This article originally appeared on Health.com.
Even if you love your current sunglasses, you still might need a new pair of shades. It seems sunglasses’ UV protection may deteriorate over time, and current industry tests are not sufficient for determining how long it’s safe to wear them, according to a study from Brazil.
Most Brazilians wear the same pair every day for about two years, the study notes, yet it has not been proven that lenses maintain the same level of protection after that type of exposure to ultraviolet radiation. The findings may have implications for the sunglass industry in the United States, as well. There is no current recommendation for when, exactly, people should retire their old pairs—and protecting eyes from the sun is important anywhere in the world, as UV exposure can lead to cataracts, retina damage, and other long-term eye problems and vision loss.
The new research focuses on Brazil’s system for classifying sunglasses by category, based on lens darkness and the level of UV protection offered. To be certified in one of these categories, lenses must pass a test in which they are exposed to a 450-watt sun simulator lamp for 50 hours at a distance of 30 centimeters. This is equal to two full days of average summer sun exposure, or four days of average winter sun exposure, the study authors write. However, because of Brazil’s proximity to the equator, the sun there is stronger than average. So in actuality, this test is only equivalent to 23.5 hours of sun exposure in the city of São Paulo, for example.
A previous survey found that Brazilians wear their sunglasses for an average of two hours a day for two years straight. The aging tests, the authors argue, should also be revised to reflect this.
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In order to represent average consumer use throughout the country, they calculate that both the time and distance of exposure in the sun-simulator test needs to change to 134.6 hours at 5 centimeters. These calculations are specific to Brazil, the authors say, but may also be helpful for other countries at similar latitudes. (Other countries around the world have similar requirements for sunglasses.)
“It’s still too soon to confirm that UV protection deteriorates over sun exposure,” study author Liliane Ventura, PhD, a professor at the University of São Paulo, wrote in an email. “If the aging test performed by sun simulator with current exposure parameters is not revised, then there are no means to guarantee that UV protection does not change over time.”
The report, published in Biomedical Engineering OnLine, suggests that in addition to UV protection, lenses’ shatterproof qualities may degrade as well.
Although the same aging standards are not used in the United States, Jeff Pettey, MD, clinical spokesperson for the American Academy of Ophthalmology (AAO), says that the new research does make important points that could be relevant to Americans.
“They’re suggesting that the way the industry currently tests sunglasses may not be adequate,” says Dr. Pettey. “There’s a lot we may be unaware of that happens over time—so while there is no official recommendation right now, it might make sense to consider looking for a new pair if you’ve worn the same ones regularly for a couple of years.”
If you’re really curious about whether it’s still safe to wear a pair of old favorites, adds Dr. Pettey, many eyeglass retailers can test lenses’ UV protection levels.
Until more is known, consumers can protect themselves by making sure they buy good glasses in the first place, by purchasing lenses that are labeled “100% UV protection” or “UV400.” Most pairs sold in the United States offer this level of protection, Dr. Pettey says, but it’s still a good idea to confirm before purchasing. (According to a 2014 AAO survey, almost half of people shopping for sunglasses don’t think to check for this language.)
Don’t take into account factors like cost, polarization, lens color or darkness, either; these don’t necessarily make a difference in UV blockage. “Even clear lenses you’d wear with a prescription can have protection, as well; it’s not necessarily about how dark they are,” says Dr. Pettey.
Size and fit, however, do matter. “Bigger is better if you’re outdoors doing activities for longer periods of time,” Dr. Pettey says. “If you’re skiing or out on the ocean and getting reflected UV light from all directions, larger wrap-around eyewear will certainly offer more protection.”
Ventura says that while there’s no way to know how often sunglasses should be replaced, she does recommend against buying them from locations where they’ve already been exposed to sunlight—from an outdoor stand on the boardwalk or beach, for example.
For now, Ventura and her team are conducting further tests on how sunglass lenses hold up over time, and hopes to report more definitive findings in the near future. “We are willing (and have proposed an effective method) to know how long UV protection lasts,” she says. “It’s a wake-up call for the sunglasses standards to be revised.”