Doesn't all clothing protect you from the sun?
What makes SunGrubbies.com clothing special?
That's a good question. And the answer is "Yes" all clothing does protect you from the sun. The big question is how much? Surprisingly, the average T-shirt allows 50% of the UV light to penetrate to the skin, and many summer weight fabrics allow even more. I once met a man who always wore a lightweight, short sleeve cotton shirt while tended a golf course in Arizona. So much sun light penetrated his shirt you could not even see a tan line on his arms.
Sun protective clothing is determined by an independent laboratory. Light is beamed across the fabric and the amount of UV radiation that passes through the fabric is used to determine that fabrics UPF rating.
How is fabric made to be sun protective fabric? Most sun protective fabrics are tightly woven/knit and may (or may not) be chemically treated with UV inhibitors. Fibers from cotton to polyester can be used, with nylon being the most common. (Interesting note: Cotton is the least UV protective of the fibers with Lycra being one of the best for UV protection.)