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      In the hot summer, watermelon, sunscreen, and sunglasses are standard equipment. Buy a watermelon every two days, and change your sunscreen every year. How long does it take for your sunglasses to change?

      The frequency with which many people change their sunglasses depends on whether there are new ones that are more suitable for them. But you know what? Sunglasses also have a "shelf life". If they work over time, they will not only fail to protect the eyes but will cause damage to the eyes.

      Researchers at the University of Sao Paulo in Brazil suggest that sunglasses need to be replaced every two years because prolonged exposure to sunlight will reduce the ability of the lenses to protect against ultraviolet rays, which can easily damage the cornea and cause vision problems. The researchers conducted simulation experiments in Europe, Australia, New Zealand, and South America. They put the sunglasses under a sunlight simulator, 30 cm away from the luminous point, and continued to "expose" for 50 hours to see the degradation of the UV protection ability of the lenses.

      The British "Daily Mail" once reported that it is still safe to wear sunglasses for two years. Sunshine varies in different parts of the world, with tropical countries and regions having the highest levels of UV rays, even in winter, which means that the UV protection of sunglasses degrades faster.

      In addition, southern hemisphere residents change their sunglasses more often than northern hemisphere residents. The researchers also said that prolonged sun exposure not only affected the UV protection of the sunglasses, but the lenses were also more likely to break, but it was still safe to wear them for two years. Study leader Professor Lillian Ventura said the experiment did not accurately assess the quality of the sunglasses, so the results were "not strong enough" and hoped that new standards for the function of sunglasses would be introduced in the future to determine the safe use period of sunglasses.

      Is this study reliable? There are two types of sunglasses:

One is that the lens is made of yellow, blue, and other colored glass, which is absorbed by the body and refracts light; the other is that the lens surface is coated with a coating, which reflects ultraviolet light. sunglasses.

      On the market, more than 80% of sunglasses are coated.

      Generally, this layer of coating is combined with glass by physical vapor deposition. Coatings are generally made of metal materials such as gold, silver, copper, etc., and metal materials will oxidize, which will cause their UV protection capabilities to deteriorate.

      However, different brands of sunglasses have different production methods. Sunglasses with high prices will be coated with several layers of coatings, or coated with alloy materials, which will have better stability and will not be prone to oxidation. As long as you hold the sunglasses to see the reflection, the newly bought sunglasses look like mirrors. If the reflection becomes weak, it means that the coating has been oxidized or even peeled off.

      In the sunglasses industry, it is generally believed that due to sun exposure, lens wear, etc. in normal use, the lenses will age to varying degrees if they are used for more than 1 year. Therefore, if the glasses are severely worn, they should be replaced in time to avoid affecting the vision.

      What are the dangers of wearing "overaged service" sunglasses for a long time? In the worst case, it is actually the long-term UV rays that directly affect the eyes.

      The World Health Organization published the article "Ultraviolet Radiation and Human Health", which pointed out that 5% of the 18 million people blinded by cataracts worldwide are caused by ultraviolet radiation. UV rays can also cause other serious eye diseases - keratitis, retinal damage, cataracts, macular degeneration, snow blindness, etc.

      Fortunately, most people will not wear sunglasses for too long, and even if the coating has fallen off, they will not face the UV light for a long time. However, sunglasses outside the shelf life are also prone to visual fatigue, dry eyes, astringent eyes, etc., and may also lead to aggravation of myopia.