The cornea is the transparent tissue at the front of the eye. It has to maintain its optical clarity to allow light and images to properly enter the eye. The cornea partly accomplish this by not having blood vessels criss-crossing it's structure. Without blood vessels and blood carrying oxygen, the cornea gets its oxygen directly from the air.
Oxygen permeability pertains to how well a contact lens material allows oxygen to pass through it. This is measured in Dk/t and the higher the number, the better. Currently, the two main types of contact lens material widely available are either Hydrogel or Silicone Hydrogel.
Hydrogel lenses uses the water content in the lens to transport oxygen and currently have Dk/t's from 10 to 40. A number of research studies have shown that Hydrogel lenses induce varying degrees of corneal hypoxia (a deprivation of oxygen supply). In contrast, newer Silicone Hydrogel lenses transport oxygen via their silicone molecules and have a permeability of more than 100 Dk/t. A number of Silicone Hydrogel designs even incorporate a high water content so that oxygenation occurs through both mechanisms.
The next time you choose your contact lenses, consider the long term effects and make an informed decision. New comers in the online market space make it easy and affordable to purchase lenses. However, what is the real cost when you take the health of your eyes into account? For example, a newly advertised Hydrogel contact lens brand only carries a Dk/t of around 20!!! Before falling for the lure of cheap online contact lenses, have a conversation with your optometrist for healthier and affordable alternatives.