Sun protection from UVB rays provided by clothing and fabrics is expressed in terms of a UPF rating similar to the SPF rating of clothing.

In very hot weather, wear loose fitting, tightly woven, clothing. The folds of loose fitting clothing provides sort of a "double layer" of fabric, thereby doubling the SPF of the clothing. The tight weave helps reduce UVR transmission when it's too hot for more effective heavier-weight fabrics.

We received a Question about the SPF Rating of our clothing so here is some information I found during my research which I found very interesting!:

Factors Affecting Fabric UV Protection Levels

There are a number of factors that affect the level of ultraviolet protection provided by a fabric and the UPF rating.
In order of importance these are:

   1. weave (tighter is better),
   2. color (darker is better),
   3. weight (heavier is better) also called mass or cover factor,
   4. stretch (less stretch is better) and
   5. wetness (dry is better).
   6. addition of chemicals such as UV absorbers or UV diffusers during the manufacturing process.

Source: Coolibar Sun Protective Clothing -UPF Ratings

Sun Protection Methods

Research on clothing and UVR protection has found that:

• One hundred percent (100%) polyester has the highest SPF of all fabrics, providing 2 to 3 times more sun protection that any other fabric.
• Darker colored clothing has a much higher SPF than light colored clothing (black clothing provides 5 times more protection than white clothing - a white T-shirt has only a 5 SPF rating).
• Knits have a higher SPF than woven fabrics, and nylon spandex knits have a very high SPF, regardless of color.
• The heavier the weight of the fabric, the higher its SPF. A double layer of fabric almost doubles the SPF of clothing. This is true regardless of fabric type (the same for cotton, silk and polyester, for example).
• More tightly woven fabrics help reduce UVR transmission to the skin, but not as effectivley as heavier-weight fabrics.
• Wet fabrics INCREASE the amount of UVR that reaches the skins - by as much as one-third.
  

Source: Belize Vacations: Sun Protection Advice for Belize

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