How ultraviolet light irritates the eye’s surface
The cornea is the clear, curved covering at the front of the eye. Its outermost layer is made of living cells, and ultraviolet (UV) light can injure those cells just as sunlight can damage skin. When enough UV reaches the cornea, the resulting inflammation is called photokeratitis—literally, inflammation of the cornea caused by light.
The injury is usually temporary, but the cornea is densely supplied with nerves. As its surface cells become damaged and begin to shed, those nerves can trigger pain, watering, redness, sensitivity to light, and the gritty sensation of a foreign object in the eye.
Why symptoms can arrive later
UV exposure does not always produce an immediate warning. The surface-cell injury and the body’s inflammatory response take time to build, so discomfort may appear several hours after the exposure. That delay can make the cause easy to miss: the eyes may feel fine outdoors, then become painful later.
Snow, water, and sand can increase exposure by reflecting sunlight toward the face. UV lamps and welding arcs can also cause photokeratitis, even when the exposure happens without a sunny day. In each case, the important factor is the amount of UV reaching the eye, not whether the light feels hot.
A small surface injury with a strong signal
Because the cornea is essential for focusing light and especially sensitive to irritation, a shallow injury can feel disproportionate to its size. The delayed gritty, painful feeling is the eye’s response to damage on a surface that must stay smooth and clear for vision.
