Many people use the same pair of sunglasses for years. However, sunglasses do have a shelf life, which is related to wearing time, lens material, wearing habits, and regional sunlight exposure. Generally speaking, the optimal lifespan for sunglasses is about two years; after that, it's best to replace them with a new pair.
First, the main areas of wear and tear on sunglasses are the joints between the lenses and the frame. There are currently three main types of lenses: resin polarized lenses, resin PC lenses (AC, CR39, and nylon are all categorized in this group), and glass lenses. Lightweight and comfortable resin polarized lenses and resin PC lenses are not scratch-resistant and will develop scratches over time. Many people are unaware that their lenses are scratched and rely on their eyes to adjust and see clearly, which increases eye strain and can lead to decreased vision. Therefore, lenses should always be stored with the lenses facing upwards to avoid scratches and abrasions.
Second, prolonged exposure to sunlight reduces the lenses' UV protection, making them more susceptible to corneal damage. Researchers conducted a simulation experiment, placing sunglasses under a sunlight simulator 30 centimeters away from the light source and subjecting them to continuous "sun exposure" for 50 hours. They found that the lenses' UV protection capabilities deteriorated. Tropical countries and regions have the highest UV levels, meaning the UV protection function of sunglasses deteriorates even faster in these areas. Even high-quality sunglasses made with special materials will age over time. If the UV protection function of sunglasses declines, it can damage the cornea and retina, leading to diseases such as solar keratitis and corneal endothelial damage.
High-quality sunglasses absorb or reflect ultraviolet (UV) rays to block them from reaching the eyes. This is generally indicated in several ways: 1. "UV400": This indicates that the lens blocks UV rays at a wavelength of 400 nanometers, meaning its maximum spectral transmittance below 400 nanometers cannot exceed 2%. 2. "UV" or "UV Protection": This indicates that the lens blocks UV rays at a wavelength of 380 nanometers. 3. "100% UV Absorption": This indicates that the lens has 100% UV absorption capability, with an average transmittance in the UV range not exceeding 0.5%. Only sunglasses with the above-mentioned markings can be considered truly protective against ultraviolet (UV) rays.
Furthermore, sunglasses belong to the plano (non-prescription) lens category and are only allowed a maximum refractive power of ±8 degrees. Products exceeding this range are considered substandard. Market testing shows that nearly 30% of sunglasses exceed this range, some even exceeding 20 degrees. Consumers with normal vision wearing such sunglasses are essentially wearing nearsighted or farsighted glasses, and after just one summer, they could be "cultivated" into nearsighted or farsighted individuals by these inferior glasses. Therefore, if you experience dizziness, nausea, or blurred vision after wearing sunglasses, you should remove them immediately.
Purchase sunglasses from reputable stores. Some cheap sunglasses not only fail to filter UV rays but may actually increase UV exposure by blocking visible light. Avoid wearing sunglasses for extended periods, as this can cause eye strain. Remove sunglasses when moving from outdoors to indoors. Avoid large-framed sunglasses, as they increase the burden on the eye's accommodation function and damage vision. (Tan Xingping, Deputy Chief Technician, Ophthalmology and Optometry Center, Xiangya Hospital, Central South University)
