Oxygen saturation is a relative measure of the amount of oxygen that is dissolved or carried in a given medium. It can be measured with a dissolved oxygen probe in liquid media, usually water.
Medical science
In medicine, oxygen saturation (SaO2) measures the percentage of hemoglobin binding sites in the bloodstream occupied by oxygen. At low partial pressures of oxygen, most hemoglobin is deoxygenated. At around 90% (the value varies according to the clinical context) oxygen saturation increases according to an S curve and approaches 100% at partial oxygen pressures of >10 kPa. A pulse oximeter relies on the light absorption characteristics of saturated hemoglobin to give an indication of oxygen saturation.
An SaO2 (arterial oxygen saturation) value below 90% is termed hypoxemia. This may be due to various medical conditions.
Environmental sciences
In aquatic environments, oxygen saturation is a relative measure of the amount of oxygen (O2) dissolved in the water. Dissolved oxygen (DO) is measured in standard solution units such as millimoles O2 per liter (mmol/L), milligrams O2 per liter (mg/L), milliliter O2 per liter (ml/L), or parts per thousand (ppt). However, as in the medical sense, oxygen saturation is calculated as the percent of DO relative to a theoretical maximum concentration given the temperature, pressure, and salinity of the water. Well-aerated water (in free interchange with the air) will usually be 100% saturated. In general, the colder the water the more O2 it can dissolve, the more saline the water the less O2 it can dissolve, and the lower the atmospheric pressure (e.g., the higher the elevation), the less oxygen it can dissolve. These generalities come from the gas laws of physics. Some examples:
- 0 °C, normal pressure, freshwater: 14.6 mg/L = 100% saturation
- 10 °C, normal pressure, freshwater: 11.3 mg/L = 100% saturation
- 20 °C, normal pressure, freshwater: 9.1 mg/L = 100% saturation
Solubility tables (based upon temperature) and corrections for different salinities and pressures can be found at the USGS web site. Tables such as these of DO in milliliters per liter (ml/L) are based upon equations that have been worked out and tested under carefully controlled laboratory conditions. Tables of DO relative to the variables of temperature and salinity (used by oceanographers) are based on an equation by Weiss (1970):
![\ln DO = A_1 + A_2 {100 \over T} + A_3 \ln {T \over 100} + A_4 {T \over 100} + S [B_1 + B_2 {T \over 100} + B_3 ({T \over 100})^2]](http://upload.wikimedia.org/math/d/3/d/d3ded7a1e7c7aea3a87ad0de64f493f2.png)
- where A1 = − 173.4292, A2 = 249.6339, A3 = 143.3483, A4 = − 21.8492, B1 = − 0.033096, B2 = 0.014259, B3 = − 0.001700, T = temperature in kelvins, and S = salinity in g/kg. DO = dissolved oxygen in ml/L. Multiply DO by 1.4276 to obtain mg/L.
Regimes of low concentrations in the range between 0 and 30% are often called hypoxic. The state of 0% saturation (no DO) is called anoxia. Most fishes can not live in water once saturation falls below 30%. Healthy ocean water is usually 80 to 110% saturated, the supersaturation (saturation greater than 100%) caused by photosynthesizing phytoplankton. Supersaturation can sometimes be harmful for organisms and cause gas bubble disease.
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