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Looking at the Science of Sunglasses

Consumers have an impressive choice when it comes to sunglasses, from rainbow tints to polarized lenses. But you don’t get more UV protection the more money you spend.

The sunglasses at the dollar store feel flimsy, yet I buy a pair. It costs me $1.43. The sticker on one of the lenses states “UV 400 Protection”. I walk over to a fancy sunglasses store and ask the employee if she would trust this pair to filter out UV light. She says no. I ask why not. “Because they’re from the dollar store.”

The cheapest pair she sells is a Ray-Ban that retails for $190. Do I need it if I want to protect my eyes?

The Sun is a harsh mistress

While we may enjoy the Sun’s warm infrared rays, its ultraviolet light, invisible and unnoticeable, is more sinister.

The total amount of UV light we are exposed to and absorb is affected by how high we are relative to sea level, how much time we spend outdoors, how high the sun is in the sky, the amount of pollution in our atmosphere, and whether or not we are taking photosensitizing drugs (medications that increase our absorption of UV rays). And while UV light can hit our eyes in a straight line, it also gets reflected on the ground and reaches us from below and from the sides.

An impressive number of eye diseases correlate with exposure to UV light, from pre-cancerous growths on the eyelid to snow blindness to conditions affecting the back of the eye. A child’s eye is particularly sensitive to UV exposure. The lens inside their eye is still clear and their pupil is wider. This means that 2 to 5% of the UV rays received by their eyes can actually reach the retina at the back. (By comparison, in adults aged 25 years and older, it’s between 1 and 2%.)

And there’s blue light. Between green-coloured light and ultraviolet light, there’s a high-energy region of the visible spectrum that has been implicated in a common form of age-related blindness, though research there is still on-going.

So we need to protect our eyes from high-energy rays of light. How do we do that?

Casting light on sunglasses

I scanned the scientific literature but also reached out to Martin Careau, an optician who teaches at CÉGEP Garneau, in order to separate useful protection from fancy preference when it comes to sunglasses.

UV protection: Glasses advertising “UV 400” will block essentially all UV rays. This is in reference to the wavelength 400 nanometres. And if you’re concerned your sunglasses may not block UV rays after all, you can visit an optician and they’ll use a spectrometer to make the determination. “If it goes up to 380 or 390,” Careau tells me, “it’s still very good but it won’t block 100% of UV light.” The UV protection itself does not diminish over time, even though the tint of the lenses could fade.

Price: While you may think that the more expensive a pair of shades is, the better the UV protection is, that is simply (and thankfully) not true. A recent study published in the Canadian Journal of Ophthalmology looked at over 200 pairs of sunglasses on sale in Quebec City in three price ranges and tested them to see if they met the standards for filtering out harmful UVs. While there are no mandatory standards in Canada, the researchers used the four existing standards in the world: United States, European Union, Australia, and New Zealand. All tested sunglasses above the 21$ price point met these standards, and almost all of the cheaper sunglasses did as well. Price had clearly no real bearing on their ability to filter out UV light, but it did have a role to play on the clarity of the visible light they transmitted.

That dollar store pair of sunglasses I bought? I took it to a major eyewear chain and asked them if they had the equipment to verify its UV protection. Their optician tested it. Sure enough, it detected 0% of UV rays coming through the lens. (My own pair of prescription eyewear let 25% of them through.) But as the optician reminded me, you may get full UV protection for $1.43, but the optical quality may lack behind. This means cheap sunglasses can create distortions in shapes, which are annoying and could be hazardous while driving. It’s like looking at the world through an old television set instead of a 4K monitor.

Tint: The colour of the lenses, including how dark they are, has no bearing on the UV protection. It boils down to preference. “If you want brighter colours, you go for the brownish, pinkish tint, like the old Serengeti lens,” Careau says. I agree from personal experience. My own regular prescription eyeglasses are tinted toward red and the colours really pop. Some people prefer grey lenses, which Careau tells me lead to a “faded, dim, dark” look without altering colours. As for really dark tints, they simply block more visible light. If your eyes are more sensitive to light, you may want to avoid lighter tints. Orange and yellow lenses offer better contrast, which is useful to tennis players, though they obviously distort colours.

Polarized: While this feature has no impact on UV filtration, it will cut down on glare coming from horizontal surfaces, like snow or the surface of a lake. But if you’re a pilot, watch out! Polarized lenses may interfere with your ability to read instruments that have an anti-glare coating and with LCD readouts, as well as reduce visibility. Hydroplane pilots in particular will have difficulty judging distances for landing while wearing polarized lenses. For everyone else? “It’s like having air conditioning in the car,” Careau suggests. “Once you’ve had it, it’s always more comfortable.”

Photochromic (AKA transition lenses): The idea of lenses that automatically get darker when go outside is very tempting, but be warned. The technology behind them is triggered by UV light. Inside a car, where the UV light is being partially blocked by the windshield, your sunglasses probably won’t darken (although some more recent technology can partly get around that). Also, the transition is not consistent across temperatures. When it’s colder, transition lenses tend to get much darker than when it’s warm, Careau tells me.

Wraparound: Because UV light can bounce around, it can reach your eyes from the side. Wraparound sunglasses thus offer better protection.

Raising a glass to sunglasses

To protect our eyes from the Sun, we should seek out sunglasses that block 99-100% of UVA and UVB rays and we should favour wraparounds. Bigger is better, as the American Academy of Ophthalmology reminds us, but darker is neither better nor worse in this respect. The colour does not matter when it comes to blocking harmful rays, and while polarized lenses cut glare, the polarization does not improve the blocking of UV rays.

As for the weather, clouds make no difference: UV light goes through them. And winter is no excuse to leave the shades behind, as the snow reflects 94% of the UV rays it receives.

But even with the best shades gracing our faces, we should not forget sunscreen and a hat. While lenses can fully block UV light, wearing sunglasses does not. Light can get reflected and enter our eyes from above, below, and from the sides.

As for my buck-fifty pair of shades, it’s not bad. I won’t be trading in my more expensive wraparounds any time soon, but these cheap alternatives do what it says on the tin. No reason to throw shade at them.

Take-home message:
- Sunglasses can protect your eyes against damaging ultraviolet (UV) light
- Even the cheap sunglasses on sale in Canada with a “UV 400” label usually filter out all of ultraviolet light


@CrackedScience

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