We are serious about testing to assure your protection from the sun’s harmful rays. The fabric used in our hats is tested by an independent laboratory each and every time we order fabric for production. You can view an example of a testing report here: hat-fabric-test-results.pdf (PDF – opens in new window). Fabrics are tested to the specification ATSM D6603 Standard as developed by the ATSM .
All of our products are rated at UPF 50+ (Ultraviolet Protection Factor):
Relationship of UPF Value to Protection Category 
|Protection Category Designation||UPR Values||% UV Blocked By Fabric|
|Good||15 - 24||93.3 - 95.9|
|Very Good||25 - 39||96.0 - 97.4|
|Excellent||40 - 50+||97.5 or more|
Here are some typical questions and recommendations from the Skin Cancer Foundation .
Clothing is our first line of defense against the sun’s harmful ultraviolet (UV) rays and protects us by absorbing or blocking much of this radiation. The more skin you cover, the better. A long-sleeved shirt covers more skin than a t-shirt, especially if it has a high neckline or collar that shields the back of the neck. Likewise, long pants protect more skin than shorts.
UPF stands for Ultraviolet Protection Factor. It indicates what fraction of the sun’s ultraviolet rays can penetrate the fabric. A shirt with a UPF of 50, for example, allows just 1/50th of the sun’s UV radiation to reach the skin.
What Is the Difference Between UPF and SPF?
UPF is a rating generally used for clothing and other fabrics that protect you from the sun. It measures the amount of UVA and UVB radiation that penetrates the fabric and reaches the skin.
SPF stands for Sun Protection Factor. It is a rating for sunscreens or other cosmetic products containing sunscreen. Theoretically, the SPF number indicates how long you can stay in the sun before your skin reddens. SPF is only used to rate a sunscreen’s ability to protect against UVB rays.
A wide-brimmed (3-inch or greater) hat covers places like the scalp where it is difficult to apply sunscreen or areas where people forget to apply sunscreen, including the tops of the ears and the back of the neck.
 Source: http://ag.arizona.edu
 Source: http://www.skincancer.org