I'm designing a floating city in my world. Ignore the physics and hard science behind this. Assume that the humans living here will have adequate food, water, and shelter. How far up should I build this city for the safety and comfortability of the humans? Obviously the most comfortable altitude is on the ground, but in the air, what would be the best altitude? Also, I don't want Everest level heights where people need oxygen tanks to not die from oxygen deprivation.
Edit: The city should be at least 100 feet (about 30 meters) above the ground.