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Light Energy and Sunscreens

By: Peter T. Pugliese, MD
Posted: March 26, 2009, from the April 2009 issue of Skin Inc. magazine.

page 4 of 13

When light shines on the skin, several processes take place. Light can be reflected, scattered, transmitted into skin and then remitted—that is, sent back out to the surface. All of these processes have an effect on how much energy enters the skin and is able to react with the tissue. First, how light interacts with the skin’s surface will be reviewed, since this is the entrance point into the skin. Figure 3 is a diagram of reflected, transmitted and remitted light. Light first strikes the surface, and it is here that sunscreen is applied on exposed skin. The angle that the rays strike the skin has a major effect on the amount of light energy that is able to enter it. This can also be seen in Figure 4, which shows various amounts of sunlight at different times of the year. Also, notice in the figure that UVB is attenuated going through window glass, but attenuated even more when passing through auto glass. The take-home message here is be careful of sun exposure two hours before and two hours after noon.

Anything that reduces the light entering the skin will also reduce the potential damage from the UV rays. Before the 1950s, many teenage girls wore pancake makeup that acted as both a scattering agent and a sunblock. Even though they had no way of knowing this, they where protecting their skin from UV radiation damage. The whole purpose of sunscreen is to prevent or markedly reduce the effects of UV radiation on skin. After entering the skin, the UV light can react with the biochemicals in the skin. At this point, the domain of photochemistry is entered—an area that few dare to tread. Yes, it is a complex topic, but if you have an understanding of the basic principles of photochemistry, you have all the knowledge you need to use sunscreens both effectively and wisely.c

Essential photochemistry

Photochemistry covers many fields, but the branch of molecular photochemistry needed for this article involves how photons interact with organic matter. Even further, the focus on light will be limited to a wavelength between 200–700 nm, which covers both UV and visible electromagnetic radiation. You learned earlier in this article that, in the physics of light, photons travel in a wave and have both an electric and magnetic component, and that light travels in these waves at different frequencies. At this point, organic molecules will be discussed in order to get a bit of information to tie light interaction to the skin.

All organic molecules have energy—more specifically, bond energy—the energy required to hold one atom to another to form the molecule.d As a result, some of these molecules and their composite atoms are vibrating at set frequencies, or oscillations. Some organic molecules have dipole oscillations within their atomic structures, and those molecules in the skin that can interact with light have dipole oscillations that match the oscillations in light waves. Only those molecules that have identical dipole oscillations of the light wave can absorb the energy from the light. That phenomenon is called resonance. In general, only organic molecules that are unsaturated can absorb UV light higher than 220 nm. This list includes both cyclic (one ring) or polycyclic (more than one ring) compounds, such as purine and pyrimidine bases in deoxyribonucleic acid (DNA).e It includes the cyclic amino acids tyrosine, tryptophan and phenylalanine; and the noncyclic amino acids cytosine and cysteine. The biochemicals melanin and hemoglobin are also very important. When UV light interacts with the aforementioned compounds, energy is transferred from the light via a photon and passed to the organic molecule, either temporarily or permanently changing that molecule.

When a molecule absorbs a photon, it is electrically excited. This is an unstable state, and the molecule must return rapidly to its pre-excited state to survive. It can do this by four possible mechanisms.

  • Give up the energy as heat. Heat is a form of energy that is easily transfered from one object to another, or just radiated into the air. The loss of heat in molecules means it returns to lower energy, known as the ground state.
  • Give up the photon as light. When molecules do this, the light emitted is always a lower energy than what was initially absorbed. This light is called emission fluorescence, and it is a technology used by many branches of science to study everything from simple measurements in blood to quantum mechanical exploration.f
  • Undergo a permanent structural change in the molecule. This structural change allows the molecule to retain the extra energy from the photon, but a new molecule is formed. Some of these molecules are quite harmful to the body.g
  • Form a new molecule. The excited molecule can react with an appropriate molecule nearby and form a new molecule.