I am writing a space sci-fi short story series, and I am trying to figure out what the range in pressure a human can survive AND breathe. I am trying to find the lowest, safest air pressure a human can breathe in where the oxygen isn't leaving the blood. The atmosphere is the same as Earth's.
If the atmosphere on your world has the same composition as Earth’s, then your answer probably is “as high as people can live on Earth”. That seems to be up to 5,000 meters: According to Wikipedia there are multiple villages and even the city of La Rinconada, Peru (pop. 50k) around that altitude.
At that altitude, the pressure is about 50% of sea level.