Lately I’ve been seeing specialty clothing with SPF ratings like this one
http://www.coolibar.com/01800.html
What’s the deal? Has anybody ever been sunburned through (non-wet) clothes?
Is it just that such clothes are cool enough to wear long sleeves even in the heat?
Or is it just marketing?
As I’ve had skin cancer before, and I’m out every day in the Arizona sun climbing mountains, my dermatologist recommended this stuff several years ago, and I use it all the time.
I have one shirt from the site you linked, but before bought several items form an outfit called Sun Precautions (800-882-7860). Their website is here
I have seen no studies on the efficacy, but I’ve been wearing pants, shirts and cap with neck drape for years now. No sunburn, no cancer, it sure seems to work.
This stuff is obviously hotter than shorts and T-shirt, but not so much that I can’t stand it (hey, I climb in +100 degree F days). The shirts have mesh vents.
Oh yes. If you’re out on the boat all day on the Fourth of July, and you’re only wearing a t-shirt, oh yes, you can get sunburned right through the t-shirt. Cite if ya don’t believe me. 
But it isn’t hard to make fabric absorb ultraviolet light well. Things in nature most often have colors that absorb shorter wavelength and not longer ones, relative to other colors. That is, there are reds and oranges and yellows and browns. The challenge with ultraviolet light is not absorbing it. It’s hard to make things very white, or other colors that don’t absorb blue, let alone nonabsorbent of ultraviolet which is on the far side of blue. Even things like paper and our eyes yellow with age - that is, they gradually cut off radiation shorter than a wavelength limit that gets longer with age.
A normal T-shirt only has an UPF of 4. I’ve been burned through my jeans here in Tucson.
There are several companies that sell the protected clothing, including Sportsmen’s Warehouse, Solumbra, and our local University hospital’s Cancer department. They do tend to be expensive.
An alternative is RIT’s protectant that you put in the washing mashine with your clothes. It’s cheap, effective, and lasts something like 25 washes.
A friend of mine is the recipent of an organ transplant. Something about the meds required to maintain that leaves him very suseptable to damage from sunlight, to the point where if he is going to be outside, he almost always wears long pants and long sleeves. He has a some of these type of shirts for times when he will be outside, but not in the shade.
As in KlondikeGeoff’s case, it’s not always a sunburn that is the problem. For some people, even non-burning levels of sun can be a problem.
Considering the number of “whiteshirted”, “cottontailed”, “whitesocked” individuals I see at the beach regularly, ain’t most clothes sunblocking anyway?
As Alice the Goon said, I have also heard that most clothing only has about an SPF 4. I’ve heard many dermatologists recommend sunscreen under clothing for people at increased risk for skin cancer. Just because you don’t look burned it doesn’t mean you aren’t getting sun damage.