Re: do clothes like solarweave and solumbra really block out uv rays ?

I must admit I was unfamiliar with these products until you asked about 
them, but a quick tour of their Websites gave me an idea of their use and 
construction.

UV blocking is not that difficult.  Even a thin layer on a pair of 
sunglasses can block (reflect) or absorb UV light.  The trick is to do it 
without either destroying the fabric (by absorbing UV light) or reflecting 
it in a disturbing way (imagine a T shirt that was reflective like a pair 
of mirrored sunglasses) or being so bulky that you felt hot in it.

If you stood under a mesh screen, part of the screen would block the sun 
and the holes would let the sun through.  If you make the screen fine 
enough, more sun gets blocked.

If you add a second screen over the first, part of the sun that gets 
through the first screen will be blocked by the second.  Adding a third 
screen would stop even more.

Fabric is like a screen in that it is a weave of fibers.  If you make the 
weave fine or add layers, it will block the sun out.  The trick is to make 
it "breathe" by letting the moisture from your skin escape, otherwise you 
will feel hot.  Try wearing a nylon jacket in the sun and you will quickly 
know what I am talking about.  The fiber doesn't let your skin breathe like 
a cotton T shirt. Making the holes too small prevents the moisture from 
getting out.

The fabric must also withstand any UV it absorbs.  I have seen fabric left 
out in the sun for days and weeks fade and degrade with the solar exposure 
here in Florida where I live.   I imagine that these special fabrics are 
treated in such a way to avoid this.  See the web site for solarweave.

The advantage for people who are sensitive to skin cancer, etc is that they 
don't have to wear lotions that can sweat away after exposure.  But they 
are expensive clothes.  When you have had a bout with skin cancer, it 
probably makes sense even at $75 for a shirt.  Could you get the same 
protection by wearing 4 or 5 cotton t shirts on top of each other?  
Perhaps, but you wouldn't be comfortable in the hot sun either.