Oxygen saturation measures the percentage of oxyhaemoglobin (haemoglobin that is bound to oxygen) in the blood. To understand how it works— every haemoglobin molecule contains four heme groups that can bind to molecular oxygen in the blood. Thus, a haemoglobin molecule can bind to four oxygen molecules during transport in the blood. A low oxygen saturation percentage can lead to acute adverse effects on vital organs in the body such as the brain, heart and kidneys. The normal SpO2 level for adults is 95 to 100%.