Sunscreen: Keeping the old UVBs out

Project 365 #127: 070510 Those Little Essentials by comedy_nose on Flickr. Used under Creative Commons License. Closeup of bottle of SPF50 sunscreen for children


Last week we talked about sunburn, how a form of ultraviolet light called UVB damages DNA, which causes inflammation in the skin, resulting in redness, heat, and pain.

Time to talk about what we can do about it….sunscreen!

All of those sunscreens on the market do one of two things (sometimes both of them): absorb incoming UV light or reflect it. There are 17 different FDA-allowed compounds that manufacturers can use as the active ingredients in sunscreen.

Catch and release

The UV absorbers, which have melodic names like cinoxate, homosalate, sulisobenzone, or tolamine salicyalte, are generally aromatic compounds with carbonyl groups attached. You’ve met carbonyl groups before in my post on fats — they’re one of the building blocks of fatty acids.

And while we associate the word “aromatic” with something that smells nice, chemists use that word in a different way. For a chemist, aromatic has nothing to do with smell, but describes organic molecules in which some of the carbon or other atoms are arranged in one or more six-sided rings.

Some aromatics absorb UVA, some UVB. When a molecule of one of these compounds is hit with UV light, the electrons in its atoms absorb the energy and get all excited — something electrons don’t like to be. To calm themselves down, they release that energy as heat (which is another way of saying they release it as infrared light — so UV absorbing sunscreens basically turn high-energy UV light into lower-energy IR light).

Solar barricade

Side by side photos in visible and UV light of a man with sunscreen on his face

If you look at the same person in visible and UV light, can you tell which side of his face has been sunscreened? (Spigget/Wikimedia Commons)

The two widely used UV reflectors are titanium dioxide and zinc oxide. These metal oxides (aka molecules with atoms of a metal bound to one or two atoms of oxygen) help form thick, opaque creams that block out UV altogether. As such, they can really be called sunblocks instead of sunscreens.

Both are considered pigments in their own right, like the melanin in our skin. Titanium dioxide, in the form of titanium white pigment, has been called the “whitest of whites.” And in that form , it’s found in paints and even food.

And as a side note: zinc oxide + iron oxide (aka rust) = calamine lotion, that great reliever of bug bites and rashes. It’s also antibacterial, and is added to a broad range of materials and products for its chemical, electrical, and physical properties.

The right amount

The best sunscreens block both sunburn-triggering UVB light and also UVA light, a higher-energy form of UV that doesn’t cause sunburns, but which penetrates the skin more deeply than UVB and with long-term exposure can damage and age skin.

So how much sunscreen is enough? The FDA-recommeded dosage of sunscreen is 2.2 milligrams per cubic centimeter (mg/cc) of exposed skin. (Which Wolfram Alpha translates to 0.00127 ounces per cubic inch.) Or, as Wikipedia puts it:

Provided one assumes an “average” adult build of height 5 ft 4 in (163 cm) and weight 150 lb (68 kg) with a 32 in (82 cm) waist, that adult wearing a bathing suit covering the groin area should apply 29 g (approximately 1 oz) evenly to the uncovered body area. Considering only the face, this translates to about 1/4 to 1/3 of a teaspoon for the average adult face.

Doing the SPF math

We should touch on the ubiquitous sun protection factor, or SPF, for a moment. SPF is a measure of how effective a sunscreen is. And it’s calculated basically by letting pasty people get sunburns.

The accepted — and FDA required — method for determining a sunscreen’s SPF is to take 20 people, all of whom are sun-sensitive, shine a UV light on them (it’s actually a device called a “solar simulator”), and see how long it takes for them to get a sunburn. From this you can calculate a “minimal erythemic dose,” or MED. They then put on the sunscreen being tested, go through the same process again. Divide the sunscreened MED by the unprotected MED, round down to the nearest five, and voila, you have your SPF.

(If you want to read all of the gritty details about SPF testing, I direct you to the FDA’s announcement in the Federal Register of Labeling and Effectiveness Testing; Sunscreen Drug Products for Over-the-Counter Human Use, and in particular the addendum to 21 CFR 201.237. Start on page 42. It’s a fascinating read. Not really.)

Don’t we have built-in sunscreen?

We do — it’s the melanin in our skin. But it can only do so much, especially if you’re fair-skinned.

Melanin is a pigment produced by skin cells called melanocytes, and is stored in intracellular packets called melanosomes. It’s also found in a couple of other places in the body, like nerves in the brainstem or adrenal glands, and also helps color our irises and hair. There are a couple of different forms of melanin, but the most common is called eumelanin.

So how much protection do we get from melanin? It depends, naturally, on how much we make. Everyone has roughly the same concentration of melanocytes in the skin — that is to say, roughly the same ratio of melanocytes to other cells. But the genes that control melanin production are more active in some people than in others, explaining in part the variety of skin tones seen among the human species.

Which leads to the question…

…do black people and others with dark skin burn? Yes. Emphatically yes. Don’t listen to what Kanye West says. Someone of African descent may be able to take more UVB exposure and stay out in the sun longer before getting a sunburn, but they’ll still get burned.

ocean sunset

(Axel Schwenke/Flickr)


2 responses to “Sunscreen: Keeping the old UVBs out

  1. Pingback: Vitamin D versus sunburn: A tale of skin tones | You've Got Some Science On You·

  2. Another question for your blog: why are we ticklish? Why some people more than others — or why can some of us control our response to being tickled better than others? Why are certain areas (neck, belly, back of the knees, bottom of the feet) so much more ticklish than others? And why can’t we tickle ourselves? Does being ticklish serve any kind biological purpose or function, or is it some evolutionary leftover like an appendix?

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