For the period of two years the UV rays damage the lenses and they begin to let it through, according to Daily Mail.
The human eye can be damaged without a good UV filter to protect it as the first symptom is swelling. It can lead to distortion of vision, and later even damage the cornea.
During the penetration of the UV rays it is possible for an eye curtain to appear. The prolonged exposure to these rays can damage and macula – part of the retina, which is responsible for the vision.
After an examination done by specialists from the University in Sao Paulo on how a deterioration of sunglasses appears when exposed to sunlight, it was found that the control test which is currently used for calculating the level on which the sunglasses lose their strength is not effective enough and does not examine correctly the quality of the lenses. The reason behind this is the fact that it does not report correctly the time and the season the sunglasses are used. During this test, the glasses are put under a sun stimulator for 50 hours in 30cm. distance.
Additionally, the experts state that one sunny day during the summer can wear down the sunglasses as much as four days in the winter.
Source: Daily Mail Online