How good are clothes at protecting you from the sun?

There are actually a number of studies looking at the UV protection factor (UPF) of clothing. Europe even has a standardized method of determining the UPF of clothing using spectrophotometry. It is recommended by the European guidelines that clothing have a UPF of 30 or higher for sun wear.

Summary:
1. Cotton, linen, and viscose are not great at sun protection
2. With UV-protective coating, the UPF of viscose greatly increases
3. Water can either increase OR decrease the UPF of clothing, depending on the material
4. Cotton's UPF can be increased via washing with soap and water (tightens the weave) or by washing with the UV-protective coating

I found one study that measured the UPF of various summer clothes. Clothing made from cotton, linen, and viscose were most likely to have UPF less than 30. But if they were black, navy-blue, white, green, or beige, they had higher UPF values in general. These were clothes taken from the summer collection of a department store. I don't really understand how white clothes can have higher UPF ratings unless the material is actually thicker or more tightly woven because it is white.

BGambichler T, Rotterdam S, Altmeyer P, Hoffmann K. Protection against ultraviolet radiation by commercial summer clothing: need for standardised testing and labelling. MC Dermatol. 2001;1:6.

In a study comparing normal viscose with viscose that is treated with special UV-protective coating, a statistically significant difference was found between the two. The UV protective coating was found to be effective in raising the UPF of the lightweight fabric, suggesting that wider usage of the coating is in order. Also, the in vivo (in life) and in vitro (in the laboratory) measurements of UPF varied depending on the material and whether or not it was treated. So the UPF measurement is not necessarily accurate, depending on the material.

Hoffmann K, Kaspar K, Gambichler T, Altmeyer P.In vitro and in vivo determination of the UV protection factor for lightweight cotton and viscose summer fabrics: a preliminary study.J Am Acad Dermatol. 2000 Dec;43(6):1009-16.

This next study looks at the effect of water on the UPF of clothing. For linen, viscose, and polyester fabrics, UPF significantly increased when wet. For the cotton fabrics and the polyester + TiO2 fabrics, UPF significantly decreased. For the modal + TiO2 fabrics and the polyester crepe + TiO2 fabrics, UPF significantly increased. Material should be tested both wet and dry to elucidate the effects of moisture on UPF.

Gambichler T, Hatch KL, Avermaete A, Altmeyer P, Hoffmann K.Influence of wetness on the ultraviolet protection factor (UPF) of textiles: in vitro and in vivo measurements.Photodermatol Photoimmunol Photomed. 2002 Feb;18(1):29-35.

"Laundering with detergent and water improves UPF slightly by causing fabric shrinkage. Dyeing fabrics or adding a UV-absorbing agent during laundering substantially reduces UV transmission and increases UPF. More UVA is transmitted through the fabrics than UVB."

Wang SQ, Kopf AW, Marx J, Bogdan A, Polsky D, Bart RS.Reduction of ultraviolet transmission through cotton T-shirt fabrics with low ultraviolet protection by various laundering methods and dyeing: clinical implications.J Am Acad Dermatol. 2001 May;44(5):767-74.