term coined by superstitious humans. Though its origins and full nature are unknown, its been long known that its deadly to non-fey folk but perfectly fine (even re-invigorating) for fey folk. Its presence is noted by a pinkish-orange haze and an odd, slightly putrid scent. Non-fey folk require some form of protection, either magical or full body cerussite clothing (often chainmail) to not suffer the negative effects.
Although referred to as "dark magic", elves often referred to heavy "dark magic" as "the feywild" and view it as their original home. The strength of the dark magic in an area is proportional to the amount of unusual, fantastical flora and fauna that appear there. Whether or not dark magic is truly "magic" has not been fully explored, but that seems to be the case based on fey folk's and fey animal's natural affinity for magic.
Humans and dwarves often depend on cerussite, a dense heavy metal.  The Western human-majority City-State Confederacy built its ecenomy on selling cerussite ore to Lagos and Tayos.  
On the surface, the dark magic ebbs and flows in some areas, and has been slowly dissipating outwards, particularly northward and westward.  This has opened up vulnerable elven and orcish lands to human and dwarven invasion.  Subterranean realms dissipate magic much slower, and the dark magic is usually much, much heavier the lower down into the planet one goes.
No/Light dark magic: Non-fey folk can live here safely and comfortably. Most of the human lands of The Kingdom of Tayos and The Republic of Lagos have little to no dark magic on its surface.
Mild dark magic: Non-fey folk can live in such places long term, but its unpleasant and unhealthy.  Poorer areas on the edge of the dark magic borderlands tend to have mild dark magic, or subterranean areas immediately below light dark magic surface lands.  These settlements tend to have more tieflings and hobbits, though causation has not been established.
Moderate dark magic: Non-fey folk can access these areas with sufficient protection (cerussite armor or recently developed magic spells) but may suffer long-term.  Dwarves can also live happily in moderate dark magic, though do prefer areas of mild or lighter dark magic.  
Heavy dark magic: Non-fey folk experience significant illness and death if they are directly exposed to heavy dark magic.  Only recently have newly developed protective magics been shown to protect humans for longer periods in combination with cerussite armor.  Fey folk, however, live and thrive happily in areas of heavy dark magic.