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The known health effects of UV: What are the effects of UV on the eye? Questions •	What are the effects of UV on the eye? The eye occupies less than 2 per cent of the whole body surface area, but it represents the sole organ system to allow the penetration of visible light deep into the human body. During human evolution a number of mechanisms have evolved to protect this very sensitive organ against harmful effects of the sun's rays: The eye is recessed within the anatomy of the head and shielded well by the brow ridge, the eyebrows and the eyelashes. However, these anatomical adaptations are of limited use in UV protection under extreme conditions such as sunbed use or strong ground reflection from snow, water and sand. Constriction of the pupil, closure of the eyelids and the squinting reflex minimize the penetration of the sun's rays into the eye. These mechanisms are activated by bright visible light and not by UV radiation – but on a cloudy day UV radiation exposure may still be high. Therefore, the effectiveness of these natural defences in protecting against UV damage is limited. Photokeratitis and photoconjunctivitis Photokeratitis Ultraviolet radiation may cause photokeratitis, a painful inflammation of the cornea of the eye. The spectral responsivity for photokeratitis - the action spectrum - follows a characteristic curve, the maximum of which lies at 288 nm according to Coga and Kinsey, at 270 nm according to Pitts et al. The threshold dose is between 50 and 100 J/m2. The threshold exposure times are evaluated, within the range of validity of Bunsen-Roscoe law, for some sources. The data obtained by various authors are compared and the discrepancies are discussed. Photoconjunctivitis Ultraviolet radiation can also cause photoconjunctivitis, a painful inflammation of the conjunctiva, the tissue coating the eyelid and part of the eyeball. The spectral responsivity of photoconjunctivitis - the action spectrum - follows a characteristic curve, whose maximum lies, according to Schulze, at 260 nm, and resembles closely the absorption spectrum of DNA damage. The threshold radiant exposure (threshold dose) is about 50 J/m2. The opinion expressed by several authors, that the same action spectrum should exist for photoconjunctivitis and photokeratitis, cannot be shared by CIE Division 6, since the cornea and the conjunctiva of the human eye are different from both anatomical and histological standpoints. Pterygium Pterygium is a raised, wedge-shaped growth of the conjunctiva. It is most common among those who live in tropical climates or spend a lot of time in the sun. Symptoms may include irritation, redness, and tearing. Pterygium may extend over the centre of the cornea and thereby reduce vision. Pterygiums are nourished by tiny capillaries that supply blood to the tissue. For some, the growth remains dormant; however, in other cases it grows over the central cornea and affects the vision. As the pterygium develops, it may alter the shape of the cornea, causing astigmatism. If the pterygium invades the central cornea, it is removed surgically. Since pterygiums are most commonly caused by sun exposure, protecting the eyes from sun, dust and wind is recommended. Instilling artificial tears liberally is also helpful to decrease irritation. In some cases, steroid drops are prescribed to reduce inflammation. Cataracts Cataracts and Other Eye Damage Cataracts are a form of eye damage in which a loss of transparency in the lens of the eye clouds vision. If left untreated, cataracts can lead to blindness. Research has shown that UV radiation increases the likelihood of certain cataracts. Although curable with modern eye surgery, cataracts diminish the eyesight of millions of Americans and cost billions of dollars in medical care each year. Other kinds of eye damage include pterygium (i.e., tissue growth that can block vision), skin cancer around the eyes, and degeneration of the macula (i.e., the part of the retina where visual perception is most acute). All of these problems can be lessened with proper eye protection from UV radiation.