It's a familiar scene from childhood: you rush outside your house in the summer, or you bravely singe your feet walking across hot sand at the beach, and as you run off, your mother calls you back. She needs to put sunscreen on you. She pulls out a brightly colored tube whose back is covered in mysterious 6 syllable words. You stand with your eyes tightly closed as she rubs it on your cheeks, nose, ears, shoulders, neck and arms. Finally free, you scamper away to play in the sun. It's so commonplace that we rarely stop and think, “How does this work?â€, “What are those chemicals in there?†or “Has nature come up with anything better?â€
The first sunscreens were discovered in ancient times and were natural products like olive oil or other plant extracts. Zinc oxide sunscreen has also been used for thousands of years. These two types of sunscreens illustrate the two classes of sunscreen that still exist today.
The first type is organic molecule-based sunscreens, which contain molecules that do not interact with light in the visible range, but do absorb UV light. On the other hand, metal oxides, like zinc or titanium oxide, form a physical barrier and reflect UV as well as other light radiation. Since most commercial sunscreens are of the first type, we are going to look more closely at them.
Ultraviolet Light
Sunburn is caused by exposure to ultraviolet (UV) light. UV light is emitted by the Sun and is characterized by a wavelength of 100-415 nanometers.
Wavelength is one major way we characterize light. Light travels through space like a wave, and the wavelength is the distance from peak to peak of the light waves. 400 nm is very small, about 1/100 of the width of a human hair. As the wavelength of light gets smaller, it gets more energetic.
Luckily for us, atmospheric oxygen and ozone block out most types of solar radiation below 290 nm, where the most damaging types of radiation are. UV light has a bit of a love-hate relationship with biology; UV light degrades important molecules like DNA and vitamin A, but is also critical in the biosynthesis of vitamin D and its reflection is used social signaling in many species of birds and insects.
The part of the UV spectrum that can penetrate the atmosphere is further divided into UVA (320-400 nm) and UVB (290-320 nm). UVB is the more dangerous of the two classes of UV light encountered at ground level. All UV light causes damage by breaking covalent bonds. Covalent bonds are the most important (but not the only!) way the molecules in our body are held together. UV lights breaks covalent bonds in such a way that unpaired electrons are left on atoms. Unpaired electrons like these are better known as free-radicals, which are highly reactive. DNA is particularly sensitive to damage caused by free radicals, which is why too much UV exposure has been linked to cancer.
Organic Sunscreens
The most common organic sunscreens are derivatives or close relatives of two similar classes of materials: benzophenones and dibenzoylmethanes. These molecules protect your skin from UV light by absorbing it. The structure of a molecule determines what parts of the light spectrum are absorbed and how strongly, so many sunscreens use combinations of sunscreen molecules to get good coverage of both the UVA and UVB.
When a molecule absorbs light, it must then get rid of the absorbed energy somehow. The pathway that dibenzoylmethanes use to release energy is usually to emit it as heat; benzophenones become excited to higher energy states and react with nearby molecules. Since the top layer of skin is mostly dead cells, there is little damage done by reacting with the skin there.
There is some concern that organic sunscreen ingredients are potentially toxic when they are absorbed through the skin into the bloodstream, as some animal studies have indicated that they may act like hormones and trick some of the body's natural regulatory systems. However, a recent review found that on the whole, sunscreen formulations were very effective at preventing skin cancer and have not been directly implicated in adverse human health effects.
Nature’s Sunscreen
Recently, researchers have been turning back to biology to develop new sunscreens that have less potential for toxicity. Ocean life living near the surface has high exposure to the Sun's energy, so they have evolved to produce compounds that are exceptionally good at mitigating the harmful effects of UV light.
Such marine-derived compounds include a class of amino acids known as mycosporine-like amino acids (MMAS) that have been explored in the last decade as a candidate for naturally derived sunscreen. MMAS are difficult and expensive to extract from their native source (algae), so researchers have been looking at how to make them efficiently in the lab. These derivatives are promising, and were shown to be excellent both as UV absorbers and free radical scavengers.
Summer may be winding down, but stay protected, and maybe stop and check out the ingredient list on your sunscreen. Perhaps those names aren't such a mystery anymore.
About the Author
 Jeremy Yatvin is a native of Philadelphia, PA and is currently a Ph.D. Candidate at the University of Georgia studying surface and polymer chemistry under Dr. Jason Locklin. He studies how antimicrobial polymers work, synthesizes durable fire retardant coatings for textiles, and is developing new forms and applications for attaching complex molecules to surfaces. Outside of lab Jeremy's greatest passion is various forms of rock climbing around the Southeast United States. |
