BehindTheMedspeak: Prepare to sweat — dark colors block UV rays better than light ones

Long story short: Red and blue fabrics protect better than yellow when it comes to blocking UV-B rays.

Here's Henry Fountain's October 20, 2009 New York Times "Observatory" feature with more.

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Color of Fabric Matters When Protecting Sun From Ultraviolet Rays

It takes more than sunscreen to keep the sun’s ultraviolet rays from harming your skin. The type of clothing you wear can offer protection, too — or not. Studies have shown that some lightweight fabrics do not provide enough UV protection.

But it is not just the type of fiber and the weave of the fabric that matters, but also the color. Ascención Riva of the Polytechnic University of Catalonia and colleagues have addressed the color issue, studying the effects of different dyes on the UV protection provided by lightweight woven cottons.

The researchers chose three fabrics, not dyed, with different initial levels of UV protection based on the weave and other factors. Then they dyed them in varying shades of blue, red and yellow and measured how much UV radiation was absorbed and transmitted.

They found that red and blue shades performed better than yellow, particularly in blocking UV-B rays, which are the most harmful. Protection increased as the shades were made darker and more intense. And if the initial protection level of the fabric was higher, the darker shades offered even greater improvement.

The researchers say the findings, reported in Industrial and Engineering Chemistry Research, should help fabric and garment manufacturers optimize their products for UV protection.

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Here's the abstract of the article noted above.

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Modeling the Effects of Color on the UV Protection Provided by Cotton Woven Fabrics Dyed with Azo Dyestuffs

Color is one of the most influential variables on the protection against ultraviolet radiation provided by a fabric. The protection against UV radiation of a fabric is expressed quantitatively by means of its UPF (ultraviolet protection factor). The present paper shows the results of a study about the influence of the color on the UPF of cotton woven fabrics appropriate for summer articles. Through statistical models, the research seeks to relate the level of protection achieved in dyeing with three azo dyes to the factors governing the process, the shade, and the color intensity, as well as their interaction with the initial UPF of the fabrics. The obtained models will allow the programming and optimization of the value of the UPF of a fabric by means of adequate adjustment of the variables.

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Answer here this time tomorrow.