The following four factors affect the affinity of haemoglobin for O2 → Acidity (pH): As acidity increases (pH decreases), the affinity of haemoglobin for O2 decreases, and O2 dissociates more readily from haemoglobin. → Partial pressure of carbon dioxide: CO2 also can bind to haemoglobin, and the effect is similar to that of H+.As pCO2 rises, haemoglobin releases O2 more readily. Temperature: Within limits, as temperature increases, so does the amount of O2 released from haemoglobin → 2,3-bisphosphoglycerate (BPG): This substance is found in red blood cells and it decreases the affinity of haemoglobin for O2 and thus helps unload O2 from haemoglobin.