There are two flavors of ingredients in sunscreens that gets that UV-blocking business done: the ‘organic’ blockers and ‘inorganic’ blockers. In this case, ‘organic’ doesn’t refer to how it’s produced, but rather what they are: organic blockers are carbon-based molecules, similar to the molecules that make up living things – these are also referred to as ‘chemical’ blockers. Inorganic blockers are metal-based compounds like titanium dioxide and zinc dioxide; these are also referred to as ‘physical’ blockers. These two kinds of blockers stop UV rays in different ways: the organic (chemical) blockers catch UV rays and transform them into heat; inorganic (physical) blockers do this too, but they primarily deflect and scatter UV rays. There has been some concern that sometimes when UV rays hit *specific kinds* of these organic blockers, the energy in the UV rays causes the molecular structure of the blocker to change into a different form which may be carcinogenic with long-term exposure. The most suspicious of these blockers are no longer regularly used in sunscreens, but these concerns have caused a rise in the popularity (or at least marketability) of sunscreens relying exclusively on titanium- and zinc-oxide blockers.
Old school titanium- and zinc-oxide sunscreens were opaque white (or with added colors). They were very effective at reflecting and deflecting the broad spectrum of UV, but unless you wanted an 80′s-pseudo-tribal look, the bright white color was a little unappealing (fun fact: titanium dioxide is also used as the pigment in white paints because it reflects light so well). However, when these particles of titanium and zinc oxides are made into nano-sized particles, they don’t scatter visible light as much–appearing tran
sparent–but they still block UV. Since these nano-sized particles behave a little differently than the larger particles, there were some early concerns about their safety, but so far a lot of research suggests these compounds are safe and don’t penetrate the outer protective layers of the skin. Titanium- and zinc-oxide sunscreens have another plus for those swimming in tropical waters: many of the organic UV blockers may damage coral reefs, so if you are swimming near those you should choose an inorganic-blocker sunscreen.
PS: For further reading about sun protection and the active ingredients in sunscreen, I recommend this review paper: Wang, S. Q., Balagula, Y., Osterwalder, U. Dermatologic Therapy, Vol 23, 2010 31-47 (if you don’t have access online, you may be able to get access through your local college library).
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