Polycarbonate is the standard material for impact-resistant eye protection because it is lightweight and has incredibly high tensile strength-it can stop a projectile traveling at 300 feet per second. However, polycarbonate has a critical molecular weakness: it is highly susceptible to UV degradation. When exposed to direct sunlight, the ultraviolet rays break the polymer bonds in the plastic.
Workers routinely commit the cardinal sin of eye protection: taking their safety glasses off during a break and leaving them on the dashboard of their truck or in the sun directly on the site. Over the course of a single summer, the intense UV radiation cooks the polycarbonate. The plastic doesn't change color significantly, but it becomes brittle. The elasticity is gone.
When a brittle, UV-damaged safety glass is struck by a flying chip of grinding grit or a nail, it does not simply flex and stop the object. It shatters, often spiderwebbing or breaking into shards that strike the eye and eyelid. The very object designed to protect the worker turns into glass shrapnel caused by the impact.
High-quality safety glasses have a hardcoat layer that includes UV inhibitors, but these inhibitors wear off over time if the lenses are not maintained. You must store glasses in a microfiber case away from direct sunlight when not in use. Furthermore, you must inspect lenses for the "crazing" effect-tiny, microscopic cracks in the hardcoat that look like fog or dried mud. If you see crazing, or if you know the glasses have been left in the sun for months, destroy them immediately. Polycarbonate has a shelf life; baking it in the sun turns a life-saver into a weapon.
