Science question

Here is something I have been contemplating for a while now: how does sunblock work? I burn pretty easily and, sometimes, pretty badly. I have been very careful to lather myself in this cream every morning and I am not sunburned. But, I just don’t get how a creme that my skin absorbs blocks the sun.  Any explanations beyond ‘it is magic’?



  1. There are 2 different ways that sunblock can work: chemical, and physical.

    95%+ of the sunblocks you buy work chemically. I can’t explain it beyond that, except that my dermatologist told me, and I am allergic to whatever the chemistry is. I think it is some kind of chemical reaction that actually needs the sun to do anything, but that’s a theory based on my not getting a rash from chemical sunscreen unless I’m actually in the sun, not based on anything from an expert.

    Physical sunblocks work like they sound. Old fashioned zinc oxide is the best known example — think of people at the beach with big white smears on their noses.

    Modern chemistry and the allergy-management industry has actually managed to make physical sunblocks that are more-or-less invisible, allowing people like me to enjoy beach vacations and walks in the sun. The only brand name with which I am familiar is “vanicream” and if you are reading this and considering trying it, I recommend the “sport” style (SPF 35) as the others rinse off too easily when you sweat.

  2. Oh yeah, and different kinds of clothing material have different SPFs. If you do your laundry in these “brightening” detergents, it actually increases the reflectivity of the cloth, and hence, the SPF.

    I have a lightweight beach cover up that we jokingly call my “beach burqa” which is SPF 25 and covers all but my face, hands, feet & ankles, but you can get specially designed clothing with SPF up to 50+.

  3. I find myself completely unsurprised that you are the one to know the answer to this Liza. Thank you. I have never been a big sunscreen person, partly because i have never been a big out in the sun person. However, I am experimenting with several here. I seem to be doing best with the Aveeno hypoallergenic one. Everything else makes me break out like crazy. The other thing that has been helping in the not getting burned category is that the sun sets so early so I am out and about in it less. The downside is that I walk around in the dark a lot.

  4. Clare,

    Zinc oxide works as Liza described think tin foil, but newer ones absorb the Ultra Voilet (longer) rays of the sun more like bounty. The energy dispersed in the process is done so through molecular changes in the chemical absorbing them.

    The process of rubbing a lotion on your skin, it could be any lotion, (my father often advocated seasoned manure products unseasoned manure can burn plants too much uric acid), reinvigorates the melanin in your subcutaneous regions which in turn stimulates your sweat glands, causing your lower large intestine to absorb more water (if the upper intestine absorbed the water you would get impacted so stay hydrated) causing greater thirst and making your go to the bar, cabana, tienda, bodega, vending machine, wateringhole, oasis, shady spot with running water where you might get bit by a insect, causing you to once again rub lotion on your skin stimulating your subcutaneous……..

    The easiest explanation: it’s magic.

  5. Steven, yes, you wouldn’t have to worry about that now would you? That said, if you ever have me over for coffee in Kenya, I will tell you all about how I can look like a tomato in a number of hours.

  6. Slight edit: UV rays are actually shorter. Infrared rays are longer. (UV have a higher frequency, frequency x wavelength = c, c is a constant…have fun math types!) Also, apparently most sunscreens are a mix of organic and inorganic materials. The inorganic materials scatter the UV rays and the organic ones absorb them and dissipate the energy as heat. This is important because UV light’s energy can directly damage DNA, but more often produces oxygen radicals that are capable of harming most cell components and lead to cancer. And sunburns.

    Clare, you knew I’d have to chime in.

  7. Bif- Yes. I knew you would have to chime in. I just thought it would be sooner and there would be more big words that I had too look up in the dictionary. Hahaha. Thanks for bring out the math nerd in me though!

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