The cornea is avascular: it has no blood supply and draws almost all of its oxygen directly from the atmosphere, dissolved in the tear film. Placing a contact lens on the eye inserts a barrier between the air and the cornea, so how much oxygen still gets through is one of the most important safety properties of any lens. That property is captured by two related figures you'll see in the ODAtlas tables: Dk and Dk/t.
The short version
- Dk = oxygen permeability of the material (independent of the lens).
- Dk/t = oxygen transmissibility of the actual lens (Dk ÷ thickness), which is what the cornea experiences.
- Thicker (and higher-powered) lenses transmit less oxygen, even from the same material.
- Overnight wear needs far more oxygen than daily wear, the main reason silicone hydrogels exist.
- Water content is not a proxy for oxygen in silicone hydrogels.
Dk: oxygen permeability (a property of the material)
Dk describes how readily oxygen moves through the lens material. "D" is the diffusion coefficient (how fast oxygen travels through the material) and "k" is the solubility coefficient (how much oxygen the material can dissolve). Their product, Dk, is an intrinsic property of the polymer and does not change with how the lens is manufactured. It is reported in "barrer" units (formally 10⁻¹¹ (cm²/s)(mL O₂ / mL × mmHg)), but for daily practice the important thing is simply that higher Dk means more oxygen-permeable material.
Because Dk is a property of the material, two lenses made from the same polymer share the same Dk even if one is thick and one is thin. That is exactly why Dk alone can mislead.
Dk/t: oxygen transmissibility (a property of the finished lens)
What the cornea actually experiences is transmissibility: Dk divided by the lens thickness (t), where t is usually the centre thickness in centimetres. Because thickness sits in the denominator, a thicker lens transmits less oxygen even when made from an identical, high-Dk material. Dk/t is quoted in units of 10⁻⁹ (cm/s)(mL O₂ / mL × mmHg).
One consequence matters at the chair every day: power changes thickness, so it changes Dk/t. A minus lens is thin centrally and a plus lens is thick centrally, and higher powers of either sign are thicker overall. So the same product can deliver noticeably less oxygen in a −10.00 than in a −1.00. When you fit high powers, treat the headline Dk with appropriate caution, because the effective transmissibility is lower.
EOP: equivalent oxygen percentage
A related, more physiological way to express oxygen performance is EOP: the effective percentage of oxygen reaching the cornea under the lens, compared with the ~20.9% available in open air. It is a useful concept when reading the literature, though catalogues quote Dk and Dk/t rather than EOP.
How much oxygen is "enough"? The published criteria
Two landmark bodies of work give the transmissibility targets that shaped modern lens design. You don't need to memorise the numbers; the pattern is what counts.
| Criterion | Daily wear | Extended (overnight) wear | Goal |
|---|---|---|---|
| Holden & Mertz (1984) | ~24 | ~87 | Limit corneal oedema to physiological levels |
| Harvitt & Bonanno (1999) | ~35 | ~125 | Avoid anoxia throughout the full corneal thickness |
The practical takeaway: overnight wear demands roughly three to four times the transmissibility of daily wear. This is precisely why conventional hydrogels were never truly safe for continuous wear, and why silicone hydrogels transformed extended-wear physiology.
Silicone hydrogel vs conventional hydrogel
In a conventional hydrogel, oxygen is carried by the water in the material. The only way to raise oxygen was to raise water content, and that route hits a hard ceiling (and brings dehydration and handling downsides). Silicone hydrogels changed the mechanism: oxygen passes through the silicone component, decoupling oxygen performance from water content. The result is dramatically higher Dk and Dk/t, often at lower water content. This inverse relationship is a common point of confusion:
- Conventional hydrogel: higher water → higher oxygen (up to a limit).
- Silicone hydrogel: oxygen comes from silicone, so a lower-water SiHy can vastly out-perform a high-water hydrogel on Dk/t.
Silicone hydrogel is now the default for most new fits and essential for any overnight wear. The trade-offs historically were surface wettability (managed with wetting technologies) and, in early materials, higher modulus (stiffness). See how to read a specification.
Recognising chronic hypoxia
When oxygen delivery is chronically inadequate, the cornea shows it. Signs to look for and act on include:
- Limbal and bulbar hyperaemia (redness), often the earliest visible sign.
- Corneal neovascularisation: new vessels growing in from the limbus.
- Epithelial microcysts and increased corneal staining.
- Corneal oedema: acutely, striae and folds; chronically, stromal thinning.
- Endothelial changes: polymegethism and pleomorphism over the long term.
Any of these is a prompt to reassess the material, the fit, and especially the wear schedule, moving to a higher Dk/t silicone hydrogel and/or reducing overnight wear.
Practical selection at the chair
- For overnight or extended wear, prioritise high Dk/t silicone hydrogel materials that meet the extended-wear criteria.
- For high prescriptions, remember transmissibility drops with thickness, so favour higher-Dk materials for high myopes and hyperopes.
- Don't judge silicone hydrogels by water content; judge them by Dk/t, wettability, modulus and comfort.
- Pair oxygen selection with sensible replacement and hygiene, because oxygen is necessary but not sufficient for healthy wear.