UV400 means a lens blocks all ultraviolet light wavelengths up to 400 nanometers, which covers 99% to 100% of the sun’s UVA and UVB rays. It’s the highest UV protection rating you’ll find on sunglasses, and it’s the number to look for when shopping for eye protection.
Why 400 Nanometers Is the Cutoff
The ultraviolet spectrum spans wavelengths from 100 to 400 nanometers, according to the World Health Organization. It breaks down into three bands: UVC (100 to 280 nm), which is absorbed by the atmosphere and never reaches your eyes; UVB (280 to 315 nm), the rays most associated with sunburn; and UVA (315 to 400 nm), longer wavelengths that penetrate deeper into tissue.
Sunglasses labeled “UV400” block the entire range, all the way up to where ultraviolet light ends and visible light begins at 400 nm. That’s why it’s considered the gold standard. A label that says “blocks UVB” or “UV protection” without specifying the 400 nm threshold may leave you exposed to UVA rays, which are just as capable of damaging your eyes over time.
What UV Rays Do to Your Eyes
Both UVA and UVB radiation contribute to cataract formation. Neither wavelength is necessary for sight, so blocking them costs you nothing in visual function. UV exposure is also a risk factor for retinal damage, particularly in children, whose lenses transmit more UV light than adult lenses do.
A study published in the journal Molecular Vision concluded that removing wavelengths below 400 nm from ocular exposure “will greatly reduce the risk of early cataract and retinal damage,” and specifically recommended sunglasses marked 400 as the way to do it. The same research noted that for people over 50, filtering short blue visible light (400 to 440 nm) with specially designed lenses could further reduce the risk of age-related macular degeneration, though standard UV400 lenses don’t cover that extra range.
The skin around your eyes is vulnerable too. An analysis of over 74,000 cases of eyelid basal cell carcinoma over a 26-year period found that while skin cancer rates increased on most parts of the body, eyelid rates held steady, likely because sunglasses were already common. Larger lenses and wraparound styles protect more of that thin, sun-sensitive skin.
UV400 vs. Polarized: They’re Not the Same
This is one of the most common points of confusion. UV protection and polarization are two completely separate features. A lens can have one, both, or neither.
UV400 refers to blocking invisible ultraviolet radiation. Polarization refers to reducing visible glare. Polarized lenses contain a filter that blocks horizontal light waves, the kind that bounce off water, snow, car hoods, and pavement. Think of it like a venetian blind for your eyes: it cuts the harsh reflected light that makes you squint.
Here’s the important part: polarized lenses that lack UV protection can actually be worse than wearing no sunglasses at all. Because they reduce glare, your pupils open wider in bright conditions. If UV rays are still passing through the lens, more of that radiation reaches your retina than it would if you were simply squinting in the sun. Always check that polarized sunglasses also carry a UV400 label.
How Lens Material Affects Protection
Some lens materials block UV light naturally, while others need a coating to do it. Polycarbonate, the most common material in sport and safety eyewear, inherently blocks UV radiation up to 400 nm without any additional treatment. Glass lenses block most UV as well, though some allow a portion of UVA through unless they’re specially coated.
Acrylic lenses are also UV resistant, and they tend to hold their clarity longer than polycarbonate, which can develop a yellowish tint over years of sun exposure if it isn’t treated with a UV-stabilizing coating on its outer surface. For everyday sunglasses, the material matters less than the label. If it says UV400, the lens meets the standard regardless of what it’s made from.
How to Verify Your Sunglasses Actually Work
A UV400 sticker is only as trustworthy as the manufacturer behind it. Cheap sunglasses from unverified sellers sometimes carry the label without meeting the standard. Dark tinting is no guarantee either. Lens color controls how much visible light gets through, not how much UV is blocked. A pale yellow lens with proper UV coating can outperform a nearly black lens without one.
If you’re unsure about a pair, take them to any optical shop. Most opticians will test them in a photometer for free, and it takes less than 30 seconds. This is especially worth doing for children’s sunglasses bought online, where quality control can be inconsistent.
Wraparound Fit Matters More Than You’d Think
Even with UV400 lenses, UV light can enter from the sides, top, and bottom of standard frames. The research recommending UV400 lenses specifically noted that wraparound styles are necessary to prevent reflected UV radiation from reaching the eye around the edges of the lens. This is particularly relevant near water, sand, or snow, where UV bounces off surfaces at angles that slip past flat-front frames.
You don’t need to wear glacier goggles to the grocery store. But if you spend significant time outdoors, especially at higher elevations or around reflective surfaces, a close-fitting or wraparound frame paired with UV400 lenses gives you the most complete protection available.