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Under normal conditions in humans at rest, haemoglobin in blood leaving the lungs is about 98-99% saturated with oxygen. In a healthy adult at rest, ''deoxygenated'' blood returning to the lungs is still approximately 75% saturated.<ref>[http://home.hia.no/~stephens/ventphys.htm Ventilation and Endurance Performance]</ref><ref>[http://groups.msn.com/TransplantSupportLungHeartLungHeart/oxygen2.msnw Transplant Support- Lung, Heart/Lung, Heart] MSN groups</ref> Increased oxygen consumption during sustained exercise reduces the oxygen saturation of venous blood, which can reach less than 15% in a trained athlete; although breathing rate and blood flow increase to compensate, oxygen saturation in arterial blood can drop to 95% or less under these conditions.<ref>[http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1464731 J Physiol. 2005 July 1]</ref> Oxygen saturation this low is considered dangerous in an individual at rest (for instance, during surgery under anesthesia): "As a general rule, any condition which leads to a sustained mixed venous saturation of less than 50% will be poorly tolerated and a mixed venous saturation of less than 30% should be viewed as a medical emergency."<ref>[http://www.manbit.com/PAC/chapters/P30.cfm The 'St George' Guide To Pulmonary Artery Catheterisation]</ref>

 

 

Substances other than oxygen can bind to the hemoglobin; in some cases this can cause irreversible damage to the body. [[Carbon monoxide]] for example is extremely dangerous when absorbed into the blood. When combined with the hemoglobin, it irreversibly makes [[carboxyhemoglobin]] which reduces the volume of oxygen that can be carried in the blood. This can very quickly cause suffocation, as oxygen is vital to many organisms (including humans). This damage can occur when smoking a [[cigarette]] (or similar item) or in event of a fire. Thus carbon monoxide is considered far more dangerous than the actual fire itself because it reduces the oxygen carrying content of the blood.