What Does UV400 Mean?

Of all the specs on a pair of sunglasses, UV400 is the only one about your healthrather than your style — and it’s the one people most often get wrong, because the marketing pushes lens darkness and forgets to mention that darkness protects nothing. Here’s what UV400 actually means and why it’s the number to insist on.

What UV400 blocks

UV400 means the lens blocks all ultraviolet light with wavelengths up to 400 nanometers. Ultraviolet light comes in bands: UVB (280–315 nm) and UVA(315–400 nm) are the ones that reach and harm your eyes. Because all of that harmful UV falls below 400 nm, a lens rated UV400 effectively blocks essentially 100% of it. That’s why UV400 and “100% UV protection” describe the same real-world result: blocking up to 400 nm covers the whole range.

Why it matters for your eyes

Cumulative UV exposure is linked to serious eye conditions. Eye-health and cancer authorities connect long-term UV to cataracts, growths on the eye, macular degeneration, and even eye cancers; the WHO attributes a meaningful share of cataract cases worldwide to UV exposure. Sunglasses with proper UV protection are a simple, effective defense — and unlike sunscreen, you don’t have to reapply them. The American Academy of Ophthalmology and the Skin Cancer Foundation both recommend blocking 99–100% of UV, which UV400 meets.

The dark-lens trap

Here’s the counterintuitive part that’s worth repeating: a darker lens does not mean more protection. Tint and UV filtering are completely separate. In fact, a very dark lens with no UV filter can be worse than wearing nothing: the darkness makes your pupils dilate to let in more light, and without a UV filter that means more unfiltered ultraviolet reaching your retina. The FDA is explicit that lens darkness does not indicate UV protection. Never buy sunglasses on how dark they look — buy on the UV label.

How to check a pair really has it

Look for a clear claim: “UV400,” “100% UV protection,” or “blocks UVA and UVB.” Reputable retailers state it because disclosure is expected under sunglasses regulations, and the international standard for general-use sunglasses (ISO 12312-1) covers UV filtering among its requirements. If a cheap fashion frame makes noUV claim at all, assume it doesn’t protect and skip it — the good news is that real UV400 costs almost nothing to include, so even budget frames on our affordable acetate list have it.

The bottom line

UV400 is the non-negotiable spec. Get the frame shape, size and material right for style and comfort — but never compromise on UV protection, because it’s the one thing on a pair of sunglasses that’s protecting something you can’t replace. Every frame in our best acetate sunglasses roundup states its UV protection, so you can shop on looks knowing the safety box is already checked.