1:40 p.m. on April 29, 2012 (EDT)
Something everyone who took high school or college chemistry should know and remember - if it is matter and not energy, it is "chemical". There is no such thing as a sunscreen or any other substance that is "chemical-free". What people usually mean by this misnomer is that the sunscreen does not contain certain organic compounds (notably PABA).
The most effective broad-spectrum sunscreen ingredients are zinc oxide and titanium dioxide. When I started high altitude climbing, we used to use "Clown White" make-up, which is zinc oxide in some sort of base (usually very oily/greasy). You can get straight zinc oxide ointment at your friendly local drug store a lot cheaper than the brand-name sunscreens. Note that the term "sunblock" is deprecated, and in fact currently NOT approved for use on sunscreen products.
SPF refers only to blocking one part of the UV spectrum (UPF is the acronym used in many European countries). Unfortunately, this is the part of the UV spectrum that produces tans, and NOT the part that promotes skin cancer. So look for "broad spectrum" on the label. ZnO and TiO2 are broad spectrum by their basic nature, so look for products that contain one or both. It should be noted that many of the current sun products containing the oxides of zinc and titanium have those encapsulated as nano-particles. There is still a lot of controversy on the safety of nano-particles, since they easily penetrate cell walls. So far, no scientific proof of any risk, though.
The UPF and SPF number refer to the factor by which you can extend the sun exposure compared to bare skin. So SPF 30 means that in 30 minutes you get the same UV with the product compared to 1 minute on bare skin. However, because some of the products are absorbed into the skin and all of them wear off and lose effectiveness with time, you still have to renew every couple hours (that includes Clown White).
If you read the pamphlets passed out by your dermatologist, you will see that the recommended amount to apply is far far greater than most people apply - a small shot-glass full to cover the face, ears, and neck (that's a big tablespoonful). If you put a half-teaspoon on (the amount most people seem to apply), your SPF30 is the equivalent of SPF5 or less. According to my skin docs, the vast majority of people neglect to properly coat their ears. On snow, remember the underside of your chin and nose, as well as wearing appropriate sun glasses on snow (glacier glasses at high altitude - "snow blindness" is sunburned eyeballs). The most painful burn of all is the roof of your mouth, as my partners and I discovered on Denali, when we were "mouth-breathing" and didn't realize how much sun gets reflected from the glacial ice.
In my personal experience at high altitude (18-20 thousand feet and higher), there is a definite difference between SPF30 and SPF50 or SPF80 (highest I can find on a regular basis. Consumer Reports and other publications note that SPF 30 blocks something like 90%, while SPF80 blocks something like 95%, only 5 percentage points more. Yeah, but ... at 18,000 ft, the UV is something like 10 times more intense. So blocking 90% leaves something like 10% of the intensity, which at 18kft leaves you with the sea level amount gettin through. Thus blocking 95% gets you down to half the amount at 18kft than you get on bare skin at sea level. So for us blonde, blue-eyed "white-fellas" do gain a significant benefit (ok, so my hair, what little is left, is white with some strands of grey - which means I need a hat all the time).
My personal practice is to wear long pants and long-sleeve shirts with head coverings all the time in the mountains, especially when on snow. I didn't do that when I was growing up in the Sonora Desert. And we used to say that you might get a sunburn the first couple days in the Sierra, but that would turn to a tan by the 3rd day, so no need for sun cream (most sun creams in those days were intended to speed up tanning!). And now, I visit my dermatologists every 3 or 4 months for liquid nitrogen treatments, biopsies, and the occasional removal of a basal or squamous cell (no melanoma so far, but Barb's father died from melanoma that metastasized). Yes, that is "dermatologists" plural. One does the month to month stuff, the other removes the active stuff (does a great job sewing up the craters left by removal).
Another thing to consider if you go to Africa or someplace else where you are taking medications - if you are taking an antimalarial (which you should in most equatorial countries), a side effect is increasing sun-sensitivity. Other drugs have the same side effect. Read the label on all medications carefully!
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