Many outdoor clothing items advertise themselves with a UPF (Ultraviolet protection factor) of 30 or even 50. How does that compare with a normal cotton T-Shirt or a pair of jeans? Do I have to use sunscreen even when using these clothes? Or is it all a marketing scam?
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The short answer is, it varies. The three factors that most influence the UV transmission factor of clothing are kind of obvious:
In addition, there are also "invisible dyes" that can absorb UV without darkening the visible color of the fabric. Some UV-absorbent laundry additives are sold specifically for that purpose, but many laundry detergents also contain fluorescent compounds called optical brighteners that absorb some UV light and re-emit it in the visible spectrum. Also, getting the fabric wet will change its UV transmittance, typically increasing it.
For some numerical values, I did a quick search on Google Scholar and found a paper titled "Clothing as protection from ultraviolet radiation: which fabric is most effective?" by Sandra Davis, Linda Capjack, Nancy Kerrand Robert Fedosejcvs, published in 1997 in the International Journal of Dermatology 36(5). Alas, the on-line version of the paper is behind a paywall, but I'll quote a few relevant sample SPF values from their table 3 below, courtesy of my university library's on-line subscription:
The measured SPF value of 166 for blue denim is basically off the scale: for practical purposes, blue jeans don't let any UV through at all. The relatively low SPF of the white cotton twill is a bit surprising, though, and indeed is what prompted the authors to test the effects of the blue dye:
Still, even an SPF of 12 is nothing to sneer at, particularly since the SPF values for clothing are not subject to the same kind of inflation as those marketed for dermal sunscreens (which are typically only valid if the product is applied thickly and frequently reapplied). You might be able to tan a little bit under white jeans if you spend a lot of time in sunlight, but probably not so you'd notice. The SPF 4 measured for the white T-shirt fabric is pretty low, though: it's a lot better than nothing, but it might not be enough to prevent sunburn if you spend a lot of time outdoors.
However, this study did not cover the effects of laundry additives; for that, we can look at "Improving Knit Fabric UPF Using Consumer Laundry Products: A Comparison of Results Using Two Instruments" by Jihyun Kim, Janis Stone, Patricia Crews, Mack Shelley II and Kathryn L. Hatch from 2004.
In this study, the authors tested the effects of two off-the-shelf laundry detergents containing optical brighteners, one laundry additive sold as an optical brightener and one sold as a UV-absorber. While their numerical values don't seem directly comparable, they do report a more than 3-fold increase in SPF for all the products, comparable to the effects of the blue dye in the Davis et al. study, although, for the detergents, the full effect was only achieved after the cloth had been washed over a dozen times.
So my rough, unscientific conclusion from all this would be that, if you're planning to wear jeans and a white T-shirt while hiking in the mountains or sailing, you should either buy a special UV-protective T-shirt, wash your normal T-shirt with a UV-absorbent laundry additive, or just apply sunscreen under it. But you don't need to worry about the jeans — there's no way you'll get sunburn under those.
this link seems to suggest 7 when dry, and 5 when wet. No word on where those numbers are obtained from, but my gut instinct was about 5-10 so that would be about right in my mind.