Dk stand for oxygen permeability of the contact lenses material while t represents the thickness of the lenses. Dk/t has its unit in (cm/sec)(mlO2/ml x mmHg).

As contact lenses act as a barrier to oxygen in reaching the cornea, oxygen permeability (Dk/t) is an important factor to consider while selecting your contact lenses. It will determine the amount of cornea swellings you might get from it.