Imagine you're in some lowland way below sea level were the oxygen partial pressure is higher than at sea level but still lower than the 0.5atm limit. Would their be any effect that isn't deadly or harming even if you stay there for weeks but very uncomfortable like the light hypoxia when you're at 3000m and can't do any effort without fainting ?
Well you can imagine anything. But earth's atmosphere has a pretty consistent composition. The largest variation would be much higher humidity at lower altitudes because of lower temperatures at high altitude.