9:25 p.m. on August 10, 2012 (EDT)
purplechesscat said:
....I loaded up on SPF 50 and SPF 100 and lip protection with spf 45. I don't want to get all red out there.
I grew up in the middle of the Sonora Desert (a village on the Gila River Reservation until I was 12, then in the Big City of Phoenix, 150,000 at the time). All us kids "grew up to be cowboys", and to answer your question about long sleeves and such, we wore long sleeves, bandannas, and broad brimmed hats for sun protection, not just to look like the movie cowboys.
Now, about the SPF stuff - in those days (1940s and 50s) we all believed in "healthy tans". The stuff sold for out in the sun was "suntan lotion", intended to enhance the tan that "everyone should have and get lots of vitamin D". I and my compadres are now paying the price. I make regular (3-month schedule) visits to my dermatologist who sprays me with liquid nitrogen and cuts off pieces of skin. So for a while I used "sunblock" until the the feds outlawed the term. "Sunscreen" is still an allowed term.
Now, the truth (ok, the "truth", since the story keeps changing by the month) about SPF - I had a long discussion with several of the sun protection manufacturers at the OR Show, plus have been keeping up with the literature (guess why? My dermatologist keeps buying Ferraris. Add to that the rather disturbing fact that Barb's father died from melanoma that metastasized, so we are running scared). The UV spectrum is talked about as if there are two distinct and separate kinds of UV rays. Wrong! The ultraviolet is a continuous part of the electromagnetic spectrum. For convenience in designating what part is more or less penetrating and has more or less harmful effect, it is divided into three segments - UVA (wavelength range 400-315 nm, which us Old School types think of as 4000 to 3150 Angstroms), UVB (315-280 nm), and UVC (280-100 nm). If you are interested in what wavelengths birds see, for example, there are other ways of dividing up the UV spectrum. The UVC portion is used for germicidal purposes (for example in the SteriPen and AllClear water purification systems), and is of no real concern - the segment from 200nm and shorter is virtually all absorbed by the oxygen in the atmosphere. Well, actually, UVC is now also considered to be of concern - for one thing, do not activate your Steripen, AllClear, or fancy "black-light" psychedelic light show when anyone can receive the "illumination" directly of by reflection. And stay far far away from tanning beds - the skin cancer rate, especially melanoma the super-deadly kind of skin cancer, is skyrocketing especially among young women under 25, and including teenagers.
UVA mostly just activates the melanin in our skins, producing that wonderful suntan, though when you get too much (as us blonde blue-eyed types get in just a few minutes a day at sea level, and in seconds at high altitudes), you can get several forms of skin cancer, or at least, like real cowboys (and even movie cowboys like Clint Eastwood), LOTS of skin wrinkles. UVA causes indirect damage to the DNA of the skin surface layers, which can end up with skin cancers. The melanin is activated by the UVA, not produced. So while some protection is provided by the activated melanin, there is a limit, especially in fair-skinned people. UVB is much more effective at producing skin cancers, especially melanoma (and if you believe the canard that black folk do not get skin cancer, I had a good friend and colleague when I was a university professor in Mississippi who got melanoma, despite being a very dark-skinned African-American - luckily it got caught in time). UVB causes direct breakage of DNA strands.
And, by the way, exposure to UV helps promote glaucoma - wear your sunglasses (and at altitude, your glacier glasses and/or ski goggles with sufficient blockage, including side shields).
So, SPF is not the full story. You need to read the label carefully to see that the sun cream is "wide spectrum". SPF is based on the UVA reduction only on most of the sunscreens available in the US. There has been a move toward a modified SPF and several other designations that would reflect the whole UV spectrum. The formal definition of SPF is the following formula:

However, I am sure no one here is going to bother (or be able) to gather the data on their favorite sunscreen to feed into the formula. So, a few rules of thumb:
SPF30 blocks over 99% of the UV. Going to higher SPF gets a little closer to 100%, but SPF100 is still a half percentage point from complete blockage.
Since SPF is non-linear, applying a double coat does not double, nor does it even multiply. For example, two layers of SPF 10, does not give SPF 20, nor as you might expect, does it give 10x10 or SPF100. This is because of the variation of the sunscreens across the UV spectrum.
You should always renew the sunscreen every 2-3 hours, especially at high altitudes or where you get lots of reflection (off water, light-colored sand, or snow or ice on a glacier).
Titanium dioxide and zinc oxide are very broad spectrum and good ingredients to have in your sunscreen. However, there is some evidence that TiO2 and ZnO in nanoparticle form can enter skin cells - it is not clear what risk is associated with this.
When I first started going to high altitudes (above 5000 ft), I had old-timers (yes there were lots of old-timers even before me) tell me to use
Clown-White grease-paint. Mostly we used it on our noses. That looked weird, but not as weird as covering the whole face. Clown-White is zinc oxide in a grease-paint makeup base. It does work, but it is really messy.
Wear long sleeves and long pants, and in hot areas, use microfiber or other wicking materials. It seems non-intuitive, but have you noticed what the desert people of the Middle East wear? Most wear long white or light-colored robes, which do a good job of blocking the UV and reflecting the infrared (heat rays) as well. Watch Lawrence of Arabia on the TV re-runs sometime.
The Arab robes are loose to allow lots of air circulation. Modern wicking materials will wick away your sweat, providing a cooling effect (growing up in the desert, we had "swamp coolers" before refrigerated air conditioners became inexpensively available - they use the evaporation of water for cooling - hmmm, that was before all the people moving to Phoenix and Tucson from the East had to have "English lawns" which boosted the summertime humidity from 5 or 10% to the 30% range - only remedy now is air conditioners that use greenhouse gases for their cooling fluid).
One thing, though, about SPF - at low altitudes, anything above SPF 30 is pretty much a waste of money, as long as you apply the right amount (a shot-glassful) and renew every 2-3 hours, more frequently if you go in the water, since no sunscreen on the market now is truly "waterproof". When you go above, say 15,000 ft as I frequently do, then SPF 80 or 100 does make a difference. That's because the air above you is thinner and does not reduce the UV as much as the 15,000 ft of air below you (18,000 ft is the half-way point is air mass above and below you). Still, you must wear clothing that filters the UV, sunglasses, and a hat (especially for us follicularly challenged folks.
Oh, and as DrReaper noted, keep your alcohol consumption way down on very hot days. Although a small amount of beer does speed up the hydration, more than a little tends to start blocking rehydration. I will leave the discussion of how that works to others.