Damage to the eyes from sun exposure can take two forms: short-term damage and long-term damage. Short-term damage ("solar retinopathy") occurs when a person stares directly at the sun, and is a result of too much ultraviolet light flooding the retina. In extreme cases this can cause blindness, but is so painfull that it is rare for someone to be able to stare at the sun for that long. Typically, eye damage from staring at the sun results in blurred vision, dark or yellow spots, pain in bright light or loss of vision in the center of the eye (the fovea). Permanent damage to the retina has been shown to occur in ~100 seconds, but the exact time before damage occurs will vary with the intensity of the sun on a particular day (clouds and haze can weaken the sun's rays) and with how much the viewer's pupil is dilated (decongestants and other drugs will dilate pupils). Obviously, no one has done a study to measure the exact time before damage or blindness occurs, but doctors collect information after events like the 1999 solar eclipse, when people who stared directly at the sun for several minutes went to see their doctor. In one study, about half had permanent damage. Short-term damage can be easily prevented: never look directly at the sun. During sunsets, when the intensity of ultraviolet light is lower due to scattering, avoid looking directly at the sun for more than a few minutes at a time. Sunglasses and camera filters do not prevent short-term damage, and may make it more likely by dilating your pupil and/or allowing you to stare at the sun longer without pain.
Long-term damage is also caused by the effects of ultraviolet light, and can result in cataracts (cloudy spots on the eye lens), corneal sunburn and growths on the surface of the eye. This type of damage can be the result of a lifetime of sun exposure, and the effects usually don't show up until people are in their 50's or later. Long-term damage is harder to prevent, especially in people who spend a lot of time outside. The best prevention is to wear sunglasses that block UV light (both UVA and UVB).
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