You are your sunglasses worst enemy. All sunglass lenses will get a few scratches over time from normal use and exposure to the environment. (And from occasionally getting dropped or misplaced.) Sunglass lenses are scratch resistant, not scratchproof. It’s up to you to care for them properly in order to extend the life of your sunglasses.
Cleaning: For optimum performance, rinse your sunglasses daily in warm water. Use a mild liquid dish soap (clear dishwashing soap such as Dawn works best) to wash each lens surface. Dry using a clean, soft, absorbent cloth. Do not use paper-based products to clean your lenses. Do not use abrasive cleaners, soaps or detergents that may leave a deposit on the lens. Do not use tissues with added lotions, lanolin, silicone or other cleaners; they will leave a film on your lens. Do yourself a favor and make this cleaning routine a regular event. A few minutes every few weeks will keep your sunglasses from accumulating so much gunk, and will extend the life of your sunglasses.
Chemicals: Certain household chemicals will react negatively with the frame material and the metal oxides used on the coatings of your lenses. Avoid contact with acetone (nail polish remover), caustic solutions (such as glass or ammonia-based cleaners), hair sprays containing methylene (listed on label), chlorine (from swimming pools or unfiltered water) or glue.
Hard Water: Hard water on your lenses will leave visible spots that are difficult to remove and may be damaging to your lens coatings. Clean and thoroughly dry your lenses immediately if hard water such as pool, unfiltered sprinkler or ocean water comes into contact with your sunglasses.
Scratches: The multiple coatings on your lenses are resistant to light scratching; however, heavy, abusive scratching can break through the coatings and cause visible marks. Removing dirt and other particles with good cleaning practices and keeping your sunglasses in their case when not in use will significantly help prolong the life of your lenses. Good cleaning practices will help minimize scratching.
Heat: Excessive heat may deteriorate your lenses or mis-shape your frame. Avoid placing your sunglasses where you might expect excessive heat, for example, on the dashboard of your car.