I had the idea to create oxygen this way because the world's air sucks IMO
Haha, I love your reasoning
Anyway I severely doubt that you're creating dangerous levels of Mg this way. Ater looking at the wikipedia article it seems manganese poisoning comes over chronic exposure from activities like welding (but I guess it is Wikipedia, so y'know)
I guess if you really think that's it it may be good to wear a mask, or do it under a fume hood. Better yet, I'm sure you could just go buy a tank of oxygen!