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The degree to which a polymer allows the passage of a gas or fluid. Symbol: Dk. Oxygen permeability (Dk) of a material is a function of the diffusivity (D) (that is the speed at which oxygen molecules traverse the material) and the solubility (k) (or the amount of oxygen molecules absorbed, per volume, in the material). Values of oxygen permeability (Dk) typically fall within the range 10-150 ✕ 10−11 (cm2 ml O2)/(s ml mmHg). A semi-logarithmic relationship has been demonstrated between hydrogel water content and oxygen permeability. Unit: Barrer.The International Organization for Standardization (ISO) has specified the permeability of contact lenses using the SI unit hectopascal (hPa) for pressure. Hence Dk = 10−11 (cm2 ml O2) /(s ml hPa). The former unit can be converted to this new SI unit by multiplying it by the constant 0.75. The ISO has established a classification of contact lens materials based on a range of values of oxygen permeability Dk. Group 1: 1-15; group 2: 16-30; group 3: 31-60; group 4: 61-100; group 5: 101-150; group 6: 151-200; group 7: 201-250; higher codes can be added in bands of 50. See Barrer; oedema; equivalent oxygen pressure; critical oxygen requirement; refractometer.