50 participants (25 female, 25 male) who are physically active for at least 150 minutes per week, at a moderate activity level.

Participants will perform a graded exercise test on either a cycle ergometer or treadmill. The testing protocol will begin with a 5- 10 min warm-up of either cycling or running/walking at the participants preferred pace.

If the graded exercise test is on a bike, exercise will begin at wattage that elicits ~50% of their heart rate max, HRmax = 208 - 0.7 × age and increase by either 10W, 15W, or 20W every two minutes in a stepwise fashion until symptom limitation. The step of the wattage increase will be at the discretion of the investigators.

If the graded exercise test is on a treadmill, the test will begin at a speed that elicits ~50% of predicted HRmax. Following, the speed of the treadmill will increase 0.5 mph (0.8 km/hr) every two minutes in a stepwise fashion until symptom limitation. If the participant maxes out the speed of the treadmill, we will increase the grade 2% every 2-minutes until symptom limitation.

The PaO₂/FiO₂ ratio is a widely used measure of oxygenation efficiency in the lungs. It compares the arterial oxygen partial pressure (PaO₂, measured in mmHg) to the fraction of inspired oxygen (FiO₂, expressed as a decimal). The ratio provides insight into the ability of the lungs to transfer oxygen into the bloodstream. A PaO₂/FiO₂ ratio of 300-400 is considered to be mild hypoxemia. Participants are considered as poorly tolerating hypoxia if their ratio is <400 and good at tolerating hypoxia if their ratio is >400.