Seeking some new shades? There is a whole lot more than style at stake, experts say.
"Most people think the darker, the better," Edward Kondrot, MD, an ophthalmologist in Dade City, Fla., told 乌鸦传媒. "But that's not the case."
Darker shades that don't protect against the sun's ultraviolet (UV) light actually allow more of these higher-energy waves into the eye, since the pupils dilate in the low-light conditions created by the heavily tinted lenses -- potentially leading to greater damage, Kondrot said.
"Dark shades cause more harm because you lose that protective reaction: you don't squint, your pupils don't constrict," he said. "And you get excessive amounts of sunlight into the eyes."
The key, experts say, is to choose a pair that guarantees to shut out UVA and UVB rays.
Penetrating Eyes
The wavelengths of visible light are squeezed into a small spot on the electromagnetic spectrum, ranging from about 400 nanometers (nm) to 700 nm -- in terms of color, that's from violet to red.
Beyond red wavelengths are longer and lower-energy waves, like infrared and radio waves; but beyond violet light are shorter wavelengths that eventually turn into the ionizing radiation of X-rays and gamma rays.
In between these DNA-damaging wavelengths and visible light is ultraviolet, generally in the range of 100 nm to 400 nm. Ultraviolet technically extends all the way down to 10 nm, but is ionizing in that range and thus tends to be absorbed by the earth's atmosphere, eliminating it as a threat to human tissue.
Most of the UV light that reaches the earth's surface is UVA, in the range of 315 nm to 400 nm. Nearly all of UVC (100 nm to 280 nm) and 90% of UVB (280 nm to 315 nm) is absorbed by the earth's atmosphere, according to Jason Yam, MD, an ophthalmologist at The Chinese University of Hong Kong.
Even though the UVA and UVB that makes it to the earth's surface isn't ionizing, it can still alter chemical bonds in molecules -- as evidenced by sunburn.
Like the skin, the eye also absorbs UV, with shorter wavelengths -- the more dangerous ones -- being caught on the front lines by the cornea, and longer wavelengths potentially reaching the lens and retina, according to Yam.
That's why ophthalmologists urge that UVA/UVB protection is more important in sunglasses than the extent of the tint.
"You should look for labels that say UV absorption 99% or 100%," Majid Moshirfar, MD, of the Moran Eye Center at the University of Utah, told 乌鸦传媒. "You may also see a label that says UV absorption up to 400 nm. That also means the absorption is up to 100% of UV."
Moshirfar added that polarized lenses don't do anything to stop UV light from hitting the eye; they just help to cut down on glare. Still, that can come in handy in glare-heavy situations, like driving or fishing.
The Purists
Although the vast majority of evidence supports the use of sunglasses, some clinicians, like Kondrot, tell their patients to drop the shades altogether.
Kondrot maintains that since the light-sensing cone cells in the retina have receptors for UV light, it may be an essential part of eye health.
He only recommends sunglasses to patients with certain eye diseases, after cataract surgery, or to those who are going to have long-term exposure to the sun during activities like fishing or hiking. Other patients can just minimize their time in the sun, he said.
"Our bodies have natural protective mechanisms: you squint, the pupils constrict, and you look away from the sunlight," Kondrot told 乌鸦传媒. "It's the natural physiological response that has helped people survive for tens of thousands of years."
None of the experts contacted by 乌鸦传媒 said they would recommend against sunglasses. But for some ocular conditions, the balance of evidence is shifting away from UV being a major cause of disease.
Specifically, recent work has shown that age-related macular degeneration (AMD) may have more to do with blue wavelengths in visible light than with UV light.
In a recent review of the evidence, published in May in the journal International Ophthalmology, Yam and colleagues concluded that there's "insufficient evidence to determine whether AMD is related to UV exposure."
"The lack of a clear association between UV exposure and AMD is not surprising, because the lens absorbs almost all UVB and only very small amounts of this waveband can reach the retina," they wrote. "Blue light has been shown to be on the portion of the visible spectrum that produces the most photochemical damage to animal retinal pigment epithelium cells."
But that review still found strong evidence for the association between UV exposure and a multitude of other eye diseases, including basal cell carcinoma, squamous cell carcinoma, and photokeratitis.
And few patients expect ophthalmologists to be recommending against sunglasses any time soon.
"I believe that sunglasses, with the proper UV protection, are helpful tools in protecting the eyes from damage that can be caused by the sun," Moshirfar said.