Over time, even the most durable lenses will become less effective in shielding your eyes from UV radiation. Once the efficacy of their UV protection decreases it can’t be restored, so your lenses will eventually have to be replaced.
But why don’t they last forever? Why aren’t lenses capable of withstanding UV rays in perpetuity? To answer this question, you’ll have to understand how UV protection works in the first place – while the material of a lens itself does offer some UV resistance on its own, much of the protective efficacy of your sunglasses comes from an external coating of dyes and pigments which are able to reflect and absorb ultraviolet radiation. These constitute a physical barrier which prevents the penetration of UV wavelengths.
But like all physical barriers, the protective coating can (and will) break down over time. All the usual wear and tear that’s visible on your sunglasses (that tiny scratch from a rogue tree branch, little scuff from a slip in a scree field, etc.) is also damage to the strength of your UV defenses. If the scratch resistant coating of a lens has failed to prevent a scuff, it’s almost certain the UV coating has been compromised as well. And according to Michael Ehrlich, a professor of ophthalmology at Yale interviewed by Allure, not all scratches are visible, either – if your sunglasses are stuffed in a daypack or constantly rattling around on your dashboard, they may be the canvas for an invisible web of tiny abrasions that decrease the protective properties of your lenses.
The natural environment is a factor as well – quite simply, the protective dyes and pigments can’t absorb radiation indefinitely, so the more sunlight they are exposed to, the more quickly they’ll become ineffective. All else equal, a pair of shades worn only on occasion in mild conditions is sure to remain effective longer than a pair which is heavily used in a more intensely ultraviolet environment. If you spend every day of the summer out on the water teaching sailing or guiding kayak tours, for example, the efficacy of your lenses will decrease more quickly than it would if you were only breaking out the shades once a week to sit on the porch and watch the sunset.
And naturally, time itself has a say in how long your glasses will stay maximally UV protective. The longer you own them, the more likely they are to receive scratches and scuffs (the badges of adventure!), and the longer you use them, the more sunlight and UV radiation they’ll be exposed to. This brings us to the practical question: how often should you replace your lenses?