As outdoor enthusiasts we are generally pretty fond of our eyes, for obvious reasons – they let us take in the view, help us participate in our favorite outdoor sports, and give us a convenient excuse to don a pair of stylish shades. Of course, the glasses aren’t just for looking good – they shield your eyes from harmful UV radiation to ensure that you’ll be able to study maps, follow blazes, and spot salmon jumping for years to come. Even with the most durable sunglasses available, it’s important to stay on top of lens-replacement – this is the only way to be confident that your vision is maximally protected from the dangerous effects of overexposure to ultraviolet radiation.

Whenever and however you’re recreating outdoors, maintaining maximum UV protection for your eyes should be a top priority. Long-term overexposure to UVA and UVB rays can cause forms of eye damage like oracular cancer, pterygia, pingueculae, cataracts, and macular degeneration; all of which are accompanied by discomfort, deformity, and/or different degrees and varieties of vision loss – in more serious cases, even blindness.

Even short term overexposure can result in photokeratitis, a corneal sunburn that causes temporary pain, swelling, light sensitivity, and vision loss. This might be a phenomenon you’ve experienced if you’ve ever spent too much time boating or skiing (photokeratitis is also called “snowblindness”) without eye protection. Activity in the snow and on the water can increase your risk of UV exposure because snowy/watery environments are highly reflective – this means far less sunlight is absorbed by the ground, and far more radiation will be bouncing back up toward your face. According to the World Health Organization, a fresh layer of snow can reflect up to 80% of the sun’s UV rays.

But skiers and boaters aren’t the only ones at risk – the American Optometric Association says that anyone who spends a significant amount of time outdoors could be at an increased risk of UV overexposure. The only way to be sure that you’re keeping your vision safe while enjoying the outdoors is to be conscientious about maintaining maximal UV eye protection by replacing your lenses as needed.

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?

The answer to this question won’t be the same for everyone – as the considerations above suggest, it depends on how frequently your glasses are used, how often they’re exposed to sunlight, and on the intensity of that exposure. However, a 2016 study from a duo of Brazilian scientists suggests that UV coating deteriorates after two years of two hours of daily use. Ehrlich (the professor from Yale) also cites the two year figure as the proper time for lens replacement.

For the average person, then, two years should be considered the safe benchmark for replacing your lenses if you want to remain confident that you’re keeping your vision safe from UV rays to the maximal extent. But if you’re out waterskiing or snowshoeing all day every day, sunbathing the months away on remote mountain peaks, or otherwise wearing your glasses outdoors for longer than two hours per day, it would likely be wise to replace your lenses more regularly. If you’re ever unsure, it’s better to be safe than sorry – you can bring your shades to many local sunglass retailers to test the efficacy of their UV defenses.

All of us who enjoy the outdoors want to keep our vision healthy. So treat your lenses with care, replace them at least once every two years, and keep your eyes trained on the next adventure!