Summer 2023 in the Northern Hemisphere is on track to be the hottest on record, and the sun is blazing in the sky. One way to deal with it is to slap on the sunscreen. But have you ever thought about how sunscreen actually works? It all comes down to how photons from the sun interact with our skin.
Photons are the messenger particles of the electromagnetic force—one of the four fundamental forces of nature—and are responsible for an array of phenomena including the X-rays we use to examine broken bones, the microwaves we use to reheat food, and, probably most importantly for many people, the visible light we use to see.
During summer, we receive the maximum flux of photons from the sun due to the Earth’s slight tilt in its direction. At roughly the latitude of Chicago, the flux of photons is three times greater at midday in the peak of summer than during midwinter.
The sun emits photons in all parts of the electromagnetic spectrum, but the majority are from the visible, infrared and ultraviolet segments. Ultraviolet radiation plays an essential role in maintaining plant and animal life, but it has also consistently been identified as a cause of skin cancer. Understanding why is the first step to understanding how sunscreen protects us from it.
UV radiation has a higher frequency than visible or infrared light, which means that, of the three types, UV photons have the most energy. When UV photons hit your skin, their energy has to go somewhere. (Even in the summertime, no one gets a holiday from conserving energy.) In the absence of protection, this energy is transferred to the fats and proteins in your skin. The excess energy is capable of triggering mutations in our DNA, which are a cause of skin cancer.
While our bodies do possess some natural protective mechanisms against UV radiation, the prevalence of skin cancer (along with painful sunburns) clearly demonstrates that it is necessary to enhance these mechanisms artificially.
The active ingredients of sunscreen fall into two main categories: organic molecules and inorganic crystals. Both of these components act by absorbing UV radiation like a sponge and then dissipating it safely into the environment. How does this work? It all has to do with electrons and quantum mechanics.
As you may remember from chemistry classes, electrons in atoms and molecules occupy orbitals i.e., discrete energy levels. An electron stays put in its home orbital unless it absorbs the right amount of energy to jump up to the next one. Because of this, an electron can’t contain any old amount of energy—only specific, quantized amounts. This is where the “quantum” comes from in “quantum mechanics,” which includes the study of quantized energy in subatomic particles.
The inorganic compounds in sunscreen have a crystalline structure and contain (mostly) free electrons. These electrons are constantly buzzing around and interacting, which creates a flexible orbital structure called a band gap.
The band presents a loophole to the quantized energy problem in quantum mechanics because it allows electrons to absorb a wide spectrum of energies. (After all, there’s not just a single dangerous wavelength of light from the sun.)
“In isolated atoms, you have pretty sharp, quantized transitions between atomic orbitals,” says Thomas Wolf, a physical chemist at the US Department of Energy’s SLAC National Accelerator Laboratory. “If you now have many atoms in a lattice like in an inorganic sunscreen, their atomic orbitals can overlap. This leads to many quantized transitions, which are fairly similar in energy and form bands. If light gets absorbed, electrons get promoted from an occupied to an unoccupied band across a band gap.”
When UV photons from the sun hit inorganic sunscreen, the electrons dash from the lower orbitals into the excited orbitals, each jumping a distance equivalent to the energy of the photon that excited it. After a while, the excited electrons drop back down to their original orbitals, releasing the energy they absorbed as heat.
Organic sunscreens work in a similar way, but their active ingredients have no band gaps. Instead, they use the beauty of covalent bonds and hybridized orbitals.
Covalent bonds form when an electron is shared almost evenly between two atoms, and this creates orbital hybridization (the mixing and merging of two independent atomic orbitals into a new super orbital, so to speak). Organic sunscreens use rings and chains of covalently bonded carbon atoms to play with the distance between these new ground and excited states. Combining many different molecules with many different orbital configurations allows organic sunscreens to protect the skin against many different wavelengths of light.
There is ongoing research to find the most efficient mechanism for the excited electrons in sunscreen to release their energy, with researchers taking inspiration from the mechanisms that plants use to protect themselves from the sun. Scientists are also researching how to make organic sunscreens hardier, since over time and after atoms have absorbed a certain amount of energy, the bonds between them can snap.
So there you have it, the science behind sunscreen. To all you physics students out there: Even on the beach, you are still applying quantum mechanics, literally to your skin!