Final answer:
Physiological shunting is the best explanation for why the oxygen saturation of a healthy individual rarely reaches 100% on room air.
Step-by-step explanation:
The oxygen saturation of a healthy individual rarely reaches 100% on room air due to physiological shunting. Physiological shunting refers to a situation where blood bypasses the alveoli without participating in gas exchange. This can occur when there is a mismatch between ventilation and perfusion in the lungs, leading to some blood not picking up enough oxygen.