Magic in Actrein's Atmosphere

While it is unknown exactly what element causes this, the atmosphere of Actrein is full of an element that enables Actreines to use magic when ingested. However, the element does not grant the Actreine control over their magic abilities. That takes time and training, the amount and length of which depends on the individual.
Interestingly enough, this mysterious element is actually toxic to humans in high enough quantities. The human will not die immediately upon exposure, but rather due to prolonged exposure. However, this does not immediately spell death for a human. There are whispers of ways that a human's system can eventually be brought to adapt to Actrein's atmosphere, and of humans also being granted magical abilities.

Regeneration

The element does regenerate over time, naturally, which means there is some kind of substance present on Actrein that emits the magic-granting element into the atmosphere. (Otherwise eventually Actreines would be unable to use magic). However, if a large enough spell is casted in an area, it is possible to create a temporary deadzone, where magic is entirely impossible, or a lowzone, where the only spells that can be used are low-level spells (for instance, lighting a candle or healing a small papercut). This can also happen if multiple magic-users cast a large spell in the same area. A magical deadzone cannot be permanent unless the magic-emitting element itself is removed from the area, and even then eventually some would spread from areas that still have the element. The only true way to create a permanent magical deadzone would be to extract all of the magic-emitting element from the world, which would eventually turn the entire world into a magical deadzone.

Detrimental Effects on Environment

Doing this, however, would not just make magic use impossible. It would cripple and destroy the ecosystem. All of the flora and fauna of Actrein (and this includes the Actreines) have adapted to the atmosphere, and to change it so drastically would likely lead to the death and demise of many species. Some may survive and adapt to this new magic-less world, but most certainly not all of them.
Interesting to note is that the main causes of a magical deadzone are the heavier taxing spells. Less taxing spells (for instance, lighting a campfire, or teleportation) don't cause a deadzone, or even a lowzone.

Efficiency

Also interesting is the fact that a skilled mage can execute a spell such that it actually takes less magical energy to cast. Effectively, a skilled or well-trained mage can become more efficient, which both allows them to cast more spells in a shorter amount of time and is less taxing on the atmosphere. On the other side of that coin, however, rookie mages can often end up taking more magical energy to cast the same spell.
By far, the worst combination would be to have a rookie mage (or even several rookie mages) casting a very taxing and very high-level spell within the same area all at once and botching the spell. This will definitely cause a magical lowzone, perhaps even a deadzone.

Manifestation

All Actreines are able to use magic, though some Actreines are able to use stronger magic than others, or use magic more frequently than others.

Magic Poisoning
The element responsible for creating the magic in Actrein's atmosphere has been known to be highly toxic to humans in large enough doses. While a human will not die upon the first breath of the magic in the atmosphere, too much exposure will kill them.
Often, such deaths have taken several days, and in some rare cases even weeks.
Adaptation
While the rumors are not confirmed, there are whispers of the possibility for humans to adapt to the atmosphere. The way this would be achieved is not well-known, though it's assumed that the adaptation would take a long time, so as to gradually ease the human into being capable of breathing the atmosphere without suffering the negative effects.

Type

Natural