Evala is a world that isn't supposed to exist. So you can think of the Aisling as it's antithesis, an almost invisible threat eating away at it little by little.

Some people are sensitive to it from a distance. They might smell ozone, or see static arcs in the air. Or perhaps feel something more physical - burst vessels causing, at best, a bloody nose. Or at worst, a minour case of majour brain damage.