Sierra Trading Post Explores: How Does UPF Work?

Every time I see a fishing shirt, hiking shirt or base layer with a UPF rating, I wonder, "Where does the UPF come from?" I've used UPF-rated clothing before and I know it works, but I've never spent the time to figure out how. Until now! Join me as I explore how UPF-rated clothes block UV rays and protect your skin in this installment of Sierra Trading Post Explores.

Protecting yourself from the sun's damaging UV (ultraviolet) rays is a good idea for everyone in any season; but if you're fair-skinned, spending time near reflective surfaces like water or sand, or enjoying the outdoors at high altitude, investing in a variety of clothing that provides UPF protection is even more important. Sun-protective clothing is often preferred to slathering oneself or one's kids with sunscreen (although a combo of both is best) because UPF-rated sun-protective clothing takes only a moment to put on and won't wear off during the day, unlike even the best sunscreens.

What is UPF?


The UPF rating you see on sun-protective clothing is similar to the SPF rating on sunscreen in that it measures the penetration of UV rays that reach and damage your skin. UPF stands for "Ultraviolet Protection Factor" and is different from SPF in that it is the standard used to rate fabrics, not sunscreens.

UPF ratings denote the amount of UV penetration that a piece of fabric allows through; a piece of clothing with a UPF factor of 50 only allows 1/50th of the sun's radiation through, or just two percent. The table below explains what UPF ratings mean and what kind of protection they provide.

UPF ratings

Clothes with UPF ratings are put through a variety of standardized tests to determine an appropriate UPF value.

How Does Sun Protective Clothing Work?


There's a variety of ways that a UPF-rated piece of clothing can offer protection. Most UPF-rated clothing will utilize either a tight weave, special dye or chemical treatment to prevent the sun's rays from penetrating the fabric.

UPF clothes

Clothing that utilizes a tight weave or dense fabric to physically block skin from UV rays will often feature vents and moisture-wicking fabric to keep you cool in sunny, warm conditions.

Sun-protective clothing that doesn't depend on fabric construction to block UV rays tends to depend on special dyes or chemical treatments. Premium UV-disrupting dyes can be light or dark; what provides UV protection is the concentration of these dyes and their ability to deflect UV rays, not the color of the dye. UV-absorbing chemical treatments used to provide UPF-rated sun protection work in a similar way to premium dyes, and increase a fabric's UPF rating by disrupting and deflecting sunlight.

Other factors that affect sun protection are fabric type (polyester is best), stretch (stretched fabrics block less light) and wetness (wet fabrics block less light).

Not All Sun-Protective Clothing is Created Equal


While all fabric blocks some UV light, most don't block enough to provide dependable sun protection. But now you know exactly what to look for when you need to protect yourself from the sun's damaging UV rays. The best sun-protective clothing will have a UPF rating of 50 and will likely feature fabric with a tight weave, premium dye or special UV-blocking treatments (or all three). Keep an eye out for brands like Marmot, Columbia Sportswear and Mountain Hardwear, which offer stellar UPF protection as well as active comfort.

Once you've armed yourself with some UPF-rated clothes, get out there and have some fun in the sun!

­That wraps up this installment of Sierra Trading Post Explores. I hope you learned a thing or two. Join me next month for another look into the lesser known aspects of your outdoor adventures and gear!
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