Magic Intolerance

Also known as the Decay or the Wasting, the little-understood condition commonly referred to as magic intolerance is the biggest barrier between the Old World and the Shards of Genesis. It affects both living and non-living things, making it all but impossible for the two halves of the shattered world to exist as a single whole.

Magic as a Base Element

Though there's only so much that is really known about magic intolerance, the most widely accepted understanding is that it's the result of the imbalance in magical saturation between Genesis and the Old World. The background level of magic found in the shards is far higher than that outside of them, and this same quality seems to extend to beings and objects originating within Genesis; it's thought that all things have an inherent 'correct' level of magic with which they're meant to exist.
As a result, being removed from an environment with that correct level of magic is traumatic. While levels of resistance to this trauma vary, it inevitably makes its presence known in various ways. Living things will begin to feel physically ill and suffer various negative effects, and inanimate objects will slowly lose their structural integrity and break down altogether. Though some limited measures can be taken to try and stave off these effects, no permanent solution has ever been found.
There are exceptions to this rule, where individuals or objects seem to have a heightened tolerance for existing in an environment other than where they originated, but no reason or common quality that causes this has ever been identified in a way that allows the effect to be replicated.

Physical Effects

ON INANIMATE S
Magic intolerance has a profound effect on inanimate objects and materials, seeming to affect highly sophisticated or manufactured items to a higher degree than basic substances. As an object spends time in an environment opposed to the magic level of its origin, it begins to suffer damage; structural weakening typically occurs within a period of several days, and the item affected becomes gradually more brittle and fragile to the touch. Most objects reach the point of critical failure within a week's time, crumbling apart entirely from the strain of even the most delicate manipulation.
Very infrequently, objects seem to be capable of resisting this decay and travel back and forth across the boundaries of Genesis without suffering any of the usual effects of magic intolerance. Most often this seems to be items that are heavily magical in their own right, suggesting that maybe they possess enough power of their own to insulate them against the environment around them, but there doesn't seem to be any guarantee that even heavily enchanted items will take on this quality. The effect seems to happen totally at random.

ON LIVING CREATURES
Though the effects of magic intolerance on living creatures is far less dramatic than on inanimate objects, perhaps due to the fact that all living things possess a degree of internal magic, they still carry enough weight that there's little avoiding them. Living beings passing in or out of Genesis tend to experience mild nausea, lightheadedness, and headaches as the first signs of magic intolerance. Over time the symptoms intensify, leading to more pronounced nausea, loss of appetite, fatigue, and persistent low-level pain throughout the body. Mental clarify can be greatly impaired and lightheadedness can progress to fainting and hallucinations. Though a person or creature suffering the effects of magic intolerance will rarely progress beyond these symptoms, it's a miserable existence and the effects don't appear to ever diminish or abate.
The time it takes for symptoms of magic intolerance vary wildly from person to person, but a rough estimate sees the first signs appearing within a month of crossing between worlds and escalating within three months. For unknown reasons, perhaps ten percent of people seem to have a high enough tolerance that they can live relatively comfortably even in an environment other than the one they were born in; most people never have the opportunity to find this out, due to the populations of the Genesis and the Old World staying largely separate from one another.

Larger Consequences

The consequences of magic intolerance are profound, keeping Genesis and the Old World apart in ways that the magical world's creator Wynn Khajib could never have anticipated. In many ways, it's reinforced tradition to kept Genesis stuck in time; most shards lack the diversity of resources needed for major technological advancement, and the difficulty of travel through the old world combined with the timeframe of material breakdown means that there's little that can be done on any sort of significant scale. Similarly, it keeps the Genesis-born and Old World populations apart. Few from outside the shards have the tolerance needed to live in Genesis longterm, and few from inside the shards have the tolerance, means, and desire to travel beyond them.