What Does "UV Protection 97%, 81%" on the Box Actually Mean?
On the specification list of some contact lenses there is a line like this.
UV Protection: 97% UVB & 81% UVA
There are two numbers, and they are different. You start to wonder which matters more, whether higher is always better, and whether you can do without sunglasses.
The two numbers are for different kinds of ultraviolet
Ultraviolet light is divided by wavelength. Lens specifications usually list two kinds, UVA and UVB, separately. Read 97% and 81% as "the share of each kind that the lens blocks". The rest passes through the lens.
The two numbers being different is not an error. The degree to which a lens blocks each kind of ultraviolet can differ, and that is why they are listed separately.
There is a class standard in the United States
The American standard ANSI Z80.20 divides UV-blocking soft lenses into two classes.
- Class 1: blocks at least 90% of UVA and at least 99% of UVB
- Class 2: blocks at least 70% of UVA and at least 95% of UVB
A study that examined these criteria describes Class 2 as the class recommended for "general environments". Class 1 is the class that requires a higher blocking rate than that.
However, this classification is not used in every country or on every product label. Which standard a product is labelled to depends on its box and the manufacturer's material. That is why this article does not judge which class any particular product belongs to. All you need to remember is that the numbers in a specification are values published by the manufacturer.
In a study that tested commercially available lenses against the same criteria, only one of the four types examined fully met Class 2. That result concerns particular products from some time ago, and does not mean the same holds for products today. But it does show that even when a lens is labelled UV-blocking, the blocking rate can differ from product to product. That is why it is better to look at the numbers than at the wording.
What a lens covers is limited
A UV-blocking lens covers the cornea and its edge (the limbus). The study explains that, because of this, a lens may help block light coming in from the sides, which sunglasses cannot.
But there are also places a lens clearly does not cover. The eyelids, the skin around the eye and the surface of the eye outside the lens are not shielded by it.
So a lens's UV blocking and sunglasses do different jobs. The lens blocks what it covers on the eye, while sunglasses and a hat reduce light arriving around and from the side of the eye. It is hard to say that one replaces the other.
What to keep in mind when reading the figures
- Compare by the numbers. A product that says only "UV blocking" and one that lists separate UVA and UVB figures carry different amounts of information.
- Look at UVA and UVB together. One number being high does not mean the other is too.
- Do not assume one lens blocks all ultraviolet. Unless the blocking rate is 100%, the rest passes through.
- Check specifications against the manufacturer's material. It also helps to see whether the seller's specification matches what is printed on the box.
Eye health concerns related to ultraviolet light vary from person to person. Which lens and which form of protection suit you is something to discuss with a professional. Another way of reading product specifications is in The Small Numbers on the Side of the Box.
References
This article is general information and does not mean that any product prevents or treats an eye disease. Follow the examination and advice of a professional for eye protection and lens choice.