Objectives: The human eye is exposed to toxic ultraviolet radiation (UVR) from sunlight and artificial sources. The UVR-induced damage occurs in ocular tissues from the corneal surface to the retina. Although the cornea and crystalline lens provide inherent UVR protection, the anterior ocular surface and the limbus, which contains stem cells, receive toxic levels of UVR from relatively short solar exposures.
Methods: Shading headwear and some UVR-blocking sunglasses are designed to reduce direct solar exposure but may not protect the eye from diffuse ambient and surface reflected light. If the squint mechanism is reduced because of the reduction of visible light, the ocular surface is then exposed to ambient and reflected UVR. In addition, laterally incident radiation that is focused across the cornea onto the limbus, a phenomenon known as peripheral light focusing, can increase the dose at the nasal limbus by a factor of 20.
Results: The UVR-blocking contact lenses that cover the limbus provide protection from all sources of ocularly incident UVR. Although directly relating solar UVR dose to ocular damage is epidemiologically challenging, irradiation of ocular cell cultures can estimate the toxic effects of UVR exposure. The use of UVR-blocking contact lenses greatly increases the time the wearer can be exposed to solar UVR before a toxic ocular dose is reached.
Conclusions: There is a need for the development of a scientifically rigorous, clinically applicable ocular protection factor metric, based not only on the transmittance of eyewear but on the protection afforded from the total UVR field and the length of that exposure.