Final answer:
A leftward shift on an oxygen dissociation curve indicates increased affinity of hemoglobin for oxygen, which could be due to a lower body temperature, increased pH, or decreased CO2 levels.
Step-by-step explanation:
A shift to the left on a % Hb saturation vs PO2 graph indicates an increased affinity for O2. This can occur due to several environmental conditions. However, it is more likely caused by a physiological state where less oxygen is required for hemoglobin to become saturated, meaning hemoglobin has a higher affinity for oxygen and can bind it more easily and at a lower partial pressure. This might be due to a lower body temperature, increased pH, or decreased levels of carbon dioxide. Conversely, a rightward shift would suggest a decreased affinity for oxygen.