Just when we thought we’d found our dream pair of sunglasses, some scientists have thrown a spanner in the works by suggesting we get a new pair in two years.

Researchers in Brazil say the sun’s rays damage sunglasses’ lenses over time, meaning they offer less protection against damaging UV.

Writing in the journal Biomedical Engineering OnLine, the scientists from Sao Paulo University called for the safety standards which test the quality of sunglasses to be revised – and said a new test is needed to guarantee sunglasses were safe for two years.

But before you bin your three-year-old Ray-Bans, we asked Dr Nigel Best, Specsavers’ clinical spokesperson for his expert opinion and some tips for keeping our eyes safe…

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Were you aware UV protection in lenses can deteriorate over time?

“It wasn’t something we were aware of or thought was an issue. We find this [research] quite surprising, it’s news to us. If we accept the premise that exposure to UV will eventually reduce the ability of a lens to block UV, it still seems strange that you can put a nominal figure on it like two years. Because in two years, someone could have worn their sunglasses every day or someone could have worn their sunglasses half a dozen times.

"The actual exposure to UV over that two-year period will vary enormously. It’s not something we would want to be promoting.”

Do you agree we should be regularly changing our sunglasses?

“No we wouldn’t agree with that advice. We advise that sunglasses should be replaced if the lenses have become damaged or unserviceable in any way. Most of our sunglass lenses are plastic.

"If they’re used regularly, probably after two or three years you might find the lenses are starting to get scratched, so it might be appropriate to replace them, but if somebody’s looking after them very well, they could last much longer – and certainly Ray-Ban lenses can last a very long time.”

What do sunglasses protect our eyes from?

“In the short-term, exposure to UV can cause the eye to become quite swollen and painful, sometimes called snow blindness, which we associate with skiing at high altitude without sunglasses or goggles on. At the end of the day, your eyes could be quite sore and red, and that will go away after a day or so.

“That inflammation on the surface of the eye caused by acute UV exposure, is not so much a problem with the levels of the UV we get in the UK. Here, you’re looking more to problems associated with long-term UV exposure, so over the course of your lifetime, which will be a higher risk of cataracts, age-related macular degeneration, a higher risk of ocular tumours. You can get melanomas on your skin from increased UV, you can also get melanomas on your retina from too much UV.”

Should we be wearing sunglasses year-round?

“Sunny days in winter can be very bright, but we shouldn’t confuse brightness with UV exposure. Brightness and UV are quite different.

"From a UV perspective, it’s more important to wear sunglasses in spring and summer because that’s when the UV exposure is highest. In terms of protecting our eyes from glare, there are some winter days which are very bright, when of course sunglasses would be needed.”

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Who is most at risk from UV exposure?

“Children and teenagers. Most exposure to UV radiation occurs in our childhood and in our teens. There’s an estimate that by the age of 18, we’ve already had 80% of our lifetime exposure to UV. The reason for that is when we’re younger, we have larger pupils and the lens inside our eye is much clearer.

"As we age, our pupils naturally become much smaller. At 45, the lens inside my eye is not as clear as it was when I was a 12-year-old, so I get much less UV into my eye – and to some extent, most of the damage is already done. It’s quite common to see mum and dad walking down the street wearing sunglasses, holding their child’s hand, who isn’t wearing sunglasses. If your child won’t wear sunglasses, a broad-brimmed hat is a good alternative.”

What should you look for when buying sunglasses?

“When you’re thinking of buying sunglasses, make sure they’re CE marked, particularly for children’s, because it means they’ve been checked and conform with the appropriate European standard and we know they will provide UV protection. Non-CE marked glasses can be more harmful than not wearing anything at all.”

Any precautions for people who wear glasses and contact lenses?

“Your prescription glasses will actually block out UV, just due to the fact you have a lens there. So someone who’s walking around in a pair of specs, just clear lenses, is getting a lot less exposure than someone like me who doesn’t wear glasses. It obviously won’t block out as much as a sunglass, but it will still give you some protection.

"A large number of contact lens manufacturers now do UV blocking lenses. If you’re a contact lens wearer, you may as well put in one that blocks UV as one that doesn’t.”

How often do you change your sunglasses? Tell us in the comments box below.