How do Oxygen levels and my lung function work. My lung function is 38%, but my oxygen levels stay on the 90's. With any exertion my body becomes quite stressed and I have a hard time breathing. My oxygen level is approx. 98 resting and drops to approx. 92 with exertion. If my body is getting this amount of oxygen, why am I struggling so much? Am I trying to simplify the issue?