The Allergy is the common name for the reaction the Otherworld and its inhabitants appears to have towards modern technology and materials. This is embedded in folklore as the fair folk’s dislike for iron but observations of the Otherworld boundaries and of its creatures during expeditions suggest that this dislike extends to other refined metals, concretes, asphalts, synthetic cloth and plastics.
While the withdrawal of the Otherworld boundary from this reality appears to mimic the loss of ancient woodlands, there is little to no recovery equivalent to the way this reality’s nature will invade development if left untended. This suggests that the long life of modern civilisation’s material markers will continue to disrupt the Otherworld regardless of whether future developments are curtailed.
Given that the Blessed Isles were essentially united in order to prevent the use of the Otherworld to launch secret attacks on the State and that a significant part of plans for the population in the event of an invasion or a major natural disaster is the evacuation into the Otherworld, this could cause the State significant issues in future centuries.