Your cornea breathes from the air
Unlike most of your body, the front of your eye has no blood supply of its own. The cornea pulls oxygen directly from the air. A contact lens sits right on top of it — so a lens that doesn't let enough air through leaves the cornea short of oxygen. Over a long day that shows up as redness, tired heavy eyes, and faster drying.
What Dk/t actually means
Dk/t is just two ideas divided by each other:
t = the thickness of the lens.
Divide one by the other and you get Dk/t — the oxygen that actually gets through at that thickness. Higher is better.
That's the part most people miss: Dk/t depends on thickness too. Take the same material and make the lens thinner, and Dk/t goes up — the air has less distance to travel. So a thin lens in a good material can out-breathe a thick one.
Where common lenses land
Roughly, on the Dk/t scale — higher means more oxygen.
Who should care most about Dk/t
- Long screen hours — you blink less, so the eye relies more on the lens to breathe.
- Long wear days — 10–14 hours of low oxygen adds up to redness and fatigue.
- Eyes that go red or tired by evening — a classic low-oxygen signal.
- Dry environments — flights, air-con, dry climates.
If none of that is you and your eyes feel fine all day, a standard hydrogel can be perfectly comfortable — Dk/t is one factor among several, alongside moisture and fit.