One of the most important factors in the quality and convenience of contact lenses is the level of their oxygen permeability. It is denoted as Dk/t.
For the normal functioning of the cornea of the eye, it needs oxygen, it ensures its healthy metabolism and uninterrupted operation. She receives it from ordinary air, as well as from tear fluid, which the eye is wetted when blinking. Contact lenses on the eyes are a foreign body, a barrier between the cornea and the air. If their material does not allow enough air to reach the cornea, it cannot "breathe", its normal operation is disrupted, and discomfort and even some eye diseases may occur. Therefore, the lenses must necessarily be made of "breathable" material, that is, with a high degree of oxygen permeability.
This indicator is denoted by the coefficient Dk/t. "Dk" is the volume of oxygen that has penetrated through a unit area of the material per unit of time. The "t" index indicates the thickness of the contact lens material at the center. A high Dk/t means the lenses breathe effectively and are healthier to wear. Therefore, when buying, compare these values of different types and brands and choose those contact lenses that have a higher oxygen permeability index.
If oxygen does not reach the eyes in sufficient quantities, corneal hypoxia occurs. This leads to unpleasant symptoms: redness of the eyes, itching, fatigue, blurring and decreased visual acuity. If this condition lasts for a long time or happens too often, then vessels can grow into the cornea of the eye. Normally, there are no vessels in the cornea, but prolonged oxygen starvation can cause dystrophy, which will lead to their ingrowth and scarring. This process reduces the transparency of the cornea and, as a result, there may be a decrease in visual acuity, up to its complete loss.
Virtually all modern soft contact lenses have a sufficiently high oxygen permeability for safe and healthy use. In hydrogels, oxygen enters the cornea through water, which is contained in large quantities in their material. In silicone hydrogels, silicone is responsible for its supply, and through it the eye receives so much oxygen, as if nothing was being worn on it.
For each brand, it is indicated what indicator of oxygen permeability it has. Depending on this value, the maximum wearing time is determined without compromising eye health. A high value of oxygen permeability is considered to be from 80 Dk/t and above. Such lenses can be worn for 6 - 8 hours a day, and in some cases even in prolonged mode – up to 6 days without removing.
Many modern silicone hydrogel contact lenses have an oxygen permeability index of 110 Dk/t. And those that can be worn, without taking off 30 days – from 130 Dk/t.
When choosing what you wear over your eyes, be sure to evaluate this important criterion. But remember that not only this indicator is responsible for the quality and safety and for the mode in which lenses can be worn. Also pay attention to the moisture content, the resistance of the material to deposits and other important parameters. Always use only the wearing mode indicated by the manufacturer, do not overwear and change to a new pair in time. The first contact lenses must be selected together with an ophthalmologist.