Lots of times when a hiking shirt, let's say, is marketed as a "desert" shirt, or a shirt for hot / summery use in general, the matter of SPF, or UPF as I guess they're calling it now, comes up.
Now, if the shirt is long-sleeved and features a stand-up collar, then we can all readily see how that shirt - as a simple physical barrier to sunlight - will help guard against getting a sunburn. But usually, regardless of the shirt's specific feature set, these garments will be marketed as offering a specific UPF - something convincingly impressive like a 30x or 50x protection factor - with the implication being that the fabric features a tight weave that the sun's rays can't effectively penetrate.
What the marketing tends to omit, however, is that this tight weave is generally undesirable from the standpoint of ventilation and thus cooling, as it simply lacks good air flow and, often, moisture transport and evaporation. As if tacitly acknowledging the problem, those shirts of the genre that are marketing most aggressively at the "desert user" will tend to feature mesh panels along the sides, or a gusseted mesh port on the back, deep front zip, etc. These features will naturally tend to increase the garment's weight, complexity and therefore cost, but not always its objective ability to ventilate and move moisture. Sometimes this point may be overlooked by the wearer, however, specifically because of the marketing. And then there's the matter of all that mesh and various gizmos that in theory *should* work, and therefore, since we bought into the marketing, must be working. And that, by inference, the more simply featured garment must therefore be the inferior.
Now again, this is all founded on the premise that the tighter weave shirts are necessary in order to have us survive a full day in hot sunny weather without getting, well, burned. If we ignore this possibility for a moment, and just seek to find the lightest, most breathable, best wicking shirt - the one that actually performs best in all around hot weather, be it humid or dry, while we're exerting heavily, I would hazard to say we'd generally end up with one of the ~4 oz loose weave knits, and to heck with the tightly woven fabrics.
I know, I know. What's with this "we" business. Isn't comfort, just like fashion, entirely subjective? Well, okay. But will someone - anyone - who's actually gotten a sunburn while wearing the breeziest shirt in their closet please tell their story?
I just don't think I buy the hype about sun shirts. I think these companies are trying to scare me. And as a fair-skinned lad who happens to be infatuated with hot weather and desert travel, experience simply doesn't bear out the marketing claims for me.