If you've read our guide to The Best Sun Shirts, you may be wondering what all this talk about UPF is. So welcome to UPF 101.
“Ultraviolet Protection Factor is a standard that measures the effectiveness of sun protective fabrics,” says Corey Simpson, Patagonia’s communication manager of product and sport communities. “It is a specific standard for fabrics. SPF (Sun Protection Factor) is a standard specifically for sunscreens.”
It’s easy to get those similar acronyms mixed up, but it’s crucial to distinguish between the two. In a paper for the Skin Cancer Foundation, researchers Peter Gies and Alan McLennan point out that a UPF rating denotes how much ultraviolet light penetrates through a fabric. And unlike SPF ratings, which only measure for harmful UVB light, UPF ratings consider UVA and UVB, both of which can contribute to skin cancer.
When shopping for sun protective clothing, you’ll find that UPF ratings range between 15 and 50-plus. A UPF rating of 20 is decent — it means that 1/20th (five percent) of UV rays pass through a fabric. Meanwhile, 50-plus is generally the highest rating you’ll see used with regards to outdoor clothing and it references a permeability factor of 1/50th (two percent) or less.
Gies and McLennan detail how fabric density, type, color, weight and thickness are key in determining these UPF ratings. A classic white cotton t-shirt scores a measly five on the UPF scale, dropping down to three when soaked. On the far end of the spectrum, jeans earn a whopping 1700.
What’s more, they point out that initial UPF ratings can change over time: washing a garment can actually shrink existing gaps in the fabric weave, henceforth curtailing the percentage of UV rays that can pass through. Excessive wear will do the opposite, as fabric weakens, tears and stretches over time. Unsurprisingly, UPF ratings can also drop when fabrics are wet.
So just be mindful of these mitigating factors, and you'll be much less likely to get burned. Literally.