I mentioned this earlier somewhere but humans don't tolerate high levels of magic in the air well and it can be quite toxic to them. The symptoms are also way too numerous for an individual to estimate toxicity. Tolerance also varies, but one main symptom of intoxication is increased drowziness, acompanied with suffocation of the lungs generally. One may develop a cold, hallucinate, physical body changes. Sometimes they luck out and nothing happens at all, it's a gamble really.
Some do have a higher tolerance than others, and there are devices and tools to help you 'breathe' in these kinds of environments.

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